Ampacity

© 2012 Fluor CorporationCOMPANY CONFIDENTIAL

Principles of Cable Sizing and Selection

By Anthony G. Quiogue, PEE

37th Annual National Convention of the Institute of Integrated Electrical Engineers

© 2012 Fluor Corporation*** COMPANY CONFIDENTIAL ***

2

Introduction

There are several aspects of selecting cables for a given application.

Among the most important are the following: type of installationor laying conditions, voltage rating, ambient conditions, and the most important aspect, ampacity.

The more complicated issue in cable selection is choosing on thebasis of ampacity.

The Philippine Electrical Code (PEC) has specific rules for calculating ampacity requirements for branch circuits and feeders

Having calculated the required ampacity, the next step is to select the cable type. After selecting the type the next step is to carefully determine the cable ampacity corrected for variant conditions ofinstallation intended for the cable’s use.


3

Cable Selection

Cable Selection

Cable selection involves carefully considering the following factors:

Voltage Cables are rated for a certain voltage of application. Common building wires are rated for 600 Volts. In the Code, generally, rules of application are divided into for cables 0 to2000 Volts and for cables over 2000 Volts. Choose the cables appropriate for the intended use.

Insulation type The insulation types are marked on the cables. These markings identify the insulation material.


4

Cable Selection

Choosing the insulation type necessitates knowing the following: The maximum temperature the cable will be allowed to

reach in use. This is greatly influenced by: Ampacity requirements of the load. This sets the minimum

requirement in sizing a conductor for use.


5

Cable Selection

Terminal provisions of connected equipment– Those rated 100 A or less or marked for 38 sq mm or less shall have

conductors rated 60C connected to them; conductors rated 75C can be used if the ampacity is determined at 60C rating.

– Those rated more than 100 A or marked for more than 38 sq mm shall be allowed to have 75C rated conductors connected. Higher temperature rating conductors shall be allowed if their ampacities are determined at 75C.

– Higher conductor ratings may be used if the equipment is listed and identified for use with such higher temperature


6

Cable Selection

The moisture factor: whether the condition of installation is

–dry –wet–damp


7

Cable Selection

Cable Construction This factor will be determined primarily by the design installation conditions. This factor consists of the following: Whether solid or stranded, for small conductors Whether a jacket is applied over the insulation, over a

set of conductors, or over armoring Whether the cable is single conductor or

multiconductor For multiconductors, the number of core conductors For multiconductors, whether the ground wire and/or

the neutral is integrated with the cable


8

Cable Selection

Additional covering or whether shielding and/or armoring or metal sheathing is desired: shielding is basically for electromagnetic isolation, armoring is for mechanical protection while metal sheathing provides water-tight protection. For example, lead sheathed cables are common in the petroleum industry. Cable construction also determines resistance to damaging

environmental conditions. Whether the insulation or additional covering is needed to afford chemical resistance, radiation resistance, flame resistance, flame retardance, ozone, oil or fungus resistance, etc


9

Cable Selection

Ampacity The most important factor to consider is cable ampacity. Basically, it should match the requirement of the circuit, that is, the demand of the load.

After the ampacity is determined, the following conditions should then be checked and the size of the conductor adjusted as necessary: Voltage drop. The Code recommends a maximum combined

voltage drop of 5% for branch circuit and feeders. A related issue is the necessary check on the terminal voltages of control and protective device, to ensure that they will operate as required


10

Cable Selection

The basic rule in the Code is that conductors shall be protected against overcurrent at their derated ampacities. Protective device rating. The PEC states that –The next higher size of protective device rating shall be

permitted for ampacities 800 amperes or less.–The next higher size of protective device shall be used for

ampacities higher than 800 amperes.


11

Ampacity -- Definition

Ampacity is defined in the PEC (Article 1.1.1) as the current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. This definition gives us the following points to consider:

1. Ampacity is a continuous current rating of the conductor;

2.The conditions of use MUST be specified,

3. The conductor’s temperature rating MUST not be exceeded


12


The mechanics for the rise in temperature in a conductor must also be understood to fully appreciate the limitations and imposed by the conditions of use on cable ampacity.

