7/30/2019 Wiring & Protection

1/204 home power 147 february & march 2012

Chapter 2 of the National Electrical CodeWiring and

Protectionis lengthy and important, covering a number of

topics. PV installers must appropriately apply those sections

that pertain to PV systems.

Conductor IdentificationChapter 2 starts with Article 200, Use and Identification ofGrounded Conductors. Many in the PV industry refer to

the DC conductors from the PV modules as positive and

negative, but that nomenclature is of less importance in

modern systems.

In the majority of PV systems installed today, the inverters

use an isolation transformer. For PV systems using these

inverters, the NEC requires bonding one of the current-

carrying conductors on the PV side of the inverter to

ground. Typically, this is accomplished across the ground-

fault protection device located in the inverterbonding the

negative conductor to ground creates a negatively groundedsystem. Some PV systems require bonding the positive

conductor to ground. (Note that nearly all PV modules can

be positively grounded.) To top off that variability, there are

now ungrounded inverters, in which neither PV conductor

is bonded to ground. These systems are covered in Section

690.35, Ungrounded Photovoltaic Power Systems.

Given the different wiring configurations, its most

accurate to refer to the conductors in terms of grounded

current-carrying; ungrounded current-carrying; and

grounding conductors rather than negative, positive,

and ground conductors. This terminology also helps clarify

how each of these conductors should be identified, which

brings us back to Article 200, which sets the requirements for

identifying the grounded current-carrying conductors for all

electrical systems, including PV systems.

Section 200.6 details those identification methods.

Subsection 200.6(A) deals with identifying conductors

that are 6 AWG and smaller. In residential grid-tied PV

systems, it is likely that all the conductors used will fall into

this size category. Section 200.6(A) lists eight appropriate

identification methods, one of which is specific to PV systems

(discussed later). The first three methods and the PV-specific

requirement are the most commonly used. The grounded

current-carrying conductors can either be white, gray, or

marked with three continuous white stripes on top of any

color insulation other than green, which is reserved solely for

grounding conductors.

Given those requirements, for most PV systems the

negative is the grounded conductor and should follow one

of those identification methods. If the positive conductor is

the grounded conductor, it will follow those rules. And forungrounded systems, neither conductor is marked that way.

Now that you have identified the grounded current-

carrying conductor(s), you can identify and mark the

ungrounded current-carrying conductor(s). These can be any

color other than those listed in 200.6not white, gray, or

with three continuous white stripes; nor green or green with

yellow stripes. In the case of negative-grounded PV systems, I

recommend that the ungrounded color be red. This will meet

Code and helps identify the positive polarity of the conductor.

Regardless of the chosen color-coding, I also recommend

using marking tape at the termination to identify the polarity

of all of the conductors.It is important to cover the marking allowance specific

to PV systems200.6(A)(6) allows the installer to mark

small PV source circuit with distinctive white markings at all

terminations. This is an exception to the general rule for small

conductors, so you may need to bring this to an inspectors

attention. The exposed USE-2 or PV cable can be black and

marked at the terminations made inside junction or combiner

boxes. Just remember that once you transition to an interior

wire such as THWN-2, that conductor type is outside the

scope of this allowance and must meet one of the other

requirements listed.

For conductors 4 AWG and larger, 200.6(B) outlines the

rules you are required to follow. The first three methods are

identical as the requirement for small conductors. The fourth

method allows for distinctive white or gray markings that

encircle the conductor at the termination point.

Overcurrent ProtectionArticle 240, Overcurrent Protection, sets the rules on how to

properly select, locate, and enclose overcurrent protection

devices (OCPDs) to protect conductors from overload, short-

circuit, and ground-fault conditions. Section 240.4 contains

some critical information on the required methods to

properly protect conductors. Conductors must be protected

against overcurrent in accordance with the conductors

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Wiring & Protectionby Ryan Mayfield

7/30/2019 Wiring & Protection

2/210www.homepower.com

ampacity values (as listed in Section 310.15), unless otherwise

required or allowed within 240.4(A) through (G). In general,

a conductor needs overcurrent protection at a value less than

or equal to the conductors ability to carry current.

