What breaker size....???

[langkg]

Greetings. Brand new member here (so be nice)

I am wiring my new garage. I've run 2-2-2-4 aluminum USE-1 through 2" conduit between house and garage. Total wire run is about 80' max.

I have a Siemens 100 amp panel in the garage where I'll run my branch circuits from for lighting / outlets.

I'm about to do my connections on the main panel in the basement of the house and not sure what size breaker I should use. Ampacity for my wire is 100 amps the best I can tell looking at various tables on the internet. Do I need to derate my breaker on the main panel to 80% of my wire ampacity meaning I use an 80 amp max?

I had the same set up in my last barn but I put a 60 amp in the basement. I ran 2-2-2-4 aluminum there as well and ran lights, arc welder, compressor, etc and never had a problem with the main breaker and that was probably 3 times the length of wire between house and barn.

Frankly I'll probably never come close to using the total capacity but wanted to size the breaker safely, to code but not be too conservative.

any thoughts are welcome. I have a 100 amp breaker ready to go but will buy a smaller one and take this one back if necessary. I had an electrician tell me that 100 amp would be OK but wanted another opinion.

 

[LXCam]

Your fine with the hundred you have. Derating comes into play for multiple circuits in the same conduit and voltage drop and termination temperature rating. No worries.

 

[jrsulo]

Do believe its 90 amp

 

[langkg]

Thanks. One point of clarification. The wire is USE-2 NOT USE-1. It appears that in the 90 degree C column that the ampacity of the wire would be 100 amps if I'm reading things correctly. The same table lists USE wire at 90 amps in a 75 degree C column so maybe that's where I got two different replies but the wire is confirmed as USE-2

My guesss is that the changes of me loading it up to that point is next to nil now or in the future but I just like to do things right up front.

thank you again.

 

[Aceman]

You need a 90 amp breaker. Make sure your wire is also RHH RHW, etc rated if it enters the home or garage. If all of it is outside the buildings then don't worry about it.

 

[langkg]

I think I'll go with Aceman. I looked at the exact manufacturer 'Priority Wire' and they list the ampacity at 95 amps. It also is RHH / RHW rated whatever that means (other than it's OK to be used indoors I guess. When I bought the wire from a electrical supply shop I specifically asked to make sure it would be OK for use inside the house/barn and underground through the conduit. I guess I'll replace that 100 amp breaker with a 90 and sleep soundly.

http://www.prioritywire.com/specs/Aluminum RHH-RHW-2-USE-2.pdf

Thank you all so much for taking the time to weigh in on this.

 

[Charles (in GA)]

Many electricians misread the code. The code allows a 100 amp breaker with that wire provided it is the main service wire for a residence. If you are using it as a feeder to an outbuilding or a feeder to a subpanel that doesn't supply the entire residence, you are limited to 90 amp.

Forget the 90°C wire ratings. To use that column you would need to have breakers and terminals at both ends that were also 90°C rated, which they don't make. All breakers and terminals in common use today are 60°C/75°C rated and you have to use the 75° column for determining the ampacity of the wire.

 

[pattenp]

The rhh/rhw rating means the wire insulation is fire resistant, so it can be used inside.

 

[Falcon67]

I use a 70A on my MHF run. Used that because it was on the shelf at HD. Lots of stuff, no trips.