1/0 or 1 SER al.

[wwhitney]

(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be permitted
to be used beyond the point of transition, a distance equal to
3.0 m (10 ft) or 10 percent of the circuit length figured at the
higher ampacity, whichever is less.

I believe the word "length" above is being used as a synonym for "segment", as in "a length of rope".

I agree with GoldDigger that the exception applies at each transition point between a segment of higher ampacity and a segment of lower ampacity.

So you could have 5 feet at lower ampacity, 50 feet at higher ampacity, and then 5 feet at lower ampacity, use the exception twice, and use the higher ampacity for the whole thing.

Cheers, Wayne

 

[iwire]

I believe the word "length" above is being used as a synonym for "segment", as in "a length of rope".

I agree with GoldDigger that the exception applies at each transition point between a segment of higher ampacity and a segment of lower ampacity.

So you could have 5 feet at lower ampacity, 50 feet at higher ampacity, and then 5 feet at lower ampacity, use the exception twice, and use the higher ampacity for the whole thing.

Cheers, Wayne

I agree.

 

[jumper]

TRYING to remember, is there something in the code about "what if" the wire is on a 100 amp breaker? :happysad::happyyes:

Not for the scenario being discussed.

 

[Dennis Alwon]

I have read the article a few times and I thing I agree with Golddigger. It makes sense I just never read it as such. The reason why I changes was the use of the word transition.

 

[jumper]

I have read the article a few times and I thing I agree with Golddigger. It makes sense I just never read it as such. The reason why I changes was the use of the word transition.

Noooooo. Don't go over to the Dark side and leave me hanging solo.

 

[luckylerado]

I am having a hard time buying that "circuit length" means "Circuit segment" as I see both terms used together in the context of RV park feeders ala T551.73(A) and they are clearly 2 different terms in the NEC vocabulary. Although someone will undoubtedly point out the informational notes are unenforceable I believe it does provide a distinct difference in the terms.

I may be willing to be convinced that you can apply the exception more than once to the same circuit but where does it end?

Would you feel the same way if in this case the SER were in contact with insulation for 8' 3 more times along its length?

Would you feel the same way if there were 8' of #2 at both ends?

At a minimum it seems that the author felt that it was important to reserve no less than a 9 to 1 ratio of heat sink to lower ampacity

 

[GoldDigger]

BTW, with the "10% of the rest of the circuit" provision it really is a 10 to 1 ratio.

 

[Dennis Alwon]

Let me clarify. If we had a run of 110' and the last and first 10' had a different temp. rating then I would not accept that. However if that 10' were at the beginning and end of a 200' run I see that as acceptable, code compliant and logical.

The exception is what is pertinent here and it says where you have two different ampacities on adjacent portions of the circuit. Does the two sections on the 200' run meet this criteria? Nowhere does it state only one portion

 

[ActionDave]

Noooooo. Don't go over to the Dark side and leave me hanging solo.

Give in. In this case it's the right thing to do.

 

[JDB3]

AND in conclusion, the vote is that #1 SER Al. , is permissible to be on the 100 amp breaker? :?

 

[Dennis Alwon]

AND in conclusion, the vote is that #1 SER Al. , is permissible to be on the 100 amp breaker? :?

IMO, in the op's question the amapcity derating exception is not met. In this case I see 1/0 ser as being rated 60C not 75C so #1/0 is needed for 100 amps. But, as mentioned earlier if the calculated load is less or equal to 85 amps then a 90 amp breaker will work. If that works for the op.