100 amp sub panel in garage.

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kennneth1

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Hello everyone,
I'm installing a 100 amp sub panel in a detached garage. The feed is coming from and 200 amp panel at the main house. The run is 80' long and will have three conduit bodies in it. My question is, would #2 copper be the correct conductor for this project? I'm using the 75 degree table to compensate for the ratings on the breakers. The cable will be THHN.
Thanks for all your help with this matter.
Best,Ken.


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And is your THHN actually double rated, as THWN or THWN-2? You need the wet rating for conduit outside.

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kennneth1 said:
Hello everyone,
I'm installing a 100 amp sub panel in a detached garage. The feed is coming from and 200 amp panel at the main house. The run is 80' long and will have three conduit bodies in it. My question is, would #2 copper be the correct conductor for this project? I'm using the 75 degree table to compensate for the ratings on the breakers. The cable will be THHN.
Thanks for all your help with this matter.
Best,Ken.


Sent from my QTAQZ3 using Tapatalk
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#2 Cu would certainly work fine.

I don't understand the comment about having to "compensate for the ratings on the breakers". Are you trying to say the breakers are 100% rated and you need the full rating?

You really have to start with your calculated load and since you have not provided that figure it is going to be hard to help you any.

Incidentally if it goes UG or outside it needs to be THWN. These days most THHN is rated as THHN, THHN2, and THWN.

100 Amps seems like a lot for a normal garage.
 
Hi Petersonra. Thanks for getting back to me so quickly. I was under the impression that most breakers are rated at 75 degrees C, therefore I was using the 75 degree table to size the conductors. As for the calculated load, I'm not exactly sure of this. The customer does intend on installing a 240, 40A electric vehicle charger in the garage. Thanks again for your help. l mainly work on high rise deck construction so,some of this residential work is foreign to me.

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kennneth1 said:
Hi Petersonra. Thanks for getting back to me so quickly. I was under the impression that most breakers are rated at 75 degrees C, therefore I was using the 75 degree table to size the conductors. As for the calculated load, I'm not exactly sure of this. The customer does intend on installing a 240, 40A electric vehicle charger in the garage. Thanks again for your help. l mainly work on high rise deck construction so,some of this residential work is foreign to me.

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it all goes back to your load calculation. what did that calculation come up with.

most common breaker terminations are indeed rated only for 75 deg C.
 
I'm not familiar with the formula for this. Is it the standard formula for "dwellings". Pardon my ignorance on this topic. Thanks for your help.

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Pretty sure you can use #2 alum which is what myself and most guys I know use. Per NEC 310.15(B)7. #2 XHHW 75deg rated 90 amps, more than the 83% of 100 required.
 
GerryB said:
Pretty sure you can use #2 alum which is what myself and most guys I know use. Per NEC 310.15(B)7. #2 XHHW 75deg rated 90 amps, more than the 83% of 100 required.
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NEC 310.15(B)7 does not apply. A garage subpanel is a branch circuit off the main, not a feeder. There is not a discount on the wire size because the loads are not mixed like a resi main panel. The proper wire size is found in table 310.15(B)(16). It is #1 AL or #3 CU if using the 75 deg column.
 
GerryB said:
Pretty sure you can use #2 alum which is what myself and most guys I know use. Per NEC 310.15(B)7. #2 XHHW 75deg rated 90 amps, more than the 83% of 100 required.
Click to expand...
The garage is not a dwelling unit, 310.15(B)(7) can not be used. You can still run #2 aluminum if the calculated load is 90 amps or less and supply it with a 90 amp breaker though. (even if the garage panel has a 100 amp main breaker in it the feeder is still protected by the 90 amp feeder breaker.

Coppersmith said:
NEC 310.15(B)7 does not apply. A garage subpanel is a branch circuit off the main, not a feeder. There is not a discount on the wire size because the loads are not mixed like a resi main panel. The proper wire size is found in table 310.15(B)(16). It is #1 AL or #3 CU if using the 75 deg column.
Click to expand...
It is still a feeder, 310.15(B)(7) doesn't apply because we are not supplying a dwelling unit.
 
kennneth1 said:
I'm not familiar with the formula for this. Is it the standard formula for "dwellings". Pardon my ignorance on this topic. Thanks for your help.

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The only thing different about dwellings is the allowance in 310.15(B)(7), and the fact you do run into conductors at times that must use 60 degree ampacity columns. But the garage is not a dwelling unit so (B)(7) can't be used, and an outdoor feeder can't use NM cable so the 60 degree conductor requirement don't normally apply either.
 
Coppersmith said:
NEC 310.15(B)7 does not apply. A garage subpanel is a branch circuit off the main, not a feeder. There is not a discount on the wire size because the loads are not mixed like a resi main panel. The proper wire size is found in table 310.15(B)(16). It is #1 AL or #3 CU if using the 75 deg column.
Click to expand...

I agree with you - sort of. It is still a feeder though. Look up the definition of feeder and branch circuit and you will see it cannot be a branch circuit, and since it is not service conductors it has to be a feeder.
 
kennneth1 said:
Hi Petersonra. Thanks for getting back to me so quickly. I was under the impression that most breakers are rated at 75 degrees C, therefore I was using the 75 degree table to size the conductors. As for the calculated load, I'm not exactly sure of this. The customer does intend on installing a 240, 40A electric vehicle charger in the garage. Thanks again for your help. l mainly work on high rise deck construction so,some of this residential work is foreign to me.

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A 100A panel does not equal a 100A load. For some perspective my whole house never has pulled more than 80A. That is with double electric oven and two electric dryers.

Sure there's a car charger but after that what have you got? A fridge maybe, some lights, cordless battery charger....prolly another eight amps of load.

There is no formula for garages or other outbuildings you just figure up what's there and apply some horse sense. If it were me I would pull 2-2-4-6 mobile home feeder and feed it with a 90A breaker. I got a guy with a 2400sq' shop that he uses for his hot rods fed the same way. Never had a problem.
 
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