2-2-2-4 SER for 100 amp Service

nick h

Member
Location
montana
table 310.15(B)(16) formerly 310.16 shows 90A for allowable ampacity. 230.42A says service entrance conductors shall have ampacity of not less than the maximum load to be served. loads shall be determined in accordance...and so on. as long as load requirements work out with wire ampacity #2 ser is legal to use for a 100A service correct? I have very little experience in residential work, but am sure ive seen plenty of 100A services with #2 SER. i can say it would not be acceptable to feed a 100A subpanel with a 100A breaker and use #2. Im having a hard time finding anything concrete on service entrance conductors. Thanks for any input.
 

kwired

Electron manager
Location
NE Nebraska
Thanks! i figured i was just missing something.
As others have said 310.15(B)(7). It is not limited to service conductors, and at same time if there are multiple service conductors supplying a dwelling it normally can't be applied either. It does apply to any conductor that is supplying the entire load of a dwelling. So for a large apartment complex, you may have a several feeders to each unit- each one is allowed to use that section for size of the supply conductors. A single dwelling can have an outside service disconnect and then a feeder carrying entire dwelling load to a panel indoors somewhere - still can apply to this feeder because it carries the entire dwelling load.
 

nick h

Member
Location
montana
As others have said 310.15(B)(7). It is not limited to service conductors, and at same time if there are multiple service conductors supplying a dwelling it normally can't be applied either. It does apply to any conductor that is supplying the entire load of a dwelling. So for a large apartment complex, you may have a several feeders to each unit- each one is allowed to use that section for size of the supply conductors. A single dwelling can have an outside service disconnect and then a feeder carrying entire dwelling load to a panel indoors somewhere - still can apply to this feeder because it carries the entire dwelling load.
great point, that really helps. thank you!
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
310.15(B)(7) made a mess of what was once a simple chart, but that is what allows you to use #2 Al for a 100A service on a single family dwelling.

Why is it a mess. 83% is easy and it takes into consideration all the de-rating that the table did not account for. IMO, the 83% has been a great improvement and, in fact, is the same as the table. Correction and de-rating are considered with the new section where the table did not consider that at all.
 

kwired

Electron manager
Location
NE Nebraska
Why is it a mess. 83% is easy and it takes into consideration all the de-rating that the table did not account for. IMO, the 83% has been a great improvement and, in fact, is the same as the table. Correction and de-rating are considered with the new section where the table did not consider that at all.
It is a mess to some because it involves math where before it just told you what to use:)
 

kwired

Electron manager
Location
NE Nebraska
It's a mess because it needlessly complicates the code. How many hundreds of millions of services were wired using table 310.15 (B)(6)? If there was a problem it would have shown up sometime in the 1980's.
Someone though we needed to apply adjustments when conditions call for it. But yes, all those older installations seemed to be fine withought it.

Same goes for the attempt to change how we determine temp on rooftops - all those older installs that never failed are a reason to make that change, right?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
It's a mess because it needlessly complicates the code. How many hundreds of millions of services were wired using table 310.15 (B)(6)? If there was a problem it would have shown up sometime in the 1980's.

Yes, but there was constant argument about whether adjustments were needed. The table doesn't call for adjustments. I understand that if there were problems they would have shown up but imo, that is not a good enough reason to bypass some of the code requirements.

How many jobs have I seen where 14/2 was used for a switch leg with a #12 (20 amp) cir. There were never issues so should we allow it?

It just seems more consistent to do it as it is now. You can always use the table if you prefer as long as there is no correction or de-rating.
 

kwired

Electron manager
Location
NE Nebraska
Yes, but there was constant argument about whether adjustments were needed. The table doesn't call for adjustments. I understand that if there were problems they would have shown up but imo, that is not a good enough reason to bypass some of the code requirements.

How many jobs have I seen where 14/2 was used for a switch leg with a #12 (20 amp) cir. There were never issues so should we allow it?

It just seems more consistent to do it as it is now. You can always use the table if you prefer as long as there is no correction or de-rating.
Can just as easily ask why this is a problem?

If the switch leg were for switched receptacles, I could understand it required to be 12 AWG in your situation, but where you do see this often is in dwellings switching fixed luminaires that often are well under 15 amps rating. You won't have overload conditions without modifying something and you still have short circuit/ground fault protection.

If it were a motor control circuit tapped from the motor circuit a 14 AWG could be used on up to 45 amp circuit, could possibly run it with same 14-2 NM cable and same 15 amp toggle switch as used for the lighting switch loop.
 
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