Clarification in regards to Table 310.15(B)(6) and NEC 2014

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dnguyen5

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Pardon my ignorance as I'm working practice problems for the upcoming PE exam and came across an area of confusion. Thus, I'm here for your infinite wisdom.

For a 120/240V, three wire, single phase copper feeder of Type THW, I'm required to size a feeder wire given a net computed load of 68.3 amps for a single family dwelling unit. However according to NEC article 230.79(C), a 100A rating for the disconnect means is required for a one-family dwelling unit and therefore the minimum load is 100A. Now here is where things go wonky. The solution refers to Table 310.15(B)(6), which I could not find in NEC 2014. A quick search online indicates that there WAS a Table 310.15(b)(6) in a previous edition and that for a 100A feeder rating, you would select 4AWG copper as seen on this site <http://freenec.com/T124.html> and thus the answer to the problem. Case closed.

But again there is no table 310.15(b)(6) in NEC 2014. Thus I refer to section 310.15(B)(7)(2) in NEC 2014, which states that the feeder conductors shall have an ampacity not less than 83% of the feeder rating. This then becomes 83% of 100A, which is 83A. I then refer to Table 310.15(b)(16), look at 75C for THW, and identify that for a 4AWG conductor, its allowable ampacity is 85A, which would work as the answer. I used the following link as a basis for this conclusion <http://iaeimagazine.org/magazine/2014/05/04/2014-national-electrical-code-updates-wire-and-cable/>

My questions to you because ignorance is not bliss and that this will deprive me of sleep:

1. Confirm or deny - there is no table 310.15(b)(6) in NEC 2014 and that I'm not blind.
2. Was my conclusion and conductor selection accurate based on the NEC 2014 code?

Thanks for your input!
 
dnguyen5 said:
1. Confirm or deny - there is no table 310.15(b)(6) in NEC 2014 and that I'm not blind.
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Correct. I don't know how far the table goes back, as long as I have been wiring. It was removed from the '14 code.
2. Was my conclusion and conductor selection accurate based on the NEC 2014 code?
Click to expand...
Yes.
 
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Table 310.15(B)(6) Is in the 2008 code. Conductor Types and Sizes for 120/240-Volt, 3 wire, Single-phase Swelling Services and Feeders. I think it was moved elsewhere in the code for a code cycle or two.
 
Let me correct one aspect of your post:
dnguyen5 said:
However according to NEC article 230.79(C), a 100A rating for the disconnect means is required for a one-family dwelling unit and therefore the minimum load is 100A.
Click to expand...
Not true. The disconnect has to be rated for at least 100 amps, and it has to be rated for the calculated load. But that has nothing to do with the actual amount of load that the building will require. For example, consider a "tiny house" that has gas heat and gas cooking. It is possible that the calculated load might be no more than 50 amps. So if the main service disconnecting means is a fused disconnect, then the disconnect switch must be rated for 100 amps, but the fuses could be no more than 50 amps.
 
OP States this is a feeder supplying a dwelling unit, sounds like this could be a multi family dwelling doesn't sound like its a service
 
david said:
OP States this is a feeder supplying a dwelling unit, sounds like this could be a multi family dwelling doesn't sound like its a service
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More likely it is a service, and the OP is misusing the word "feeder."
 
dnguyen5 said:
My questions to you because ignorance is not bliss and that this will deprive me of sleep:

1. Confirm or deny - there is no table 310.15(b)(6) in NEC 2014 and that I'm not blind.
2. Was my conclusion and conductor selection accurate based on the NEC 2014 code?

Thanks for your input!
Click to expand...

It is true, they did eliminate this table in the 2014 NEC. They switched from a lookup table, to a master percentage. Stating that service conductors for single and duplex dwellings with split-phase 120/240V services, only require 83% of the service rating as their ampacity.

However, you generally still get the same results. You can cut the table out of your 2011 NEC, and tape it in your current version, follow the same method you were familiar with doing in 2011, and you'll get correct answers in most common situations. As long as temperature derates aren't extreme enough to govern the sizing, and as long as you have no bundling (which is rare on a service), there will be no difference.

The current method with the 83% factor now gives a calculation that considers derate factors if significant, and gives you a method to calculate what to do if you decide to parallel. Something you might decide to do on a 400A service. It also allows consideration of other-than-75C termination ratings, should they apply.
 
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