Consider a conductor with resistance R carrying an amount of impressed current I. The current will generate heat within the conductor at the rate P according to the following power formula

P = I2R.

To a lesser degree heat is also generated by induced currents in the other metallic components of the cable, if present. Among these are metallic sheaths and armors


13


Losses in the dielectric materials surrounding the conductor also contribute to heat generation (insulation, shields, screen, jacket and serving/bedding)

Other heat sources (e.g. other current-carrying conductors nearby) contribute heat to the conductor as well as to the environment surrounding that conductor

All the generated heat tends to raise the temperature of the conductor material. With a sufficient combination of current and time, (I2t), the conductor, and its insulation will attain a higher temperature than the ambient (air or soil).


14


The difference in temperature gives rise to the transfer of heat from the cable to the environment. This transfer occurs by way of conduction, convection and radiation

– Conduction is the transfer of heat from molecule to molecule and generally occurs predominantly if the material surrounding or touching the cable is solid, such as earth

– Convection occurs if the material surrounding the cable is a moving fluid such as air, oil or water. Convection is the transfer of molecules (and therefore with it, heat) either by bulk or significant diffusion

– Thermal radiation is another mode of heat transfer although a less significant one under normal temperature ratings


15


Generally all modes of heat transfer operate. Conductors in a raceway in earth will have convection operating in the air within the duct and conduction from the duct to the earth

The rate at which heat is transferred to the environment is affected by temperature difference as well as the thermal barriers between the conductor and the environment. Thermal barriers include the insulation, jacketing, air inside raceway, the raceway itself, soil, concrete, building materials, and any thermal insulation applied such as fireproofing.


16


If an equilibrium is attained between the rate of heat transfer and the rate of heat generation, a steady state temperature is achieved by the conductor which is also the temperature at the boundary with the insulation.

The conductor can normally withstand the temperature developed within it. However, the insulation enclosing the conductor has temperature limits as determined by its thickness and composition


17


This limit is termed the conductor’s maximum operating temperature.

The object therefore of defining the ampacity of a specific conductor is to set the value of current at which the maximum operating temperature will not be exceeded.


18

Factors Affecting Ampacity – Inherent Cable CharacteristicsThe PEC recognizes inherent cable characteristics, determined bydesign and manufacture, that set the fundamental parameters for a cable’s ampacity rating. Among these are:

Conductor characteristicsa. Conductor material b. Conductor size

Insulation characteristicsa. Type of materialb. Thickness

Constructiona. Single conductor or multi-conductorb. Presence of jacketing, coverings, sheathing and armoring


19

Factors Affecting Ampacity – Inherent Cable Characteristics

These characteristics in turn, determine the following

Application provisions (voltage, location and type of installation)

Temperature rating (maximum operating temperature)


20


Conductor Material The material of the conductor affects the ampacity

because the resistivity, and hence the resistance, for a given size is determined by the type of material

The formula for resistance is as follows:R = L/A

Where R is the conductor’s d.c. resistance, is the conductor material resistivity constant, L is the length and A is the cross-sectional area.


21


Conductor Material The most common conductor materials are copper,

with a resistivity of 1.68 x10-8 ohm–m at 20oC, and aluminum with a resistivity of 2.82 x10-8 ohm–m at 20oC


22

Factors Affecting Ampacity – Inherent Cable Characteristics Conductor Size

From the same formula it is noted that resistance is inversely proportional to the cross sectional area. Therefore, the smaller the wire diameter, the greater the resistance for any given length, all other factors being equal.

Wire sizes in the PEC are designated in terms of square millimeters (mm2).


23

Factors Affecting Ampacity – Inherent Cable Characteristics Insulation types

Electrical conductors are normally coated or wrapped in insulating materials to isolate conductors from one another as well as provide some sort of mechanical protection

There are various insulation material types used for conductor insulation. Among the common ones are:– Natural polymers such as paper, cotton and natural

rubber – Synthetic polymers such as thermoplastic materials (e.g.

