Derating S.E.R. cables to sub panels

[howardrichman]

I'm having a liitle trouble trying to get hold a towns local Electrical Inspector to see if he'd be strick on using 1/0 SER aluminum for a 100 amp sub panel. Usually I'd install a 100amp sub panel from a main service panel using #2 SER service cable, and just install a 90 amp breaker to meet the 75c columb of art.310-16. Acording to the NEC and many local inspectors, any cables ( including romex) leaving the 1st distribution panel, must meet the derate cable of the 60c columb. This is why i can install a 4/0 SER from a 200A service disconnect to the 1st main dist. panel. I think for conductors up to #10 gu cu, the amp ratings are higher for derating purposes. 1/0 alum, is not stocked by most supply houses, and #1 SER is non existant, and 3-3 or 2-3 romex copper way too expensive for most jobs for a sub panel. Not sure but; To meet the code entirely; even the main feeder from a service disconnect to a sub panel is supposed to be derated to the 60c columb, because it being a shiethed cable assembly, but most inspectors are letting it go. Not surewhat to make of that??

Any clarification appreciated;
HR...

 

[Dennis Alwon]

1/0 alumin. is good for 100 amps at 60C-- what's the issue

 

[infinity]

I wouldn't call using a cable at it's 60? C ampacity derating, that seems too confusing when derating is typically used to reduce a conductor ampacity according to the tables in the Article 310. Yes you are correct under the 2008 SE cable must be used at it's 60? C ampacity for interior instllations. There is some change to this in the 2011 NEC when the cable is not installed in thermal insulation you can use the 75? C ampacity. At 75? C #2 Al is good for 90 amps.

 

[Twoskinsoneman]

#1 SER is non existant, and 3-3 or 2-3 romex copper way too expensive for most jobs for a sub panel.

Ah yes wouldn't I be a happy camper if 1-1-1-6 SER al was stocked locally. I think most inspectors mistakenly allow #2 for sub panels based on table 310.15(B)(7). Otherwise I would think there would be a large demand for the #1

 

[Dennis Alwon]

I must be missing something. The op asked if 1/0 alum would be allowed for 100 amp.

I'm having a little trouble trying to get hold a towns local Electrical Inspector to see if he'd be strick on using 1/0 SER aluminum for a 100 amp sub panel.

 

[Twoskinsoneman]

I must be missing something. The op asked if 1/0 alum would be allowed for 100 amp.

I think everyone was going on the assumption of typo thinking he meant #1

 

[infinity]

Sounds like he has no chance of finding #1 SER.

1/0 alum, is not stocked by most supply houses, and #1 SER is non existent

 

[Dennis Alwon]

I think everyone was going on the assumption of typo thinking he meant #1

See, you all are smarter than I.

 

[Twoskinsoneman]

BTW why do they make #2 SER most commonly in the 2-2-2-4 configuration?
Even using table 310.15(B)(7) #2 al is only good for 100a and the EGC would only need to be #6 right?

That why I posted 1-1-1-6 would be nice but I see it's made in the 1-1-1-3 configuration.... What am I missing?

 

[infinity]

BTW why do they make #2 SER most commonly in the 2-2-2-4 configuration?
Even using table 310.15(B)(7) #2 al is only good for 100a and the EGC would only need to be #6 right?

That why I posted 1-1-1-6 would be nice but I see it's made in the 1-1-1-3 configuration.... What am I missing?

My guess is that it allows yo to use over-sized cables where you would need to increase the size of the EGC.

 

[Dennis Alwon]

My guess is that it allows yo to use over-sized cables where you would need to increase the size of the EGC.

I think it is sized for a 3 phased service with a reduced neutral.

 

[Twoskinsoneman]

My guess is that it allows yo to use over-sized cables where you would need to increase the size of the EGC.

That would seem a waste for a uncommon situation.

I think it is sized for a 3 phased service with a reduced neutral.

that never crossed my mind but makes sense.

 

[infinity]

I think it is sized for a 3 phased service with a reduced neutral.

That works for me too.

 

[howardrichman]

I must be missing something. The op asked if 1/0 alum would be allowed for 100 amp.

No; what I mean is since #2 SER is not allowed(derated) as to use as 90 amp@75c, Are Inspectors enforcing 1/0 be used for 100 amps, and are we allowed yet to use the 75c colum if installed w/no insulation interior residence as in the 2011 code?
HR...

 

[Twoskinsoneman]

No; what I mean is since #2 SER is not allowed(derated) as to use as 90 amp@75c, Are Inspectors enforcing 1/0 be used for 100 amps, and are we allowed yet to use the 75c colum if installed w/no insulation interior residence as in the 2011 code?
HR...

As was stated it is not derating. If it is installed in contact the insulation "the ampacity shall be in accordance with the 60?C (140?F) conductor temperature rating"

If you can't get #1 than I guess you have to use 1/0.

What you seem to be asking is are the inspectors reinforcing this. The answer will obviously vary based on where your at. In my county I would say no they don't enforce it and either believe or appear to believe subs can be fed IAW table 310.15(B)(7) even though everyone on this forum seem to understand this to be a violation.

 

[Dennis Alwon]

No; what I mean is since #2 SER is not allowed(derated) as to use as 90 amp@75c, Are Inspectors enforcing 1/0 be used for 100 amps, and are we allowed yet to use the 75c colum if installed w/no insulation interior residence as in the 2011 code?
HR...

I would say that an inspector who is doing his/her job would require 1/0 if the sub does not carry the entire load of the panel. It also depends on which code cycle you are in.

2005 you could use 75C, 2008 you had to use 60C, now 2011 depending if it is in insulation or not would be 60C or 75C.

 

[infinity]

No; what I mean is since #2 SER is not allowed(derated) as to use as 90 amp@75c, Are Inspectors enforcing 1/0 be used for 100 amps, and are we allowed yet to use the 75c colum if installed w/no insulation interior residence as in the 2011 code?
HR...

Not sure that I understand, at 75? C, #2 al is a 90 amp conductor.

 

[david luchini]

and are we allowed yet to use the 75c colum if installed w/no insulation interior residence as in the 2011 code?

Yes, if you are under the 2011 code, you can use the 75deg C column for your SER IF it is not installed in thermal insulation.

338.10(B)(4) Installation Methods for Branch Circuits and Feeders

(a) Interior Installations. In addition to the provisions
of this article, Type SE service-entrance cable used for interior
wiring shall comply with the installation requirements
of Part II of Article 334, excluding 334.80.

Where installed in thermal insulation, the ampacity shall be
in accordance with the 60?C (140?F) conductor temperature
rating. The 90?C (194?F) rating shall be permitted to be
used for ampacity adjustment and correction purposes, provided
the final derated ampacity does not exceed that for a
60?C (140?F) rated conductor.