310.15(B)(7)(3)

[augie47]

Posts in another thread brought to mind a question that, after all these years and posts should be clear but is not.. senility I guess
I understand the 83% rule but looking at 310.15(B)7)(3) and the fact that it refers back to (1)&(2) does (7) mean that in no case shall the feeder be required to be larger than the service OR only when the "entire load" is still in play.
Assume a 200 amp residential service where I take advantage of the 83% rule. I add a 20 amp or 40 amp 240v load at the service panel. The feeder no longer carries the "entire load" and I have lost my load diversity. Can I still run a feeder the same size as the service based on (B)(7)(3) or not ??

 

[infinity]

Posts in another thread brought to mind a question that, after all these years and posts should be clear but is not.. senility I guess
I understand the 83% rule but looking at 310.15(B)7)(3) and the fact that it refers back to (1)&(2) does (7) mean that in no case shall the feeder be required to be larger than the service OR only when the "entire load" is still in play.
Assume a 200 amp residential service where I take advantage of the 83% rule. I add a 20 amp or 40 amp 240v load at the service panel. The feeder no longer carries the "entire load" and I have lost my load diversity. Can I still run a feeder the same size as the service based on (B)(7)(3) or not ??

If you had a set of SEC's feeding a panel sized at 83% and then a feeder to another panel the feeder conductors are not required to be larger than the 83% SEC's even if they do not carry the full load. So for example with 100 amp SEC's sized at 83% you could have a 100 amp feeder to another panel sized also at 83%.

 

[LarryFine]

Posts in another thread brought to mind a question that, after all these years and posts should be clear but is not.. senility I guess
I understand the 83% rule but looking at 310.15(B)7)(3) and the fact that it refers back to (1)&(2) does (7) mean that in no case shall the feeder be required to be larger than the service OR only when the "entire load" is still in play.
Assume a 200 amp residential service where I take advantage of the 83% rule. I add a 20 amp or 40 amp 240v load at the service panel. The feeder no longer carries the "entire load" and I have lost my load diversity. Can I still run a feeder the same size as the service based on (B)(7)(3) or not ??

 

[LarryFine]

(Took too long to edit my comments to the above quote.)

I am obviously of the belief that the answer is 'yes' as long as the SECs are still large enough with the added load. Any new outside loads can only mean that the feeder can never be asked to carry more current than the SECs.

What other situation could the allowance in 310.15(B)(7)(3), to never have to exceed the SECs, be referring to?

 

[Dennis Alwon]

This is an example of the craziness of this section. If, for example, the code stated that in this case the sec would need to be increased to accommodate the feeder being properly sized. The whole point is to not allow the feeder to be sized smaller.

So if you run the feeder at the size of the sec then would you have to use a 150 amp breaker? No where does it state that I can still protect those feeder conductors at 200 amps.

 

[augie47]

I think I need to borrow sting's scratch pad
Change my scenario a little due to the next size up rule.
100 amp service with feeder to interior panel.
#2 AL SE cable to 100 amp M/B exterior panel with a 20 amp 2 pole load and feed thru lugs to interior.
It seems Larry is saying #2 AL is acceptable for the interior feeder based on (B)(7)(3) but Dennis is saying if #2 AL is used it would need to be protected by a 90 amp breaker. Am I misreading the two replies ??

 

[Dennis Alwon]

I think I need to borrow sting's scratch pad
Change my scenario a little due to the next size up rule.
100 amp service with feeder to interior panel.
#2 AL SE cable to 100 amp M/B exterior panel with a 20 amp 2 pole load and feed thru lugs to interior.
It seems Larry is saying #2 AL is acceptable for the interior feeder based on (B)(7)(3) but Dennis is saying if #2 AL is used it would need to be protected by a 90 amp breaker. Am I misreading the two replies ??

Next size up rule works if the load is not greater than 166 amps.

in the 100 amp example yes- that is what I am saying and I believe Larry and Trevor are in agreement on that issue.

