Objectives

• Finish residential electrical systems• Solve some example

• Compare electrical systems for residential and commercial building

Residential system sizing

Typically 3 wire 1 phase system 120/240 V

• Procedure defined by NEC article 220:

• Identify characteristic electrical loads• Add lighting (3W/ft2)• Add two 20 Amps circuits for kitchen• Add one 20 Amps circuit for laundry• Electricity for HVAC equipment based on requirement for

heating or cooling

Homework 5 - problem 2

• Sizing the residential electrical system

• Defined by NEC article 220-30• Similar to the example we worked out • You need to collect the data for several

characteristic consumers• Includes conductor sizing

Summary of Power Sizing

• Include enough capacity for heating and cooling

• Work through kitchen and then other rooms

• Include any items that are unusual

• Take first 10 kVA and then 40% of remaining load

Panel Sizing

• Find panel power usage and neutral usage

• Size panel conductors and associated conduit and switches

Electric panel - 3 wire 1 phase 120/240

Commercial buildings-Differences

Defined by NEC and local electric code!

For example:

No NMC (non metallic sheeted cables) wiring allowed

Other differences?

• Three phase

• Multiple panels

• More attention to expansion possibilities

• More voltage levels available• For example: 277/480 and 120/208

Typical Building Electrical ServicesVoltage/Phase[V]

Current

[A]

Building Type

120/240 1 (3w)

120/208 3<200 Residential

120/240 1 (3w)

120/208 3200-800 Small

Commercial

220/380 3277/480 3347/600 3

400-2000 Large Commercial

5-15 kV low Very Large Commercial

Building Needs: Lighting and Duplexes

• Lighting• Typically use lighting power densities for building type

• Sometimes actual lighting load

• Requires neutral

• Duplexes• NEC/local electrical code specifies number that are

required for different applications

• Requires a transformer

• Requires neutral wire

Building Needs: HVAC/Other/Auxiliary

• HVAC from system sizing• Don’t forget fans, reheat coils, etc.

• Building equipment• Transportation (elevators etc.)• Food service• Specialty uses• No neutral conductor on power panel typically

• Auxiliary• Energy management, security and safety, data transfer,

telecommunications

Electrical Systems for Commercial

• Example:

• Duplexes (receptacles)

• Phase total 60 kVA, 14kVA (neutral)

• 30 poles

Panel D

Transformer capacity

• 200∙208∙√3 = 72 kVA ~ 75 kVA

• 200∙208 / 480 = 86 A• 100 A switch

• Neutral 117*208/480 = 50 A

• Three #1 and one #6 aluminum (could use #4)

• 100A fuse - could use 90, but likely same price

Building Needs: Lighting and Duplexes

• Lighting• Typically use lighting power densities for building type

• Sometimes actual lighting load

• Requires neutral

• Duplexes• NEC/local electrical code specifies number that are

required for different applications

• Requires a transformer

• Requires neutral wire

• Duplexes (receptacles)• Main lugs only (remote protection)• 3 Phase total 60 kVA, 14kVA (neutral)• 30 poles• 60 kVA / (208V √3 ) = 167A• Select: 200 A panel• Neutral 14 kVA /120V = 117A (or if not specified 167A)• Table 11-2 for (200A, aluminum 75C, 117 neutral, but we

expand to 130A like phase)• Hot wires should be 250 MCM, Neutral #2/0 aluminum

• 2½ inch conduit

Panel D

Transformer capacity

• P = I V ∙√3 =200∙208∙√3 = 72 kVA ~ 75 kVA

SD switch selection: • For transformer: I1E1=I2E2 • I2 = I1E1/E2 = 200∙208 / 480 = 86 A

• Select switch rated for 100 A

• For 100 A rated switch select: Three #1 (aluminum)

and one #1 for neutral (aluminum)• 90A fuse for phases

Lighting Panel

• 100A - 20 pole spaces

• Hot: 65kVA/(480V∙√3) = 78 A - #2 Aluminum

• Neutral: 22 kVA /277V = 79 A - #2 Aluminum

• 100 A three pole fused switch

• 1½ inch conduit

Power Panel

• No neutral? – all three pole circuits• 33 poles required – chose 42 standard size and allows

for expansion

• 165kVA/(480 √3) = 198 A

• Choose 225A switch (could use 200 A, but you count on expansion)

• 3 300 MCM aluminum wires – 2 inch conduit(in the book table you don’t have MCM 300, however it exist)

Wireway

• 198A + 86 A + 78 A = 362A

• 400A would allow for some expansion, some would probably go to 600A

• 400A three-pole switch

• Six 250 MCM (unlikely to find conductors big enough to just have 3) and one #2/0 aluminum

• Neutral current: maximum = 117 A, you can expand neutral too (130 A)

Summary Panel Sizing

• Find panel power usage and neutral usage

• Size panel conductors and associated conduit and switches