NCEES Question #510 - Power Exam Sub Forum - Engineer Boards
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TakiTaki

NCEES Question #510

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A 50-hp, 460-V, 3-phase induction motor is served by a single feeder conduit... and it is rated for continuous duty... Starter running overload protection is available in 5-A increments... the initial maximum sized protection device (amperes) that can be applied is most nearly:

 

This might be a very dumb question but where is the current 65 [A] coming from? I understand the solution and I know why we need to multiply it by 1.15 and round down to get the final answer but maybe I am missing a step here. Any help is appreciated.

 

Answer from NCEES: 65(1.15) 74.75 [A] using 430.32(A)(1)

Round down and it equals 70 [A]

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To find the maximum sized protection device, you need to know the motor nameplate full load current.  The problem does not provide this, so you have to look it up based on the information they give you.

Use Table 430.250 (for 3 phase motors) to get the full load current at 50HP, 460V to get 65A.

Edited by MEtoEE
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On 9/30/2019 at 9:49 PM, TakiTaki said:

A 50-hp, 460-V, 3-phase induction motor is served by a single feeder conduit... and it is rated for continuous duty... Starter running overload protection is available in 5-A increments... the initial maximum sized protection device (amperes) that can be applied is most nearly:

 

This might be a very dumb question but where is the current 65 [A] coming from? I understand the solution and I know why we need to multiply it by 1.15 and round down to get the final answer but maybe I am missing a step here. Any help is appreciated.

 

Answer from NCEES: 65(1.15) 74.75 [A] using 430.32(A)(1)

Round down and it equals 70 [A]

To be honest, this is a question I always wonder if NCEES made a mistake on. The NEC is pretty clear on using the name plate full load amps to size separate overload protection. The NEC is also very clear that motor FLC table values are for sizing OCPD and conductors. I guess since NCEES left out the nameplate FLA, you are forced to use the NEC FLC instead, but I still think it is a slight error. 

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