1. Quiz in Progress 1)Homework 1 was? Easy Hard Easy except for problem 1

2. Selecting Electrical Systems
MLO/MCB
Aluminum wiring

3. 2) Why do we need a transformer for panel D?
Lower voltage power is more efficient
Lower voltage power reduces peak demand
Lower voltage power is required for duplexes
Lower voltage power is safer
Transformer manufacturers lobbied to make transformers required under the NEC.

4. Panel D Duplexes (receptacles) Main Lugs Only (remote protection)
Phase total 60 kVA
14 kVA (neutral)
30 poles

5. Panel D Duplexes (receptacles) Main lugs only (remote protection)
3 Phase total 60 kVA, 14kVA (neutral)
30 poles
60 kVA / (208V √3 ) = 167A
200A panel (standard size)
Neutral 14000/120 = 117A
Text Table 12.2, NEC, Tao and Janis (2001) Table 11-2
Hot wires should be 250 MCM, Neutral #2/0 aluminum
2½ inch conduit

6. 3) How do you calculate current flow through a neutral conductor in a 3 system?
I =√3 E P
I =P/ (√3 E )
I =P/ E
I= E P

7. 4) Why is neutral load not 1/3 of hot load?
Because not all power goes through the neutral conductor
Because there can be different amounts of current on each hot conductor
Because the neutral is oversized for safety reasons
A. and B.
A., B., and C.

8. Selecting Electrical Systems
MLO/MCB
Aluminum wiring

9. 5) What is the current in the hot conductors upstream of the transformer (I1)?
E1 = V1 = 480V, E2 = V2 = 208V, I2 = 200A
86 A
100 A
462 A
Can’t be determined
149 A

10. Transformer capacity 200∙208∙√3 = 72 kVA ~ 75 kVA 200∙208 / 480 = 86 A
100 A switch
Neutral 117*120/277 = 50 A
Three #2 and one #6 aluminum (could use #4)
1¼ inch conduit (probably actually 1½ inch)
100A fuse - could use 90, but likely same price

11. Selecting Electrical Systems
MLO/MCB
Aluminum wiring

12. 6) What is lighting panel hot conductor current?
65kVA/(480V∙√3) = 78 A
65kVA/(480V) = 135 A
65kVA/(277V∙√3) = 135 A
65kVA/(277V) = 235 A

13. 7) Does the neutral conductor carry more or less current than the hot conductor?
More current in neutral
Less current in neutral
No current in neutral

14. Lighting Panel 100A MLO 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

15. Selecting Electrical Systems
MLO/MCB
Aluminum wiring

16. 8) The power panel has no neutral and therefore has how many wires in the conduit at point 4
3 hot + 1 neutral from panel = 4
3 hot + 1 ground = 4
3 hot = 3
3 hot + 1 ground + 1 neutral = 5
1 hot = 1

17. 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)
3 300 MCM aluminum wires – 2 inch conduit

18. Wireway
198A + 86 A + 79 A = 363A
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 #3/0 aluminum
Neutral current = 130A

19. Summary Panel Sizing
Find panel power usage and neutral usage (we will learn how to calculate in a minute)
Size panel conductors and associated conduit and switches

20. NEC summary (Handout) Minimum capacity Grounding necessary
Only approved wires (voltage/temperature/location)
200, 210, 215, 220 demand calculations
Conduits, raceways required in commercial buildings (usually rigid)
310 - Number of conductors in raceway
Restrictions (to residential) on use of NM and NMC conductors
Connections and splices must be approved and inspectable
Need overcurrent/overvoltage/and overload protection
Need disconnect switches (also 11.)
Not too many wires in a conduit

21. Branch Circuits (Handout)
From panel to loads
Balance loads on each phase of three phase panel
NEC 430

22. Design Guidelines No wire smaller than 14 AWG in residential
12 AWG for commercial (2)
Continuous load per 120 V circuit limited to
1.2kW for 15 A and 1.5kW for 20 A breaker
Continuous load for heavy duty circuits
Demand (kW)
Conductor Size (AWG) 2 10
2.5 8 3 6

23. Do not exceed branch circuit rating by:
Panelboard to first lighting outlet
Use next highest conductor size if >75 ft
Only allowed >100 ft if <2% voltage drop (7)
Panelboard to first receptacle
If >100ft, use bigger than 10 AWG conductors
Can’t use branch circuit for convenience outlets and display lighting
80 %
Portable appliances
70 %
Inductive lighting
50 %
Fixed appliances

24. Ref: Tao and Janis (2001)

25. Ref: Tao and Janis (2001)