PVC and fluorinated plastics) and thermosetting materials (e.g. synthetic rubber, cross-linked polyethylene, and silicone rubber)

– Mineral insulation


24

Factors Affecting Ampacity – Inherent Cable Characteristics Insulation types

The different insulation types offer different characteristics which, together with insulation thickness, determine the application conditions for which these types will be allowed. Among these characteristics are:– Dielectric strength– Resistivity– Flame and Heat resistance– Resistance to abrasion, cracking, crushing and impact


25


Insulation thickness Cables are wrapped in varying thicknesses of

insulation. The PEC specifies required insulation thicknesses for listed insulation types.

The thicker the insulation, the better it can withstand an applied voltage before breaking down, so generally, the higher the conductor’s voltage rating the thicker the insulation.

Aside from providing electrical isolation, insulation should also provide some mechanical protection


26


The PEC Tables 3.10.1.13, 3.10.1.61, 3.10.1.62, 3.10.1.63and 3.10.1.64 list approved insulation type and thickness for various cable insulations. In fact the code defines “insulated conductor” as one encased within material of composition and thickness that is recognized by the code as electrical insulation. This recognition is documented in these tables. Conductors not meeting the required insulation type and minimum thickness of encasing material are classified as “covered conductors” only.


27

Factors Affecting Ampacity – Inherent Cable Characteristics Insulation Temperature Rating

Insulated conductors are characterized by a specific maximum operating temperature rating based on the type and thickness of insulation employed on the conductor.

As mentioned in above, the object of defining the ampacity of a specific conductor is to set the value of current at which the maximum operating temperature will not be exceeded.


28

Factors Affecting Ampacity – Inherent Cable Characteristics Insulation Temperature Rating

Higher-temperature rated insulation allows a conductor of the same size to be used at a higher ampacity than a conductor with lower-temperature rated insulation.– Example: A copper conductor of size 8 mm2 has an

ampacity of 40 amperes if the insulation is rated 60oC (Types TW, UF) but it has an ampacity of 50 amperes if the insulation is rated 75oC (Types RHW, THHW, THW, THWN, XHHW, ZW) as per PEC Table 3.10.1.16


29

Factors Affecting Ampacity – External Conditions Ambient Temperature

Ambient temperature is the temperature of the material surrounding the conductor, and can either be the air for aboveground installations, or the earth for underground installations.

The ambient temperature determines the temperature gradient about the conductor. It therefore affects the rate of transfer of heat from the conductor to the environment.


30

Factors Affecting Ampacity – External Conditions Ambient Temperature (cont.)

Ampacity ratings are defined based on a given ambient temperature. If there is a significant difference between the actual (operating) ambient and the ambient at which the cable is rated, ampacity is correspondingly adjusted.


31

Factors Affecting Ampacity – External Conditions Installation Conditions

Installation methods affect the way generated heat is transferred to the environment and can be one of the following:

– Free air– Directly buried in earth– Enclosed type of installations restrict the flow of air


32

Factors Affecting Ampacity – External Conditions Installation Conditions (Cont.)

Free air – A cable installation that is not enclosed in a raceway nor routed underground and allows the air to flow freely. With this type of installation, cable conductors can more easily cool to the ambient air, temperature. Installation of this type can be one of the following:– Open Wiring on Insulators - An exposed wiring method

using single insulated conductors supported on insulators run in or on buildings.

– Messenger Supported Wiring - An exposed wiring support system using messenger wire to support insulated conductor.

– Cable Tray – Ladders and ventilated troughs allow free flow of air around the conductors (However, covered solid bottom cable trays restrict the flow of air by practically enclosing the conductors)


33


– Directly buried in earth

Cables directly buried in earth tend to cool to the ambient earth temperature. Ambient earth temperature varies with location, season and depth. Typical values in temperate countries are 15oC in winter and 25oC in summer. In the NEC tables, the reference ambient earth temperature of direct buried cables is 20oC.

Depth of installation also affects cable ampacity of direct buried cables. A 6% derating in ampacity (that is multiply the ampacity by 94%) for every 300 mm increase in depth of laying from the value specified in the Code.


