# NEC 240.21(B)(1)(2)(3) and (4) Explanation



## doubleoinfo07 (Oct 10, 2016)

Hello. Can someone explain to me the meaning of Article 240.21 (B)(1-4) in the NEC. The wording confuses me a bit, and I don't understand the rule as applied.

Thank you in advance.


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## knight1fox3 (Oct 10, 2016)

doubleoinfo07 said:


> Hello. Can someone explain to me the meaning of Article 240.21 (B)(1-4) in the NEC. The wording confuses me a bit, and I don't understand the rule as applied.
> 
> Thank you in advance.


What portion(s) of this Article are you having trouble understanding? It's basically an explanation of how large branch circuit conductors need to be relative to their length from the corresponding tap (i.e. power source).


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## doubleoinfo07 (Oct 10, 2016)

Ok let me give you an example. This is an example that I was asked to solve in a class:

_Determine the max length of a 20A rated tap off of a 100A feeder circuit._

What would be the though process as you answer this problem? in going through the precepts in 240.21(B) and applying it to problems such as the one noted, that's where I seem to have my issues.


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## allgood (Oct 10, 2016)

I don't believe the question is very accurate. Is this an indoor or outdoor installation? I'm going to assume it's an indoor installation and not supplying a transformer.

240.21.B.2.1

100A/3=33.3A &lt; 20A. The ampacity of the tap conductors is less than a third, so you wouldn't meet this requirement.

I'd say 10', assuming you meet the other conditions of 240.21.B.1


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## doubleoinfo07 (Oct 12, 2016)

Thank for the response and your help


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