Pennsylvania Electrical Suppliers - #2 SER for 100A sub??

[emf10]

I called four electrical suppliers looking for SER or any cable for that matter large enough for a 100amp sub-panel. Not one had any in stock and they didnt seem interested in ordering it because they would need such a large quantity. It seems everyone in PA is confused about the proper size cable for a 100A sub panel. Nearly everyone around here seems to be misapplying 310.15(B)(6) by using it to size a sub-panel. It has arguably never been allowed for a sub-panel and newer revisions of the code reflect this by clearing up the wording (the cable must supply "all loads that are part or associated with the dwelling unit"). At best this would require #1AL SER with 75deg connectors and when the next code cycle (NEC 2008) is in effect in PA we will be further restricted to the 60deg column which works out to SER #1/0. I am very surprised how far the misinformation is spread here or did I interpret something wrong?

oh btw they don't stock 90amp breakers either.

 

[Dennis Alwon]

You are correct in your understanding of the code and 310.15(B)(6).

 

[infinity]

I'm still baffled as to why we need this 60 degree rating for SE in the first place. I'm sure it has to do with keeping SE and NM in the same line but was sizing SE at 75 degrees ever really a problem?

 

[emf10]

I'm still baffled as to why we need this 60 degree rating for SE in the first place. I'm sure it has to do with keeping SE and NM in the same line but was sizing SE at 75 degrees ever really a problem?

or why is NM-B restricted to the 60deg column? I guess for an extra margin of safety. I assume they have observed a significant enough number of problems in the past that led them to conclude the 60deg requirement, thereby upsizing the wire, was worth it.

 

[infinity]

or why is NM-B restricted to the 60deg column? I guess for an extra margin of safety. I assume they have observed a significant enough number of problems in the past that led them to conclude the 60deg requirement, thereby upsizing the wire, was worth it.

But the NM 60 degree requirement has been around for years and so has the SE 75 degree requirement. Is there an evidence that the 75 degree requirement for SE is a hazard? If not then why change it?

 

[Dennis Alwon]

I'm still baffled as to why we need this 60 degree rating for SE in the first place. I'm sure it has to do with keeping SE and NM in the same line but was sizing SE at 75 degrees ever really a problem?

Rob, my understanding was that the wire manufacturers wired se cable going thru insulation with a 100% load on the cable for many many hours and there was evidence of it breaking down.

Now how often is se cable installed with a 100% load on it's ampacity. Usually there is a 125% factor on it already for heat pumps etc. It certainly does not have 100% load in a residence.

 

[electricmanscott]

I can say with certainty that I am the only one in my area using 1/0 AL SER for 100 amp sub panels. My supplier always has to get it special for me. I showed them the "book" the other day exaplaining what the deal was. I think they get it but I doubt they care. It really isn't their problem.

 

[infinity]

Rob, my understanding was that the wire manufacturers wired se cable going thru insulation with a 100% load on the cable for many many hours and there was evidence of it breaking down.

Now how often is se cable installed with a 100% load on it's ampacity. Usually there is a 125% factor on it already for heat pumps etc. It certainly does not have 100% load in a residence.

You might be right but I thought that someone posted an ROP for the change a while back that stated they wanted to have both NM and SE at the 60 degree rating.

 

[iaov]

We had a disscusion about this a while back and Iwire explained to me the error of my ways. I think a lot of electricians have been using the wrong table to choose wires for sub panels for a very long time. Like Dennis pointed out though, rarely is this wire ever subjected to anything close to 100% loading and so it rarely is ever a problem.

 

[don_resqcapt19]

7-88 Log #2639 NEC-P07 Final Action: Accept
(338.10(B)(4)(a))
____________________________________________________________
Submitter: James M. Daly, General Cable
Recommendation: Delete the phrase “excluding 334.80” and change the
comma after “Article 334” to a period.
Substantiation: When Type SE conductors are used for interior wiring, as a
replacement for Type NM cable, the ampacity of the conductors should be the
same as permitted for NM cable since the insulations used are the same both
NM and SE conductors.
Panel Meeting Action: Accept
Panel Statement: This action will modify the action taken on Proposal 7-84.
Number Eligible to Vote: 14
Ballot Results: Affirmative: 14
____________________________________________________________

There is no technical substantiation for the change but it was proposed by a cable manufacturer. There were no comments on this proposal.

 

[peter d]

There is no technical substantiation for the change but it was proposed by a cable manufacturer. There were no comments on this proposal.

Wow.

It's stuff like that which really makes me lose my faith that the NEC is driven solely by safety.

 

[Jim W in Tampa]

Wow.

It's stuff like that which really makes me lose my faith that the NEC is driven solely by safety.

Just might be a few kickbacks behind it .

 

[Flex]

I having trouble understanding. Is #1 aluminum se cable the right size for 100A subpanel?

 

[Dennis Alwon]

I having trouble understanding. Is #1 aluminum se cable the right size for 100A subpanel?

No , 1/0 aluminum would be required unless that subpanel carried the entire load of the service. This is based on 2008 NEC

 

[SEO]

Dennis what if you had a 100amp disconnect nippled on the meter fed with #2 al and the subpanel was located in the house. The ser feeding the subpanel carries the entire load. Now based on section 338.10(B) (4) type Se cable used for interior wiring shall comply with the requirements of Part II of art 334 the ampacity shall be in accordance with the 60 C conductor rating. In this case you would have to use a 1/0 ser cable for the subpanel .

 

[Flex]

Im still confused, If I add a 100A subpanel in a basment for, say 3 circuits in an addition, I am wrong to use #1. It needs to be 1/0?

 

[jimport]

So am I to understand that SE doesn't breakdown when it carries the entire load of a dwelling, but, somehow does degrade when it only carries a portion of the load?

 

[Flex]

So am I to understand that SE doesn't breakdown when it carries the entire load of a dwelling, but, somehow does degrade when it only carries a portion of the load?

Thats whats confusing me. Plus I think I had the wrong size wire in the first place.

 

[LarryFine]

So am I to understand that SE doesn't breakdown when it carries the entire load of a dwelling, but, somehow does degrade when it only carries a portion of the load?

No, that the SE cable never will be asked to carry its ampacity, due to diversification.

 

[emf10]

So am I to understand that SE doesn't breakdown when it carries the entire load of a dwelling, but, somehow does degrade when it only carries a portion of the load?

I think where this comes from is basically that the demand factor for a dwelling as a whole is known well enough that #2AL SE will normally meet the requirements. When a 100A subpanel from a 200A main panel is installed for example it's possible that the sub is loaded up so that it is nearer to 100amps load for a much longer period of time. (ie it does not supply cooking equipment which in a dwelling is only used for a short time each day.) Maybe for example the sub ends up with a bunch of electric heat, heated pool and spa, etc and this dwelling is located in a cold part of the country where these things will run a lot.

As far as #2AL SE being adequate for 100amps for a long period of time (decades), well that's another issue and would depend on many factors. I am skeptical of the change saying it isn't. The NEC still says it is adequate to supply a 100A service, where it could have the sun beat down on it all day. If that's the case I would think a short run of #2AL SER in a dark cool basement would also be adequate for a 100A sub even with possible greater demand, but apparently the manufactures are saying otherwise? It definitely means they'll be selling more wire.