34


– Enclosed type of installations restrict the flow of air Raceway - metal conduit or nonmetallic conduit,

tubing, wireways and ducts, Cable installations using raceways could either be

aboveground or underground. Ambient air temperature is the reference for aboveground installations while ambient earth temperature is the reference for installations underground

Likewise for underground ducts, depth of installation also affects cable ampacity of direct buried cables. A 6%derating in ampacity (that is multiply the ampacity by 94%) for every 300 mm increase in depth of laying from the value specified in the Code


35


Grouping with other cables.

– When several wires or cables are grouped together in a run, each one gives off heat and affects ampacity by

Transferring generated heat directly to other cables, and/or

Raising the immediate ambient air or earth temperature, thereby diminishing the temperature gradient or difference


36


Grouping with other cables.

– The net effect is to decrease the individual cable’s rate of heat transfer This necessitates a reduction of cable ampacity so as not to let the temperature of the conductor rise to a level injurious to the insulation.

– Each of the PEC ampacity tables define under what grouping conditions the listed ampacity values hold true. When actual grouping condition differ from what is defined in the ampacity tables, adjustment factors for derating or reduction of ampacity rating is required by the code


37

Ampacity Determination The PEC allows two methods of determining ampacity of cables

1. By use tables of cable ampacities2. By calculation under engineering supervision using the

Neher-McGrath equation There are twenty six (26) ampacity tables in the PEC, six for

cables 0 – 2000 V, and twenty for cables over 2000 V


38

Using the PEC Ampacity Tables The use of the PEC ampacity tables require

understanding the conditions under which the tables were developed. Deviations from the defined conditions require the application of adjustment factors, specifically for ambient temperature and installation condition including grouping.


39

PEC Ampacity Tables There are six ampacity tables for cables rated 0 – 2000V in the

PEC. Below is the tabulated data for ampacity Tables 3.10.1.16 to 3.10.1.21

400CRaceway or cable

Copper/Nickel or nickel-coated

copper/Aluminum or copper-clad

aluminum

Insulated conductors, not more than three current-carrying

conductors

150-2500C3.10.1.18

Copper/Aluminum or copper-clad

aluminum


aluminum

Conductor Type

60-75-900C

60-75-900C

Conductor Temperature

300C

300C

Ambient Temperature

Free airSingle-insulated conductors3.10.1.17

Installed in raceway, cable, or

earth (directly buried)


conductors

3.10.1.16

Installation Condition

Construction / GroupingTable No.


40

PEC Ampacity Tables

400CFree air,

610mm/sec wind velocity

Copper/AAC Aluminum

Bare or covered conductors800C3.10.1.21


aluminum



aluminum

Conductor Type

75-900C

150-200-2500C


400C

400C

Ambient Temperature

Supported on a

messenger

Not more than three single-

insulated conductors

3.10.1.20




Ampacity Tables for Cables rated 0 – 2000V (cont.)


41

PEC Ampacity Tables

There are twenty ampacity tables for cables rated 2001 – 35,000V in the PEC. Below is the tabulated data for ampacity Table 3.10.1.67 to3.10.1.86

Aluminum

Aluminum

Aluminum

Aluminum

Copper

Copper

Aluminum

3.10.1.76

90-1050C3.10.1.74

3.10.1.72

3.10.1.70

3.10.1.68

400CIsolated conduit in air

Insulated three-conductor cables90-1050C

3.10.1.75


CopperInsulated triplexed or three single-

conductor cables

3.10.1.73

400CIsolated in airInsulated three-conductor cables90-1050C

3.10.1.71

400CIsolated in airCopperInsulated single

conductor cables90-1050C3.10.1.69

400CTriplexed in airCopperInsulated single

conductor cables90-1050C3.10.1.67

Conductor TypeConductor Temperature

Ambient Temperature

Installation ConditionConstructionTable No.


