2020 NEC Article 310 Single Family Dwelling Ampacities

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
This section has been re-written a bit and the Table for SFD has been brought back and can be used if no adjustment factors are needed. Also the ampacity table number is back to T310.16 again.


310.12 Single-Phase Dwelling Services and Feeders.

For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.12(A) through (D).

For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of two ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.12(A) through (C).

310.12(A) Services.

For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

310.12(B) Feeders.

For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

310.12(C) Feeder Ampacities.

In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).

310.12(D) Grounded Conductors.

Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, if the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.

Where correction or adjustment factors are required by 310.15(B) or (C), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor.

Informational Note No. 1: The service or feeder ratings addressed by this section are based on the standard ampere ratings for fuses and inverse time circuit breakers from 240.6(A).

Informational Note No. 2: See Example D7 in Annex D.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Good. The table should have never been removed to start with.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Good. The table should have never been removed to start with.
Yeah but the table caused problems because in the past you were not required to apply de-rating factors. Now if there are no adjustment to the conductor then you can use the table. BTW, the table was still in the NEC only hidden in the back of the book. The table is the same as 83%
 

infinity

Moderator
Staff member
Location
New Jersey
I didn't have a problem with the 83% rule it's simple math and you could still use the old table from a previous version.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I don't believe load calculation or diversity* is the reasoning for this code section.
I would have to disagree with this statement. Diversity is what the section is about. Loads in a commercial establishment are usually pretty constant- lights etc... but in a home the diversity of the loads are why we don't have tremendous loads on a building. 95% of the time loads on a 200 amp service are well below 40 or 50 amps.
 

Devin Hanes

Member
Location
United States
I would have to disagree with this statement. Diversity is what the section is about. Loads in a commercial establishment are usually pretty constant- lights etc... but in a home the diversity of the loads are why we don't have tremendous loads on a building. 95% of the time loads on a 200 amp service are well below 40 or 50 amps.
I agree real world residential service load calculations are very conservative and I agree with your statement here "95% of the time loads on a 200 amp service are well below 40 or 50 amps."


Do you believe I am misreading or misapplying the Information I posted? Or that the reasoning has changed since the allowance's inception?


If you look at the 1956 table I posted, that note allowance was not just for residences.
 
I agree real world residential service load calculations are very conservative and I agree with your statement here "95% of the time loads on a 200 amp service are well below 40 or 50 amps."


Do you believe I am misreading or misapplying the Information I posted? Or that the reasoning has changed since the allowance's inception?


If you look at the 1956 table I posted, that note allowance was not just for residences.
Which goes to my point... We should not be using art. 310 to see what size wire should be used for dwelling service or any other service for that matter, that's what art. 220 should be for.
 

Devin Hanes

Member
Location
United States
Which goes to my point... We should not be using art. 310 to see what size wire should be used for dwelling service or any other service for that matter, that's what art. 220 should be for.
In my post #7 I provided some references that I believe point toward ampacity, my references are from 1953-1957 to the code sections that I see as the predecessors of our current section discussed in this thread.

I believe I've read most if not all of the panel comments from 1956 to present day and there are varying reasons given for this allowance, but the only ones I have come across that say this section doesn't change ampacity are more recent ones, I'm thinking 90's.This section was stagnant for a long time.

I have come across more recent textbooks and IAEI articles that say this section doesn't change ampacity. But from my look into the evolution of this section, that appears to be incorrect unless you can have an allowance for 40 years and then just change the reason with no substantiation and keep it in the ampacity section when if the reason changed, the location should have as well. I have not come across any statement about changing the reasoning behind this section, just forty years later someone stated a reason that was not consistent with previous ones and it appears other people just went along.

And if that's the case, I agree, if the reasoning is not ampacity then this is the wrong section for it. By the way I read recent articles and textbooks that discuss this section I actually think it might be better in 240, around 240.4(G). That's another reason why I believe the reason at the inception of this allowance still stands, ampacity, that's why it was and is in the ampacity section. Always glad to learn something new though if someone wants to offer something I am missing.

You are probably aware, a few years ago there were many proposals to move this section like you are suggesting, I believe one was from George Stolz on here.
 
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