#2 AL can't carry 100 amps

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Stevenfyeager

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United States, Indiana
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electrical contractor
I'm running #2 AL SER inside the house, then changing to underground conduit with single #2 AL conductors to a subpanel for a detached garage. #2 AL will carry 100 amps right? I had one inspector a few years ago that said no. He said the single conductors would but the cable would not. 310.15 services for dwellings says 100 amps. But he and his county did not allow the #2 SER cable to be used for 100 amps. He finally said the reason was: a detached garage is NOT a dwelling and therefore table 310.15 does not apply for the cable. Any thoughts ?? Thank you
 
Stevenfyeager said:
I'm running #2 AL SER inside the house, then changing to underground conduit with single #2 AL conductors to a subpanel for a detached garage. #2 AL will carry 100 amps right? I had one inspector a few years ago that said no. He said the single conductors would but the cable would not. 310.15 services for dwellings says 100 amps. But he and his county did not allow the #2 SER cable to be used for 100 amps. He finally said the reason was: a detached garage is NOT a dwelling and therefore table 310.15 does not apply for the cable. Any thoughts ?? Thank you
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Residential service feeder! Is good for 100 amp.

Sent from my SM-T350 using Tapatalk
 
The inspector is correct to turn this down. #2AL in the conduit is only rated at 90A @75c.

Also, you can only use the #2AL at 100-amps feeding the service disconnect for the residence. Any feeder after that needs to go by 310.15(B)16 (which has #2AL limited to 90A at 75c), if the sub panel carries 100% of the dwelling load, the feeder will not be required to be larger than the service conductors.
 
ptonsparky said:
It does seem strange that the service to the house can be 2 @ 100 amps but the feeder to the from that panel to the garage must be larger. O well, they do make 90 amp CBs.
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Not quite. A dwelling with an NEC calculated load of 100A may be fed with a 90A set of conductors. This doesn't magically make them 100A conductors; rather it is an allowance for the fact that _dwelling_ service characteristics permit a smaller ampacity conductor to safely serve the load.

Feeders from a service need not be larger than the service conductors. If you have a 100A dwelling service that includes a 100A feeder to a garage (not a dwelling) then you would not need larger conductors in the feeder than the service.

But if you have say a 200A service to a dwelling with a 100A feeder, that feeder would need to be full size.

-Jon
 
Not always seems logical, but the way things are worded the only time you get to use the smaller conductors allowed by 310.15(B)(7) is for conductors that supply the entire load of a dwelling.
 
Since you are using SER I assume you have a feeder involved and as such, depending on the code year, ser may only be rated 60C, This is why the inspector is saying that individual conductors would be rated at 75 C but not ser cable.
 
Dennis Alwon said:
Since you are using SER I assume you have a feeder involved and as such, depending on the code year, ser may only be rated 60C, This is why the inspector is saying that individual conductors would be rated at 75 C but not ser cable.
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Before losing the table and just going with the 83% of 310.15(16), there was no temp ratings to deal with, old table simply said #2 aluminum or #4 copper was good for 100 amps regardless of cable or conductor insulation type.
 
kwired said:
Before losing the table and just going with the 83% of 310.15(16), there was no temp ratings to deal with, old table simply said #2 aluminum or #4 copper was good for 100 amps regardless of cable or conductor insulation type.
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Was the table removed because it was IMHO one of the misused/abused parts of the code?
 
ptonsparky said:
It does seem strange that the service to the house can be 2 @ 100 amps but the feeder to the from that panel to the garage must be larger. O well, they do make 90 amp CBs.
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Diversity and conservatism. Never thought I'd use those two in the same sentence. :lol: :p A 100amp service when derived from NEC sizing calcs will most likely never reach 100amps. Even if it somehow did, it would be for a very short period of time due to the immense diversity of load. On the other hand, something like an electric furnace that lacks real diversity could easily reach its rated current for hours on end.
 
norcal said:
Was the table removed because it was IMHO one of the misused/abused parts of the code?
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Dennis Alwon said:
I believe it was very confusing for many because it did not take into consideration the de-rating that may occur.
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That was my understanding.

Before you could have had a 100 amp conductor needed, but table just said you needed either 4 copper or 2 aluminum. No real information on how or if you needed to make ampacity adjustments.

Now it is more clear you must apply adjustments.

If no adjustments are necessary you still end up with same size conductor as the old table had.
 
kwired said:
That was my understanding.

Before you could have had a 100 amp conductor needed, but table just said you needed either 4 copper or 2 aluminum. No real information on how or if you needed to make ampacity adjustments.

Now it is more clear you must apply adjustments.

If no adjustments are necessary you still end up with same size conductor as the old table had.
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How did we manage to wire houses safely before all these ampacity adjusments?
 
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