42

PEC Ampacity Tables

Aluminum

Aluminum

Copper

Aluminum

3.10.1.82

3.10.1.80

3.10.1.78

200CDirectly buried in earth

Single insulated copper conductors90-1050C

3.10.1.81

200C

In underground

electrical ducts (one cable per electrical duct)

CopperThree insulated conductors cabled within an overall covering (three-conductor cable)

90-1050C

3.10.1.79

200C

In underground

electrical ducts (three

conductors per electrical duct)

Copper

Three single-insulated

conductors90-1050C

3.10.1.77


Ambient Temperature



43

PEC Ampacity Tables

Aluminum

Aluminum

Copper

3.10.1.86

90-1050C3.10.1.84

200C

Directly buried in earth, RHO of 90, 100% load factor

Three triplexed single insulated

copper conductors90-1050C

3.10.1.85

200CDirectly buried in earth

CopperThree insulated conductors cabled within an overall covering (three-conductor cable)

3.10.1.83

Conductor TypeConductor Temperatur

eAmbient

TemperatureInstallation ConditionConstructionTable

No.


44

PEC Ampacity Tables There are eight other ampacity tables included in Appendix B

of the 2009 PEC. These tables, summarized below, are not part of the Code per se but they provide ampacity information for conditions not described in the tables within the Code.

200C (earth)In Non-magnetic

Underground duct (one conductor per

duct)

Single Insulated conductors 750CB-310-5

60-75-85-900C

(cable type TC, MC, MI,

UF and USE)

60-75-900C


400C

300C

Ambient Temperature In

air (UON)

In Free airMulticonductor with at most 3

Insulated conductors

B-310-3

Installed in raceway in free air

2 to 3 Insulated conductors, with overall covering (multiconductor

cable)

B-310-1



Cables rated 0 to 2000V, Copper/Aluminum or copper-clad aluminum


45

PEC Ampacity Tables

200C (earth)Directly Buried, 100% Load factor, RHO of

90

3 triplexed single insulated conductor

60-750CB-310-9


90

2 to 3 Insulated conductors, with overall covering (multiconductor

cable)

60-750CB-310-8


90

Three Single Insulated

Conductors60-750CB-310-10

750C

750C


200C (earth)

200C (earth)

Ambient Temperature In

air (UON)

In Underground duct (three conductors

per duct)

Three Single Insulated

conductors B-310-7

In Underground duct (one cable per duct)

Three-Conductor Cable B-310-6

Installation ConditionConstruction / GroupingTable No.

Note: For underground duct and directly buried cables, see Figure B-310-2 of the PEC for cable installation dimensions.


46

PEC Ampacity Tables The Tables of Ampacities in the PEC are characterized by the

very specific parameters described above. Some of these (voltage, maximum operating temperature, conductor material, construction) are either inherent characteristics of the cable or are designated by the manufacturer based on standard tests. They are the first filters to be used when selecting cables.


47

PEC Ampacity Tables Each of these parameters should be understood when applying

the ampacity tables. Any deviation from these parameters can mean one of the following:1. The table is not applicable2. The table is applicable but adjustment factors have to be used

There are two parameters for which deviations from the listed condition in the NEC ampacity tables can be compensated (or will be modified) by use of adjustment factors: 1. Ambient temperature (air or earth)2. Grouping and/or installation conditions


48

Using the PEC Ampacity Tables - Adjustment Factors for Ambient Temperature For cables rated 0 to 2000 V, the adjustment factors for deviations

from listed ambient are tabulated at the bottom of each ampacitytable. Below is an example for the factors for table 3.10.1.16


49

Using the PEC Ampacity Tables - Adjustment Factors for Ambient Temperature For cables rated over 2000 V, the adjustment of ampacity for

deviations from listed ambient are determined from the following formula:

TDTATC

TDTATCII

1

212

WhereI1 = Ampacity from tables at ambient TA1I2 = Ampacity at desired ambient TA2TC = Conductor temperature in degrees Celsius (oC)TA1 = Surrounding ambient from tables in degrees Celsius (oC)TA2 = desired ambient in degrees Celsius (oC)TD = Dielectric loss temperature rise

Note that the same formula is valid for cables rated for 0 to 2000 volts by ignoring the terms TD, the dielectric loss temperature rise.


