# SER sizing



## CopperSlave (Feb 9, 2012)

What are you using the SER for?


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## Dennis Alwon (May 9, 2009)

First off does the ser carry the entire load of the dwelling?

Then if you are limited to 60C then the inspector is correct. If you are not run thru insulation then you can use the 75C rating



> 310.15(B)(7)(3) says "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)(2)."


This has nothing to do with wire size but rather ampacity. You could need a larger conductor to get the same ampacity of a smaller conductor based on de-rating factors.


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## BSK3720 (Mar 29, 2014)

Resi feeder, but it's always in contact with thermal insulation.


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## Dennis Alwon (May 9, 2009)

BSK3720 said:


> Resi feeder, but it's always in contact with thermal insulation.


If it is run outside the insulation it can touch it but it cannot run thru it-- also look at 310.15(A)(2)


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## BSK3720 (Mar 29, 2014)

Yes, SER is usually carrying the entire load of the dwelling. That brings up another point: If you had a 400A service with 2 200A sub-panels, I guess you would have to use 250AL instead of 4/0AL for the feeders. I'm missing the logic in the 83% rule.

My supplier doesn't stock 250AL SER.


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## BSK3720 (Mar 29, 2014)

Dennis Alwon said:


> If it is run outside the insulation it can touch it but it cannot run thru it-- also look at 310.15(A)(2)


Good point. Thanks


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## Dennis Alwon (May 9, 2009)

BSK3720 said:


> Yes, SER is usually carrying the entire load of the dwelling. That brings up another point: If you had a 400A service with 2 200A sub-panels, I guess you would have to use 250AL instead of 4/0AL for the feeders. I'm missing the logic in the 83% rule.
> 
> My supplier doesn't stock 250AL SER.


You are correct that if you have 2- 200 amp panels then you cannot use the 83% rule however if you pipe between the panels you can use the 75C rating which means 4/0 is rated 180 amps. We can then use 240.4(B) and use 200 amps as the overcurrent protective device as long as the calculated load is not more than 180 amps.

The 83% rule can be used but you must also use any de-rating that comes into effect. Thus if your ser runs thru insulation then you must use the 60C rating of the wire.

200 amp service times 83%= 166 amps. This means you need a conductor or cable rated 166 amps or better after all adjustments have been made.

Look at 250 kcm-- it is rated 170 amps at 60C. This is greater than 166 amps so you can use that for a 200 amp service if it carries the entire load of the dwelling


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## McClary’s Electrical (Feb 21, 2009)

I don't understand the need for this change. 4/0 had been just fine for the past 50 years on 200 amps, so why the need for a change? Can anybody , in the entire world, offer me one single instance or one single shred of evidence that 4/0 se has been overloaded on a sfd at 200 amps? I bet there is not even one single documented case.


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## Ultrafault (Dec 16, 2012)

mcclary's electrical said:


> I don't understand the need for this change. 4/0 had been just fine for the past 50 years on 200 amps, so why the need for a change? Can anybody , in the entire world, offer me one single instance or one single shred of evidence that 4/0 se has been overloaded on a sfd at 200 amps? I bet there is not even one single documented case.


Here you go.


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## Dennis Alwon (May 9, 2009)

mcclary's electrical said:


> I don't understand the need for this change. 4/0 had been just fine for the past 50 years on 200 amps, so why the need for a change? Can anybody , in the entire world, offer me one single instance or one single shred of evidence that 4/0 se has been overloaded on a sfd at 200 amps? I bet there is not even one single documented case.



I agree that 4/0 would probably never be an issue in a residence but the change is really not much of a change. The 83% basically comes out to be just as the Table would have allowed. The difference is that now there is clarification that all ampacity adjustments must be considered. In the past, 4/0 alum. was allowed even when run thru insulation. That was not the intent but nonetheless it was not clear to many. Now that has been clarified.


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## BSK3720 (Mar 29, 2014)

I heard someone on the code making panel say that the Table was "broke" and it needed to be fixed. I guess this is what he's talking about. Thanks for your insight.


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## duaneb (Nov 4, 2014)

*SE cable*

I am installing a 3 place meter socket and feeding the 100amp panels with SE cable. my question can I run that cable under the joists in a unfinished basement?334.15(c) says you can with #8-3nm but what about se?


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## duaneb (Nov 4, 2014)

I just saw I posted a question on someones thread ooops, how the hell you post a new topic? =rookie on electalk


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## chicken steve (Mar 22, 2011)

BSK3720 said:


> I heard someone on the code making panel say that the Table was "broke" and it needed to be fixed. I guess this is what he's talking about. Thanks for your insight.


Holt is saying it's gone through quite the cmp process, new inclusion in the '14 , possible changes on the '17 horizon

Myself, i'd just like to keep using SE vs pipe if possible, but i've found only 250 SER, not SEU ....

~CS~


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## Carultch (May 14, 2013)

mcclary's electrical said:


> I don't understand the need for this change. 4/0 had been just fine for the past 50 years on 200 amps, so why the need for a change? Can anybody , in the entire world, offer me one single instance or one single shred of evidence that 4/0 se has been overloaded on a sfd at 200 amps? I bet there is not even one single documented case.


What they did, was remove the "service conductor sizing" table from NEC2011, and replace it with a factor that you apply onto ampacities from table 310.15(B)(16). Take the service amps, multiply by 0.83, then select a wire from 310.15(B)(16). The content and results haven't changed, just the way they present the procedure. The lookup table still works, and you can snip the table out of your old code book and paste it in to a blank part of your new code book.

One reason for this change would be to satisfy people who prefer formulas over lookup tables.

Personally, I'm still not satisfied with the concept that somehow 200A service conductors could be 83% the amperes of 200A feeder conductors. Is it or is it not 200 Amperes? The thermal conductivity of aluminum doesn't increase just because they are service conductors.

To check the table, here's a select few examples:
100A service, #2 AL per former table; 83A is 83% of service, #2 AL Per 310.15(B)(16) at 75C AL

200A service, #4/0 AL per former table; 166A is 83% of service, #4/0 AL Per 310.15(B)(16) at 75C AL

300A service, 350 kcmil AL per former table; 249A is 83% of service, 350 kcmil AL Per 310.15(B)(16) at 75C AL

400A service, 600 kcmil AL per former table; 332A is 83% of service, 600 kcmil AL Per 310.15(B)(16) at 75C AL


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## Carultch (May 14, 2013)

Anyone know how to delete a post? I thought I had a response that makes sense, but think I misunderstood the question.


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