IAEI study guide question, NEC 2017 question #30 pg 183

[wlancaster]

You guys have been helpful in the past, thanks. I'm trying to pass ICC electrical plan review exam and bought the IAEI study guide. So here's the question, I don't understand the answer explanation.
30. A service equipment is rated 208Y/120V, 3-phase. The busbars in the service equipment are rated 800 amperes. The calculated load is 675 amperes. The service disconnecting means consists of two 150-ampere, 3-pole; one 200-ampere, 3-pole; one 400-ampere, 3-pole; and one 300 ampere, 3 pole circuit breakers. The equipment is identified as suitable for service equipment. The service entrance conductors consist of four 400kcmil Type THW copper conductors in each of two metric designator 78 (trade size 3) rigid metal conduits. Which one of the following violations is applicable?

A. A single main disconnecting means is required
B. The ratings of the disconnecting means do not comply
C. The permitted conduit fill is exceeded
D. The service entrance conductors are undersized

The book says the answer is D. because 400kcmil CU is rated for 335 amperes. 335 amperes x (2) = 670 amperes which is less than the calculated load of 675 amperes, the conductors are undersized.
Would this not be 3 phase conductors in two parallel runs with each phase rated for 335 amperes which would be 2010 amps?? Thanks for helping me out on this one.

 

[don_resqcapt19]

Each phase is treated separately and the book is correct.

 

[infinity]

The book says the answer is D. because 400kcmil CU is rated for 335 amperes. 335 amperes x (2) = 670 amperes which is less than the calculated load of 675 amperes, the conductors are undersized.
Would this not be 3 phase conductors in two parallel runs with each phase rated for 335 amperes which would be 2010 amps?? Thanks for helping me out on this one.

That answer is correct. 335*2=670 amps which is too small with the calcuated load of 675 amps. 2010?

 

[suemarkp]

If a service, feeder, or load is rated at X amps, it doesn't typically matter whether it is single phase, 3 phase, or 100 phase... Each pole of the phase is sized for X. A residential 120/240 400A service has 2 conductors rated for 400A. A 120/208 3 phase 400A service has 3 conductors rated for 400A. There is probably also a neutral in these, but it is rarely sized as large as the ungrounded conductors.

Another clue for this is looking at a circuit breaker. Whether it is a single pole, double pole, or 3 pole breaker, if it is a "100A breaker" each individual breaker in the gang is rated 100A.

 

[wlancaster]

You guys have been helpful in the past, thanks. I'm trying to pass ICC electrical plan review exam and bought the IAEI study guide. So here's the question, I don't understand the answer explanation.
30. A service equipment is rated 208Y/120V, 3-phase. The busbars in the service equipment are rated 800 amperes. The calculated load is 675 amperes. The service disconnecting means consists of two 150-ampere, 3-pole; one 200-ampere, 3-pole; one 400-ampere, 3-pole; and one 300 ampere, 3 pole circuit breakers. The equipment is identified as suitable for service equipment. The service entrance conductors consist of four 400kcmil Type THW copper conductors in each of two metric designator 78 (trade size 3) rigid metal conduits. Which one of the following violations is applicable?

A. A single main disconnecting means is required
B. The ratings of the disconnecting means do not comply
C. The permitted conduit fill is exceeded
D. The service entrance conductors are undersized

The book says the answer is D. because 400kcmil CU is rated for 335 amperes. 335 amperes x (2) = 670 amperes which is less than the calculated load of 675 amperes, the conductors are undersized.
Would this not be 3 phase conductors in two parallel runs with each phase rated for 335 amperes which would be 2010 amps?? Thanks for helping me out on this one.

Ok, my confusion is in grasping how 3 phase amperage is calculated, I was thinking each 400kcmil is good for 335 amps x 3 phases x 2 runs would equal 2010 amps. Thanks for helping me out.

 

[infinity]

Ok, my confusion is in grasping how 3 phase amperage is calculated, I was thinking each 400kcmil is good for 335 amps x 3 phases x 2 runs would equal 2010 amps. Thanks for helping me out.

It's the number of sets that will determine the total current carrying capacity of the conductors. In this question there are two sets so the total ampacity is 670 amps.