50

Using the PEC Ampacity Tables – Grouping with other cables

Each of the PEC ampacity tables define under what grouping conditions the listed ampacity values hold true. By way of example, the following table header from the PEC table 3.10.1.16 is shown:

Thus the table is expressed as valid for one to “three current carrying conductors in raceway, cable or earth (directly buried)”


51

Using the PEC Ampacity Tables – Grouping with other cables– When actual grouping condition differ from what is defined in the

ampacity tables, adjustment factors for derating or reduction ofampacity rating is required by the code

– Table 3.10.1.15(b)(2)a lists the adjustment factors for more than three current- carrying conductors in a raceway or cable.

– Neutral conductors carrying only the unbalanced current from thesame circuit as well as grounding and bonding conductors are notcounted as current-carrying conductors specified in the table.


52

Using the PEC Ampacity Tables – Grouping with other cablesThere are exceptions to the application of the factors listed inTable 3.10.1.15(b)(2)a. The more significant exceptions are:– Derating factors apply only to power and lighting cables.

– Derating factors do not apply to conductors in nipples not exceeding 610 mm (24 inches)

– Derating factors do not apply to not more than four conductors entering or leaving an outdoor trench protected by rigid metal conduit, intermediate metal conduit or rigid non-metallic conduit not exceeding 3.05 m (10 ft)

– Cable installed in cable trays. This is discussed in detail in the topic Ampacity of Cables in Cable Tray

– Cable installed in metal wireways (and similar raceways). This is discussed in the topic Ampacity of Cables in Metal Wireways and Similar Raceways.


53

Ampacity of Cables in Cable Tray

The PEC has definite rules for cables installed in cable trays. Even in the ways of installing cables in a cable tray, is governed by specific rules.

– These rules generally specify the type, number and sizes of cables that can be laid in a certain tray type (Cable ladder, ventilated through, ventilated channel, or solid bottom) and size, and whether the cables can be laid in a single layer, or in multiple layers. Consequently these rules also determine the size of trays that should be used for a given collection of cables.

– The topics of cable installation in cable trays and the sizing of cable trays are quite broad and will not be discussed here. Suffice it to say, that compliance with the rules is a precondition of the PEC in allowing the use of available ampacity tables in the code for cables in cable trays.


54


For multiconductor cables rated 2000 volts or less, the following tables are used

B-310-3 400CIn Free airMulticonductor with at most 3

Insulated conductors

60-75-85-900C

(cable type TC, MC, MI,

UF and USE)

B-310-3



aluminum


aluminum

Conductor Type

150-2500C

60-75-900C


400C

300C

Ambient Temperature

Raceway or cable


conductors

3.10.1.18

Installed in raceway, cable, or

earth (directly buried)


conductors

3.10.1.16




55

Ampacity of Cables in Cable TrayThe conditions for using the ampacities in these tables are as

follows:– The multiconductor cables are installed in accordance with the

requirements of the code as detailed in 3.92.1.9.

– The derating factors for more than three current-carrying conductors in a raceway or cable (table 3.10.1.15(b)(2)a are applicable only to each cable and not to the total number of conductors in a tray.

– Ampacities for multiconductor cables shall be as shown in tables 3.10.1.16 or 3.10.1.18, as applicable. When a cable tray is covered for more than 1.8 meters, a derating factor of 0.95 is applied to the ampacities in the tables 3.10.1.16 or 3.10.1.18.

– When multiconductor cables are installed in a single layer, with a minimum spacing of 1 cable diameter, the ampacities in table B-310-3 can be used.


56

Ampacity of Cables in Cable TrayFor single conductor cables rated 2000 volts or less, the ampacities in following tables are used:

400CSupported on a messenger


aluminum

Not more than three single-

insulated conductors

75-900C3.10.1.20



aluminum


aluminum

Conductor Type

150-200-2500C

60-75-900C


400C

300C

Ambient Temperature






57

Ampacity of Cables in Cable TrayThe conditions for using the ampacities in these tables are as

follows:– The single conductors are installed in accordance with the requirements of

the code as detailed in 3.92.1.10, and are 50 sq mm or larger.

– The derating factors for more than three single current-carrying conductors in a raceway or cable shall not apply to the ampacity of conductors in a cable tray.

