Question on sizing a feeder

[julian padilla]

Based on the question below, should I calculate the feeder based on the (1) calculated load or (2) overcurrent protection?
Any advise is appreciated.

How many sets of THW conductors are required for a feeder with a 1,000-amp overcurrent protective device that serves a calculated load of 900 amperes?

1. two sets of four THW per raceway of 500 kcmil with grounding
2. three sets of four THW per raceway of 350 kcmil with grounding
3. two sets of four THW per raceway of 600 kcmil with grounding
4. three sets of four THW per raceway of 400 kcmil with grounding

 

[infinity]

You need a conductor or group of parallel conductors that equals or exceeds 1000 amps.

 

[texie]

You need a conductor or group of parallel conductors that equals or exceeds 1000 amps.

True, but are you gonna tell him why?

 

[julian padilla]

What NEC code specifically states not to use the "calculated load", but to use overcurrent protection?
What is the calculation the leads to the correct multiple choice answer?
Please explain,
Julian

 

[tom baker]

Start with Art 240. Section 240.4 protection of conductors

 

[petersonra]

Based on the question below, should I calculate the feeder based on the (1) calculated load or (2) overcurrent protection?
Any advise is appreciated.

How many sets of THW conductors are required for a feeder with a 1,000-amp overcurrent protective device that serves a calculated load of 900 amperes?

1. two sets of four THW per raceway of 500 kcmil with grounding
2. three sets of four THW per raceway of 350 kcmil with grounding
3. two sets of four THW per raceway of 600 kcmil with grounding
4. three sets of four THW per raceway of 400 kcmil with groundin

You always have to take both into account. However, sometimes you are allowed to use the next higher standard rating of an overcurrent device if it does not match your conductor ampacity.

 

[infinity]

As Bob stated this question seems to ask if you understand the next size up rules, 800 amps or less {240.4(B)} and over 800 amps {240.4(C)}. Since it exceeds 800 amps (B) cannot be applied so the conductors need to be a minimum of 1000 amps due to the 1000 amp OCPD. That means the only possible answer is #4.

 

[Dennis Alwon]

However we don't know if the neutral is a current carrying conductor or not. I would assume it isn't.

 

[infinity]

However we don't know if the neutral is a current carrying conductor or not. I would assume it isn't.

True, a feeder neutral is almost never a CCC. Also given the 4 answers only one works and that is without any derating.

 

[electrofelon]

For pedagogy purposes I would point the OP to 215.2(A) which states the feeder must have ampacity not less than the calculated load. Then you look to the article 240 sections mention for how to protect the conductors.

 

[julian padilla]

You guys are awesome. Thanks for your help!