#2 SER cable - 100 amps?

jeff48356

Senior Member
When the panel is located far away from the meter, a disconnect box next to the meter, as well as 4-conductor SER cable is required. Normally #2 SEU cable is good for 100A service. However, table 310.15 says that SER cable would be good for only 90 amps. Would I be allowed to install a 100A breaker in the outdoor disconnect, or would I be limited to 90A? I've seen most of these with 100A breakers in them.
 

winnie

Senior Member
#2 aluminium where the conditions of use permit the 75C rating has an ampacity of 90A. If you need an actual ampacity of 100A you cannot use it.

However historically #2 aluminium was permitted to supply the total load of a residential service with a 100A rating. This means that a residential service protected by a 100A breaker could use #2 aluminium.

Under _current_ rules, for the _total_ load of a residential service you are permitted to use 83% of the service rating as the required conductor ampacity. This means that under current rules a 100A residential service only requires conductors with 83A ampacity.

In your case if you are describing a _residential_ service, than #2 SER would be acceptable.

-Jon
 

Dennis Alwon

Moderator
Staff member
I concur with winnie as long as the ser carries the totally load of the dwelling. Of course, we are assuming this is a dwelling
 

kwired

Electron manager
310.15(B)(7) or in the past it was (B)(6) is what allows this. It must be carrying the entire dwelling unit load to comply with this section. If you had a meter/loadcenter that maybe supplied the air conditioner and then a feeder to the panel inside, sort of doesn't make any sense but in that case you can have #2 service entrance conductor but would need #1 feeder to the inside panel as is worded.

Presume your conductors are aluminum here #2 copper is good for 115 amps without this allowance but would be good for 138 amps under this allowance.
 

winnie

Senior Member
310.15(B)(7) or in the past it was (B)(6) is what allows this. It must be carrying the entire dwelling unit load to comply with this section. If you had a meter/loadcenter that maybe supplied the air conditioner and then a feeder to the panel inside, sort of doesn't make any sense but in that case you can have #2 service entrance conductor but would need #1 feeder to the inside panel as is worded.
I believe you also have an additional permission that such that interior feeders are not required to have a higher ampacity than the service entrance conductors.

So if the dwelling has a 100A service with #2 Al conductors, and then you have a 100A feeder to an interior panel, then even though that feeder doesn't carry the full load of the dwelling, it can still be a #2.

-Jon
 

kwired

Electron manager
I believe you also have an additional permission that such that interior feeders are not required to have a higher ampacity than the service entrance conductors.

So if the dwelling has a 100A service with #2 Al conductors, and then you have a 100A feeder to an interior panel, then even though that feeder doesn't carry the full load of the dwelling, it can still be a #2.

-Jon
That has been discussed here before, I think the consensus was that doesn't apply to 310.15(B)(7) applications, because one of the conditions is the conductors must be carrying the entire dwelling unit load to be able to use this, and if you are dropping off part of the load then continuing on to another panel, you no longer are supplying the entire dwelling load with that portion of the circuit.

The allowance has to do with load diversity of the entire dwelling and the intent is to only apply this allowance when the circuit carries the entire dwelling load. That said, it sort of doesn't make any sense if you are dropping off the AC then supplying the rest of the dwelling, The AC is often a load that runs somewhat continuously when it does run and is also often one of the larger loads (that runs for very long anyway) in the dwelling.

If you find something that confirms either way let us know. I will do a little looking myself and post anything if I find it.
 

kwired

Electron manager
I found 215.2(A)(3) but it isn't saying what we are talking about here. (copied from 2014 NEC)

[h=4](3)Ampacity Relative to Service Conductors.[/h]
The feeder conductor ampacity shall not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55 amperes or less.
Shouldn't be anything on load side of service disconnect that is covered in art 230, though I did take a quick glance but didn't see anything I wanted to look harder at in 230.

Only other place that I can think of is 310.15 and I see nothing there other than 310.15(B)(7) and it is pretty clear that the conductors must be carrying the entire load of the dwelling to be able to use the allowances of (B)(7).
 

jeff48356

Senior Member
I found 215.2(A)(3) but it isn't saying what we are talking about here. (copied from 2014 NEC)



Shouldn't be anything on load side of service disconnect that is covered in art 230, though I did take a quick glance but didn't see anything I wanted to look harder at in 230.

Only other place that I can think of is 310.15 and I see nothing there other than 310.15(B)(7) and it is pretty clear that the conductors must be carrying the entire load of the dwelling to be able to use the allowances of (B)(7).
OK, thanks for the clarification! This explains why I can use a 100A breaker in that situation, but I would need to use a 90A breaker if it only supplied part of the dwelling load. I worked on another job where I installed a 150A panel in a new addition, which supplied power to all the new circuits, then had to run a #2 SER cable to feed the panel in the basement for the original part of the house. I used a 90A for that, rather than 100.
 

wwhitney

Senior Member
So what about (2014) 310.15(B)(7)(3):

(3) 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)(1) or (2)
That doesn't include the verbiage about "supplying the entire load", which is present in 310.15(B)(7)(1) and (2).

