310.12 and 310.16

J

Jimmy7

Senior Member
Location
Boston, MA
Occupation
Electrician
I had a relative ask me to wire their new addition on their house. I told them I was too busy because I didn’t want to get involved ( You get it). In any case, they hired an electrician to wire the addition, I just recently got to see the completed rough while over visiting their home. I noticed that the electrician ran a 100 amp subpanel below the addition with a 4-3 romex. Obviously, this subpanel doesn’t carry the entire load of the house, so the electrician shouldn’t have used 310.12 to choose #4, correct? He should have used #3 copper (Example pipe and wire for #3 copper) from 310.16, correct? I kept my mouth shut about it. I was thinking how would I explain (Which I won’t) that you can use a #4 for a feeder if it handles the “Entire Load”, but you have to use a larger conductor when it handles a partial load?
 
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You're correct that he cannot use the 83% rule for this sub-panel. #4 NM cable has an ampacity of 70 amps so that is the maximum size OCPD permitted ahead of the feeder.
 
Jimmy7 said:
I had a relative ask me to wire their new addition on their house. I told them I was too busy because I didn’t want to get involved ( You get it). In any case, they hired an electrician to wire the addition, I just recently got to see the completed rough while over visiting their home. I noticed that the electrician ran a 100 amp subpanel below the addition with a 4-3 romex. Obviously, this subpanel doesn’t carry the entire load of the house, so the electrician shouldn’t have used 310.12 to choose #4, correct? He should have used #3 copper (Example pipe and wire for #3 copper) from 310.16, correct? I kept my mouth shut about it. I was thinking how would I explain (Which I won’t) that you can use a #4 for a feeder if it handles the “Entire Load”, but you have to use a larger conductor when it handles a partial load?
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You say it's a 100A sub panel, but what is the size of the OCPD upstream?

Rob G - Seattle
 
Btw, the table for the single family dwelling did come back in the 2023 code but IMO there needs a correction. It takes about 83% of the service and then allows us to use the table but in some cases this is an issue, IMO.. For instance, 4/3 NM as discussed above is a 60C rated cable which is rated for 70 amps however 100 amps x 83% = 83 amps. Why are we allowing a 60°C cable to be used for this table.

It used to disallow NM cable or any insulated wire that was rated 60C
 
Dennis Alwon said:
Btw, the table for the single family dwelling did come back in the 2023 code but IMO there needs a correction. . . . Why are we allowing a 60°C cable to be used for this table.
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I submitted a PI on this for the 2026 NEC, if I read the minutes correctly the CMP took some action to clarify 75C minimum for the table. It will be easier to see what they did when the First Draft Report comes out on July 10.

Cheers, Wayne
 
Dennis Alwon said:
It used to disallow NM cable or any insulated wire that was rated 60C
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If you go back to before they removed it from the code (for example below for the 2008 NEC) it never allowed 60° C conductors like TW insulation.

Table 310.15(B)(6) Conductor Types and Sizes for
120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN,
THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2
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wwhitney said:
I submitted a PI on this for the 2026 NEC, if I read the minutes correctly the CMP took some action to clarify 75C minimum for the table. It will be easier to see what they did when the First Draft Report comes out on July 10.

Cheers, Wayne
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Here is Waynes input

(A) General.
Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15(A) through (F) and 310.12. Under engineering supervision, ampacities of sizes not shown in ampacity tables for conductors meeting the general wiring requirements shall be permitted to be determined by interpolation of the adjacent conductors based on the conductor’s circular-mil area.
The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does but the resulting ampacity may not exceed the ampacity for ampacity before adjustment and correction factors for the temperature rating of the termination in accordance with 110.14(C).


Statement of Problem and Substantiation for Public Input
This proposal seeks to improve the clarity of meaning of this part of 310.15(A). The current language is open to the following misinterpretation from an overly literal reading: "First you look up the unadjusted and uncorrected ampacity at the termination temperature rating, call that A. Then you take the tabular ampacity at the insulation temperature rating and apply the adjustment and correction factors, call that B. If B > A, you can't use the insulation temperature tabular ampacity, so you need to apply the adjustment and correction factors to A to get the ampacity." Since the intention is that the ampacity is just the minimum of A and B, as shown in Annex D Example D3(a) in the section "Ungrounded Feeder Conductors," the proposed language more clearly reflects the intention and precludes the above misinterpretation.
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@wwhitney Would it not have been easier to just use 75C rated cable?
 
Dennis Alwon said:
Here is Waynes input
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(with the strike out and italicization stripped out, so it doesn't read quite right).

Dennis Alwon said:
@wwhitney Would it not have been easier to just use 75C rated cable?
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Apparently the CMP thinks so, based on their response to my PI. I'm not sure what I was thinking at the time, maybe that the text should also cover equipment with 60C rated terminals, as well as the possibility of 60C rated cable.

Cheers, Wayne
 
I think I found the one that is pertinent to the thread

Statement of Problem and Substantiation for Public Input
As currently written, Table 310.12 would allow #4 NM cable to be utilized for a 100A feeder "supplying the entire load associated with a one-family dwelling," since there is no restriction on the wiring method mentioned. However, the ampacity of #4 NM cable is limited to 70A per 334.80, so this would provide a ratio of ampacity to rating of 70%, much less than the 83% specified in section 310.12. Thus it is my understanding that the use of Table 310.12 is intended to be limited to wiring methods that can make use of the 75C ampacity columns in Tables 310.16 et al. Therefore that restriction needs to be placed on Table 310.12. Since multiple paragraphs refer to Table 310.12, putting the restriction in the table heading seems simplest.

Submitter Information Verification

Submitter Full Name:Wayne Whitney Organization:

Statement: Language was added to the last sentence in 310.12(A) to clarify that installations using conductors or cables limited to 60C ampacities are not permitted to use the table. “If no adjustment or correction factors are required, Table 310.12(A) shall be permitted to be applied for conductors or cables rated 75C or greater.
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83% is only for the service and a feeder that feeds the entire house. Can use NM on a service but for a feeder you are correct. They will have us use SER which is probably cheaper anyhow.
 
infinity said:
If they just put the insulation types back in the table like the were before the table was removed the problem would be solved. :rolleyes:
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This whole section should be mostly reverted back to the way it was in 2011 with the exception of the 'service rating' language. It should be revised so that language is removed and the table simply designates the allowed ampacities to use in the applicable situation, like any other table.

See related discussion we just had.

What Exactly Is The "Service Rating"?

Service to a single family dwelling with a 3/0 AL service entrance that hits a wireway then feeds two 125A disconnects with 125A fuses? The disconnects each feed a 125A MLO subpanel What is the rating of that service? 75C ampacity of 3/0 Al = 155A, so the rating is 155A / 0.83 = 186A. Please...
forums.mikeholt.com
 
Eddie702 said:
83% is only for the service and a feeder that feeds the entire house. Can use NM on a service but for a feeder you are correct. They will have us use SER which is probably cheaper anyhow.
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I don't think that is the intent. The next code cycle I believe you will see that 75C conductor must be used
 
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