– Ampacities for conductors 325 sq mm and larger shall not exceed 75% of the values listed in the tables 3.10.1.17 and 3.10.1.19. If the tray is covered for 1.8 meters or more, ampacities shall not exceed 70% of the listed values in these tables.

– Ampacities for conductors 50 sq mm through 250 sq mm shall not exceed 65% of the values listed in the tables 3.10.1.17 and 3.10.1.19. If the tray is covered for 1.8 meters or more, ampacities shall not exceed 60% of the listed values in these tables.


58


The conditions for using the ampacities (cont.):– Ampacities for single conductors, when installed in a single layer in an

uncovered cable tray, with a spacing of not less than one cable diameter between conductors, shall be as listed in tables 3.10.1.17 and 3.10.1.19.

– When single conductors are installed in triangular (trefoil) or square configuration, in an uncovered cable tray, with a maintained spacing of 2.15 times the conductor diameter of the largest conductor in the configuration, ampacities listed in table 3.10.1.20 can be used.


59


For multiconductor cables rated 2001 or over, the following tables can be used:

Aluminum

Aluminum

Copper

Copper

3.10.1.76

3.10.1.72


Insulated three-conductor cables90-1050C

3.10.1.75

400CIsolated in airInsulated three-conductor cables90-1050C

3.10.1.71


Ambient Temperature



60


The conditions for using the ampacities in these tables are as follows:

– The multiconductor cables are installed in accordance with the requirements of the code as detailed in 3.92.1.12.

– Ampacities for multiconductor cables shall be as shown in tables 3.10.1.75 or 3.10.1.76, as applicable. When a cable tray is covered for more than 1.8 meters, a derating factor of 0.95 is applied to the ampacities in these tables.

– When multiconductor cables are installed in a single layer in an uncovered cable tray, with a minimum spacing of 1 cable diameter, the ampacities shall not exceed the values in tables 3.10.1.71 or 3.10.1.72.


61


For single conductor cables rated 2001 or over, volts or less, the ampacities in following tables are used:

Aluminum

Aluminum

Copper

Copper

3.10.1.70

3.10.1.68

400CIsolated in airInsulated single conductor cables90-1050C

3.10.1.69

400CTriplexed in airInsulated single conductor cables90-1050C

3.10.1.67


Ambient Temperature

Installation ConditionConstructionTable

No.


62


The conditions for using the ampacities in these tables are as follows:

– The single conductors are 50 sq mm or larger and are installed in accordance with the requirements of the code as detailed in 3.92.1.12.

– The derating factors for more than three single current-carrying conductors in a raceway or cable shall not apply to the ampacity of conductors in a cable tray.

– Ampacities for conductors in uncovered cable trays shall not exceed 75% of the values listed in the tables 3.10.1.69 or 3.10.1.70 as applicable. If the tray is covered for 1.8 meters or more, ampacities shall not exceed 70% of the listed values in these tables.

– When single conductors are installed in triangular (trefoil) or square configuration, in an uncovered cable tray, with a maintained spacing of 2.15 times the conductor diameter of the largest conductor in the configuration, with the adjacent conductor, ampacities listed in table 3.10.1.67 or 3.10.1.68 can be used.


63

Ampacity of Cables in Metal Wireways and Similar Raceways

Some raceways allowed by the Code are structurally extended boxes with covers, which have larger cross sectional areas than typical conduit-type or tubing-type raceways and therefore afford larger space for conductors. The following raceway types fall under this category:

Sheet metal auxiliary gutters

Metal wireways

Strut type Channel raceways

Surface metal raceways


64

Ampacity of Cables in Metal Wireways and Similar Raceways (continued)

Generally, the rules governing ampacities of cables installed inthese types of raceways are:

The sum of the cross sectional area of all contained conductors shall not exceed 20% of the interior cross-sectional area of the raceway at any point.

The derating factors for more than three single current-carrying conductors in a raceway or cable shall NOT be applied IF the number of conductors is 30 or less. Conductors for signaling, control or communication are not counted as current-carrying conductors. For strut-type channel, and surface metal raceways, an additional condition for the derating NOT apply is that the channel or surface raceway have cross sectional area of at least2500 sq mm.

