Kaplan Sample Power Exam Problem # 10

[cabbagekid2]

Hi Everyone,

I'm not very familiar with NEC so hopefully someone can help me.

Kaplan Problem 10 states:

Given: A commercial building with an electrical power system that includes a significant number of 120V AC and 208V AC three phase loads (4 wire, wye connected). Computed, total load of 645 amps.

The size of THHN aluminum conductor required for the service when they are paralleled six times per phase is:

a) #2 AWG

b) #1 AWG

c) #1/0 AWG

d) #2/0 AWG

The answer given is B. 645Amps/6=107.5 Amps Then use Table 310.16 for THHN and Aluminum. (#1 AWG is 115A)

My question is why are the ampacities of the cables not derated since there are 6 of them per phase in parallel? Do we not need to use Table 310.15(B)(2)(a)?

When do you need to derate the cables? If you did it for this example, would it be 6 conductors or 3x6=18?

Thanks!

 

[Flyer_PE]

They didn't say anything about routing in the problem. However, if you assume they're running all of the conductors in a single tray, anywhere along the route, Table 3.15 B(2)(a) is applicable and you have 18 current carrying conductors.

 

[stage3s4]

Hi Everyone,
I'm not very familiar with NEC so hopefully someone can help me.

Kaplan Problem 10 states:

Given: A commercial building with an electrical power system that includes a significant number of 120V AC and 208V AC three phase loads (4 wire, wye connected). Computed, total load of 645 amps.

The size of THHN aluminum conductor required for the service when they are paralleled six times per phase is:

a) #2 AWG

B) #1 AWG

c) #1/0 AWG

d) #2/0 AWG

The answer given is B. 645Amps/6=107.5 Amps Then use Table 310.16 for THHN and Aluminum. (#1 AWG is 115A)

My question is why are the ampacities of the cables not derated since there are 6 of them per phase in parallel? Do we not need to use Table 310.15(B)(2)(a)?

When do you need to derate the cables? If you did it for this example, would it be 6 conductors or 3x6=18?

Thanks!

You only need to de-rate the wire when you run more then 3 current carrying conductors in a sinlge piece of conduit. Since they say 6 times per phase, I'm guessing its assumed they are asking you to size the wire as if you were to run 6 sets of conductors each in there own conduit or raceway.

 

[patelpe]

Hi Everyone,
I'm not very familiar with NEC so hopefully someone can help me.

Kaplan Problem 10 states:

Given: A commercial building with an electrical power system that includes a significant number of 120V AC and 208V AC three phase loads (4 wire, wye connected). Computed, total load of 645 amps.

The size of THHN aluminum conductor required for the service when they are paralleled six times per phase is:

a) #2 AWG

B) #1 AWG

c) #1/0 AWG

d) #2/0 AWG

The answer given is B. 645Amps/6=107.5 Amps Then use Table 310.16 for THHN and Aluminum. (#1 AWG is 115A)

My question is why are the ampacities of the cables not derated since there are 6 of them per phase in parallel? Do we not need to use Table 310.15(B)(2)(a)?

When do you need to derate the cables? If you did it for this example, would it be 6 conductors or 3x6=18?

Thanks!

You only need to de-rate the wire when you run more then 3 current carrying conductors in a sinlge piece of conduit. Since they say 6 times per phase, I'm guessing its assumed they are asking you to size the wire as if you were to run 6 sets of conductors each in there own conduit or raceway.

There are so many mistakes in Kaplan's solution. I would select 1/0 wire size. 1/0 THHN wire ampacity is 135 A multiply by derating factor .8 This will gives you 108 A which is more then 107.5 A per set of phase. As Flyer mentioned, derating factor table must apply to this problem.

Thanks,

 

[knight1fox3]

An interesting yet subjective problem. The problem certainly leaves an unknown that is needed to properly apply NEC. I work in the material handling industry (overhead and portal cranes) which sometimes requires paralleling conductors to conserve cable size when working with large hoist induction motors. With this particular problem and the solution that accompanies it, one would have to ASSUME that the cables will be installed accordingly so as not to require a derate factor. As I've seen on some cranes with large capacities, the #1 cables could be run as 3 sets in 3 pieces of 1" conduit (IMC, RMC, etc.) per phase (3 pieces of 1" conduit per phase with 2 #1 conductors in each piece). Thereby eliminating the need for deration. It would be ideal to run the conduit so as to keep the phases bundled together to avoid any unwanted induced currents. Of course paralleling six times per phase would not be the best-suited solution to this application in my mind.