So as I see it, on a dwelling unit, you could have 83A service conductors supplying a 100A main breaker (310.15(B)(7)(1)). That main breaker can supply an 83A feeder for a panelboard with the entire load of the dwelling unit (310.15(B)(7)(2)). Then that panelboard could have a 100A breaker in it (or just feed through lugs) supplying an 83A feeder to a second panelboard that carries only part of the load of the dwelling unit (310.15(B)(7)(3)).

Cheers, Wayne
 

winnie

Senior Member
So what about (2014) 310.15(B)(7)(3):

That doesn't include the verbiage about "supplying the entire load", which is present in 310.15(B)(7)(1) and (2).
That is the point I was making. In order to apply the 83% factor, the service or feeder conductors must supply the _entire_ building load. However once you've met that requirement you never have to jump up to a higher ampacity conductor any downstream feeders, even those carrying part of the load. The 83% factor only applies directly to conductors that carry the full load of the dwelling, but this factor applies indirectly to any conductors that are as large as the service conductors.

Thus if you have a 100A service to a residence, you can use 83A service conductors, and for any downstream feeder you can continue to use 83A conductors.

However if you have a 200A service to a residence (using conductors with a 166A ampacity) then a 100A feeder would require larger conductors since the 83% rule does not apply directly to that feeder, and the indirect rule just says that the feeder doesn't need an ampacity greater than 166A :)

-Jon
 

kwired

Electron manager
So what about (2014) 310.15(B)(7)(3):



That doesn't include the verbiage about "supplying the entire load", which is present in 310.15(B)(7)(1) and (2).

So as I see it, on a dwelling unit, you could have 83A service conductors supplying a 100A main breaker (310.15(B)(7)(1)). That main breaker can supply an 83A feeder for a panelboard with the entire load of the dwelling unit (310.15(B)(7)(2)). Then that panelboard could have a 100A breaker in it (or just feed through lugs) supplying an 83A feeder to a second panelboard that carries only part of the load of the dwelling unit (310.15(B)(7)(3)).

Cheers, Wayne
(B)(7)(3) says "In no case shall a feeder for an individual dwelling unit" doesn't state general feeders within a dwelling. I still say to apply the allowances here the conductor must be supplying the entire load to a dwelling unit. If that is not what they intended they needed to word it better.
 

wwhitney

Senior Member
(B)(7)(3) says "In no case shall a feeder for an individual dwelling unit" doesn't state general feeders within a dwelling. I still say to apply the allowances here the conductor must be supplying the entire load to a dwelling unit. If that is not what they intended they needed to word it better.
Your interpretation renders (3) superfluous. Given that they've included a section (3), there has to be a point to it. And the point is that it applies to any feeder within a dwelling unit, to avoid the absurdity of requiring a downstream feeder to have a higher ampacity than an upstream service/feeder.

Cheers, Wayne
 

kwired

Electron manager
Your interpretation renders (3) superfluous. Given that they've included a section (3), there has to be a point to it. And the point is that it applies to any feeder within a dwelling unit, to avoid the absurdity of requiring a downstream feeder to have a higher ampacity than an upstream service/feeder.

Cheers, Wayne
Well it needs to be worded differently if that is the intent IMO. Could be as simple as striking the word "for" and replacing it with "within".
 

Dennis Alwon

Moderator
Staff member
Your interpretation renders (3) superfluous. Given that they've included a section (3), there has to be a point to it. And the point is that it applies to any feeder within a dwelling unit, to avoid the absurdity of requiring a downstream feeder to have a higher ampacity than an upstream service/feeder.

Cheers, Wayne
Yes but a wire can be larger in size than he se conductors and still have the same ampacity after derating.
 

winnie

Senior Member
Yes but a wire can be larger in size than he se conductors and still have the same ampacity after derating.
Agreed. In earlier editions of the code, the permission was size based; if you started with #2 Cu you could always use #2 Cu.

In current code the corresponding permission is based on ampacity, so that you might have a 100A service using #2 Cu, and never need to go to a higher ampacity than that #2 Cu...but if because of installation conditions you need to derate conductors then they might be required to be larger gauge.

-Jon
 
Well it needs to be worded differently if that is the intent IMO. Could be as simple as striking the word "for" and replacing it with "within".

It might be a little more clear if you look at (3) and see how it says a feeder is not required to have ampacity greater than specified in (1) and (2), and see that (1) and (2) specify an ampacity based on the rating of the feeder/service supplying the entire load associated with the dwelling.

So, if you had 400 amp service and main feeder ratings this section wouldn't allow you to have 83amp conductors for a downstream 100amp sub-feeder, but it would allow 332amp conductors on a downstream 400amp sub-feeder.

It's to rectify the problem winnie pointed out, having higher ampacity conductors downstream of lower ampacity conductors, not to allow 83% to be used on any feeder rating in the dwelling. This is commented on quite a bit by panel members in cmp reports during recent code cycles. I've had to read the reports to wrap my head around the section.



(1) For a service rated 100 through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.

(2) For a feeder rated 100 through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating.

(3) 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)(1) or (2).
 
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