Non-metallic auxiliary gutter, non-metallic wireway are not exempted from the application of the derating factor, regardless of the fill or number of conductors.


65

Neher-McGrath Equation

The Neher-McGrath equation is another method that can be used in calculating for cable ampacity under engineering supervision as the PEC requires.

The Neher-McGrath formula is a heat transfer formula, comprising a series of heat transfer calculations, that takes into account all heat sources and the thermal resistances between the heat sources and free air.

The most common use of this formula is to calculate for the ampacity of conductors in underground electrical ducts (raceways), although the formula is also applicable to all conductor installations.


66

Neher-McGrath Equation (continued)

To understand how the formula relates to ampacity we need to examine how heat is transferred in respect to a current carrying conductor. Current carried by a conductor passes through the electrical resistance of the conductor. When this happens heat is generated.

The heat generated in the conductor passes through several thermal barriers by convection, conduction, and radiation and dissipates into the air. Possible thermal barriers are the conductor insulation, the air inside a duct, the duct wall, the soil surrounding an underground duct, and any additional thermal insulation applied.


67


The rate of heat transfer is dependent on several variables and can be described by a thermal equation that closely resembles ohms law (E=I x R), substituting heat for current and thermal resistance for electrical resistance.

In a heat transfer equation the rate of heat transfer is directly dependent on the difference in temperature between the conductor (TC) and the ambient (TA). In a heat transfer equation TC-TA = (I x I x R) x RCA, where I is current in amperes, R is electrical resistance in ohms, and RCA is thermal resistance in degrees Centigrade-cm/watt usually called thermal-ohm-feet. TC is the maximum permissible operating temperature in degrees Centigrade of the conductor. TA is the ambient temperature of the air or soil for underground installations. Solving for I:

xRCARTATCI


68


Equation No. 1 is sometimes called the Fourier heat transferequation. The equation in section 3.10.1.60(d) of the PEC, called the Neher-McGrath equation, is a more complex version of the Fourier heat transfer equation. The equation in section 3.10.1.15(c) and section 3.10.1.60(d)

of the PEC, called the Neher-McGrath equation, is a more complex version of the Fourier heat transfer equation.


69


The Neher-McGrath equation:

Where:I = Ampacity, (kA)TC = Conductor temperature, (°C)TA = Ambient temperature in °C∆TD = Conductor temperature rise due to dielectric loss, (°C)RDC = Conductor dc resistance, (µΩ/m)YC = Loss increment due to conductor skin and proximity effectsRCA = Thermal resistance between conductor and ambient, (thermal Ω-m)

RCAYCRDCTDTATCI

)1(


70


This equation was developed by J.H. Neher and M.H. McGrathand presented in a paper entitled “The Calculation of the Temperature Rise and Load Capability of Cable Systems” to AIEE (American Institute of Electrical Engineers) the precursor to the organization IEEE.

In this equation, ∆TD, is a term added to the ambient temperature, TA, to account for the heat generated in the insulation and other covering of dielectric material. It represents the temperature rise arising from dielectric loss. Itis insignificant for voltages below 2000. Another term in the NM equation, (1+YC), is a multiplier to convert direct current resistance (RDC) to alternating current resistance. For small wire sizes this term becomes insignificant.


71


Although the equation looks deceptively simple, the application to actual cable configurations can be daunting. Appendix B of the PEC contains ampacity tables developed using the Neher-McGrath equation. Power systems software like ETAP and EDSA use the Neher-McGrath equation, as well as equivalent in an IEC standard.


72

Conclusion

Wiring is a very significant part of our work. One of the basic responsibilities of the electrical engineer or electrician is toensure electrical installations are safe. Whether as engineeringdesigners, constructors, equipment manufacturers, or maintenance personnel, we have the obligation to ensure that our wiring design and installations are correct according to the Code. It behooves us therefore to make sure that cables we use are properly selected and sized for use.


73

Q & A

QUESTIONS?


74

References

Philippine Electric Code 2009

National Electrical Code, 2005

Mike Holt Enterprises Website

National Electrical Manufacturers Association (NEMA)

http://neher-mcgrath.com

Documents

Ampacity