 

[infinity]

100 amp service with feeder to interior panel.
#2 AL SE cable to 100 amp M/B exterior panel with a 20 amp 2 pole load and feed thru lugs to interior.
It seems Larry is saying #2 AL is acceptable for the interior feeder based on (B)(7)(3) but Dennis is saying if #2 AL is used it would need to be protected by a 90 amp breaker. Am I misreading the two replies ??

IMO the feeder could be protected at 100 amps. Now if you had a 150 amp SEC's to the first panel and a 100 amp feeder to the second then you would need a 100 amp conductor for the feeder.

 

[wwhitney]

IMO the feeder could be protected at 100 amps. Now if you had a 150 amp SEC's to the first panel and a 100 amp feeder to the second then you would need a 100 amp conductor for the feeder.

That's right. A side effect is that if you increase the size of an existing service, you may need to upsize existing feeders (or downsize their OCPD)..

Cheers, Wayne

 

[don_resqcapt19]

The ampacity of a feeder for a dwelling is NEVER required to be larger than the service conductors, no matter what loads are or are not served by the feeder.

In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).

 

[LarryFine]

It seems Larry is saying #2 AL is acceptable for the interior feeder based on (B)(7)(3) but Dennis is saying if #2 AL is used it would need to be protected by a 90 amp breaker. Am I misreading the two replies ??

You're reading mine correctly.

 

[Dennis Alwon]

The ampacity of a feeder for a dwelling is NEVER required to be larger than the service conductors, no matter what loads are or are not served by the feeder.

I don't think anyone is arguing that but where does it allow a feeder, in augies 100 amp example, #2 Al to be used at 100 amp ampacity unless it carries the full load

 

[Greentagger]

Get the torches and pitch forks ready, but I’ ll disagree. If a feeder does not supply the entire load, all bets are off. I interpret it this way because why include this language of supplying the entire load. It either does or it doesn’t.?

 

[infinity]

Get the torches and pitch forks ready, but I’ ll disagree. If a feeder does not supply the entire load, all bets are off. I interpret it this way because why include this language of supplying the entire load. It either does or it doesn’t.?

How so? We're not discussing supplying the entire load. It's a 100 amp feeder, the SEC's are #2 aluminum, the feeder conductors are not required to be larger than #2.

 

[don_resqcapt19]

I don't think anyone is arguing that but where does it allow a feeder, in augies 100 amp example, #2 Al to be used at 100 amp ampacity unless it carries the full load

The language in (B)(7)(3) very clearly permits that. Nothing in (3) talks about the feeder carrying the full load. (3) applies in all cases.

(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).

 

[Greentagger]

How so? We're not discussing supplying the entire load. It's a 100 amp feeder, the SEC's are #2 aluminum, the feeder conductors are not required to be larger than #2.

Yes sir , thats the way it could be interpreted ......... but 2020 NEC refers us to 310.12(C), then back to 310.12(A)&((B), and the only way you get to apply the 83% reduction, is if the service or feeder supplies the entire load? If not the entire load, no reduction?.

 

[don_resqcapt19]

Yes sir , thats the way it could be interpreted ......... but 2020 NEC refers us to 310.12(C), then back to 310.12(A)&((B), and the only way you get to apply the 83% reduction, is if the service or feeder supplies the entire load? If not the entire load, no reduction?.

310.12(C) takes away the load requirement and simply says that in all cases, no matter what load the feeder may be carrying, the feeder conductors never have to have an ampacity greater than what is required for service or feeder conductors that carry 100% of the dwelling unit load.
If it does not work that way, there would be no reason to have (C) as (B) would apply. (C) is intended for those cases where you have outside service equipment with breakers for things like an AC unit or a garage feeder, and a feeder breaker that feeds the dwelling unit panel. It is to prevent you from having to install a larger conductor for the house feeder than you did for the service itself.

 

[augie47]

It does appear 2020 removes the "confusion".

 

[Greentagger]

It does appear 2020 removes the "confusion".

Not so sure about that. That whole “ entire load” language still mucks it up to me.

 

[Greentagger]

Maybe “ sting “ will clarify this issue?