A general rule is often followed up with specific cases

and/or exceptions. In 240.4(B), an allowance is made forovercurrent devices rated at 800 A or less. When specific

conditions are met, a conductor can be protected by an

overcurrent device with an ampere rating greater than the

conductor. The next higher standard-rated device can be used

to protect the conductor if the conductors ampacity doesnt

correspond to a standard OCPD ampere rating, the next

standard OCPD doesnt exceed 800 A, and the conductors

being protected are not part of a branch circuit supplying

multiple receptacles. For PV systems, once you adjust a

conductors ampacity for conditions of use and continuous

duty, that conductor can be placed on an overcurrent device

with a rating greater than the conductor. For example, letssay you are using a 10 AWG conductor to connect a string of

modules to an inverter. After applying correction factors if

that 10 AWG conductor has an ampacity value of 21 A, you

can use a 25 A OCPD.

Section 240.4(D) lists special requirements for small

conductors, namely the limitations of OCPD ampere ratings

based on the conductor size. So, unless specifically allowed in

240.4(E) or (G), the OCPD protecting small copper conductors

cannot exceed:

15 A for 14 AWG Copper

20 A for 12 AWG Copper 30 A for 10 AWG Copper

When sizing conductors, you may have a scenario where

one of the above listed conductors must be protected by an

OCPD with a smaller rating than the conductors ampacity.

The 240.4(D) section simply defines the maximum OCPD

rating, even if the conductors ampacity exceeds the OCPD

rating. For example, after correction factors are applied, a 10

AWG conductor may have an ampacity value of 35 A. The

Code requires that this conductor is protected by an OCPD

with a maximum ampere rating of 30 A.

Article 240.6(A) lists standard OCPD ratings, the values

referenced when determining the proper OCPD for protecting

conductors. The standard values start at 15 A and go up to 6,000

A. Per 690.9(C), for OCPDs protecting PV source circuits, the

standard ratings are 1 A to 15 A in 1 A increments. After that,

the standard values listed in 240.6 are applicable. Practical

examples of this selection process, as well as conductor sizing

will be covered in an upcoming Code Corner.

Section 240.24 covers the location and accessibility

requirements for OCPDs. Generally, all OCPDs must be

readily accessible and the center of the grip on the OCPDs

operating handle may not exceed 6 feet 7 inches above

the working platform when in its highest position. Two

allowances to this rule may apply to a PV installation. One

is that OCPDs may be accessed by portable means, such as

ladders, if the OCPDs are mounted adjacent to the equipment

they are protecting. The other is not an exception listed in 240

but rather an allowance in 690. In 690.9(C), the Code allows

the OCPDs protecting PV source circuits to be accessible,

but doesnt require that they be readily accessible (per NECdefinition of readily accessibleagain we see that it is

acceptable to access particular OCPDs via a ladder and so

forth).

Section 240.24 specifies that OCPDs be located where

they are not exposed to potential physical damage. This may

require protecting the OCPDs with barriers (for example,

bollards may be used to prevent potential damage to OCPDs

by vehicular traffic). Section 240.24(D) prohibits locating

OCPDs in the vicinity of easily ignitable material, such as

in clothes closets. And finally, OCPDs are not allowed in

bathrooms [240.24(E)] or over steps [240.24(F)].

Part III of Article 240, which covers enclosures forOCPDs, affects many PV installations, and Section 240.32 is

of particular importance. If the OCPDs are located outside,

youll need to verify that the enclosure used to house the

OCPD complies with 312.2, as referenced in 240.32. This is to

prevent moisture from entering and accumulating within the

enclosure. Article 312.2 has requirements for raceways and

cables entering the enclosure, stating that if an enclosure is

being entered above uninsulated live parts, the fittings used

to enter the box must be listed for wet locations. Thankfully,

PV installers have multiple options for enclosures listed for

various applications. For enclosures that will be mounted

on an outdoor vertical surface, you can use NEMA 3R-ratedenclosures. Some manufacturers of NEMA 3R enclosures

have listed their products for installation on a tilted surface;

this should be verified on a per-model basis. Typically, for

enclosures mounted in a non-vertical orientation, a NEMA

4 enclosure should be used (see NEC table 110.28 for more

information on enclosure types).

Setting up Your InstallationThese two Code articles, 200 and 240, set the foundation for PV

installations. Be sure to read through to the end of Article 240,

as it covers the various types of OCPDs. As with nearly all of

the Code, I suggest you read and re-read these articles. Youd

be surprised how often something new jumps out at you and

changes your thinking.

AccessRyan Mayfield (ryan@renewableassociates.com) is the principal at a

design, consulting, and educational firm with a focus on PV systems in

Corvallis, Oregon. He is an ISPQ Affiliated Master Trainer.

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