NEC 310.15(B)(7) and 220.82 Dwelling Units

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Tainted

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Can 310.15(B)(7) be used with 220.82? The language is weird and 220.82 mentions there must be conductors with an ampacity of 100A or more. Does this mean I can do 83% rule and use #4 wires if the service is 100A or no? #4 wires do not have ampacity of 100A or more. Are they saying it needs to be #3 wires minimum?
 

wwhitney

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The rule in 310.15(B)(7) modifies the ampacities from the ampacity tables and can be used with 220.82.
That would be a very simple answer, but the wording in 310.15(B)(7) doesn't sound to me like it is modifying ampacities: ". . . shall be permitted
to have an ampacity not less than 83 percent of the service rating."

Cheers, Wayne
 

wwhitney

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The code is very clear...the rule in (B)(7) changes the ampacity of the conductor for dwelling unit services.
I agree the text in 310.15(B) is clear: sections 310.15(B)(1) through (7) may change the ampacity of conductors relative to the table values. But not all of them do, only (2) "Ampacities . . . shall be corrected . . ." and (3) "ampacity . . . shall be reduced," as clarified by (4), (5), and (6).

The language in (7) simply says that you can use a lower ampacity conductor than the service or feeder rating. It doesn't say that in the applicable circumstances the conductors' "ampacity shall be permitted to be increased by 20.4%" (which is 1/83% - 100%). If anything the language is clear that the ampacity is not being increased, as it is distinguishing between the ampacity and the rating.

Cheers, Wayne
 

Carultch

Senior Member
Location
Massachusetts
The code is very clear...the rule in (B)(7) changes the ampacity of the conductor for dwelling unit services.

I don't interpret it that way. I interpret it as the rule allowing the ampacity of the service conductors and any circuit directly in-line with the service conductors (i.e. whole house feeder) to be selected at 83% the amp rating of the service OCPD where 310.15(B)(7) applies, rather than the actual ampacity being modified.

You might ultimately reach the same conclusion on how large the service conductors need to be by using your interpretation, but that isn't what the wording says. The NEC doesn't cause #4/0 AL to get a rating of 216.8A just because it is used on a dwelling's service. It allows a 200A service to only need 166A worth of conductor ampaticy.
 

don_resqcapt19

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I don't interpret it that way. I interpret it as the rule allowing the ampacity of the service conductors and any circuit directly in-line with the service conductors (i.e. whole house feeder) to be selected at 83% the amp rating of the service OCPD where 310.15(B)(7) applies, rather than the actual ampacity being modified.

You might ultimately reach the same conclusion on how large the service conductors need to be by using your interpretation, but that isn't what the wording says. The NEC doesn't cause #4/0 AL to get a rating of 216.8A just because it is used on a dwelling's service. It allows a 200A service to only need 166A worth of conductor ampaticy.
310.15(B) Tables.
Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.15(B)(16) through Table 310.15(B)(19), and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) as modified by 310.15(B)(1) through (B)(7).


That language is telling me that the rule in (B)(7) modifies that ampacities found in the tables.
 

wwhitney

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That language is telling me that the rule in (B)(7) modifies that ampacities found in the tables.
"As modified" is warning you that (B)(1) through (B)(7) may have language that modifies the table values; it does not turn those sections into modifications. (B)(1) is obviously not a modification. (B)(2) and (B)(3) are. (B)(4) clarifies how to use the tables. (B)(5) and B(6) clarify (B)(3). And that leaves (B)(7) which does not use the modification language that (B)(2) and (B)(3) use. So it is not a modification.

If you still see it differently, we'll have to agree to disagree.

Cheers, Wayne
 

wwhitney

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Just a quick followup: in the 2020 NEC, what was 310.15(B)(7) has been moved to 310.12. As such it is clearly no longer subject to the "as modified" language in 310.15, and simply says you are allowed under the specified circumstances to use a conductor whose ampacity is smaller than the service/feeder rating.

Depending on your point of view, you could consider this a change from 2017 to 2020, or a clarification on the intention of the 2017 NEC.

Cheers,
Wayne

P.S. The 2023 first draft report recognizes that 240.4 needs an exception for 310.12, which allows the use of conductors of lower ampacity than the service/feeder rating. So it now has a new section 240.4(H). One could argue under the 2020 NEC that 310.12 is moot, as it has no such allowance in 240.4.
 

Carultch

Senior Member
Location
Massachusetts
Just a quick followup: in the 2020 NEC, what was 310.15(B)(7) has been moved to 310.12. As such it is clearly no longer subject to the "as modified" language in 310.15, and simply says you are allowed under the specified circumstances to use a conductor whose ampacity is smaller than the service/feeder rating.

Depending on your point of view, you could consider this a change from 2017 to 2020, or a clarification on the intention of the 2017 NEC.

Cheers,
Wayne

P.S. The 2023 first draft report recognizes that 240.4 needs an exception for 310.12, which allows the use of conductors of lower ampacity than the service/feeder rating. So it now has a new section 240.4(H). One could argue under the 2020 NEC that 310.12 is moot, as it has no such allowance in 240.4.

What is the physical basis for 310.15(B)(7), and reason for the double standard of the sizing of service conductors vs feeder conductors?

I've heard that load diversity is part of the reason behind this, but that should be already accounted for, in the calculation that sizes the service in the first place. It seems like 310.15(B)(7) is "double dipping" the load diversity argument.
 

wwhitney

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Location
Berkeley, CA
Occupation
Retired
What is the physical basis for 310.15(B)(7), and reason for the double standard of the sizing of service conductors vs feeder conductors?
Of the various explanations I've heard of, I like the idea that says for a 120/240V service or feeder, there are only 2 CCCs, so the Table 310.16 numbers are overly conservative. This would also imply that it was a mistake to expand the allowance to 120/208V services and feeders. There is an apparently well researched and documented proposal for the 2023 NEC to remove that expansion, but it was rejected.

Cheers, Wayne
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
What is the physical basis for 310.15(B)(7), and reason for the double standard of the sizing of service conductors vs feeder conductors?

I've heard that load diversity is part of the reason behind this, but that should be already accounted for, in the calculation that sizes the service in the first place. It seems like 310.15(B)(7) is "double dipping" the load diversity argument.
The real basis for the rule permitting the smaller service conductors for dwellings is the fact that the load calculation required by Article 220 results in a service that is at least 50% larger than what the actual load will ever be. I believe these reduced conductor sizes were derived from real world load studies by utilities.
 
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