# Calculating wire type/size for subpanel



## Mellow (Jul 14, 2018)

I need to buy wire to feed a 100 amp subpanel in a detached garage, approx 100 feet away from the main panel in the (separate) house. Between the subpanel and the room with the main panel is 2" PVC run underground and stubbed up. 



Based on tables 310.15(B)(16) and 310.15(B)(7) I've surmised that #2 aluminum or #3 copper would be sufficient conductor size... Is this correct?


Also, because the wire is underground in PVC conduit for approx 20 feet, does this mean the whole run should be done in something like USE cable? Or is there a less expensive code compliant alternative?



I was looking at this: https://www.homedepot.com/p/Southwi...ded-AL-Quad-****-URD-Cable-55417399/205001803


I'm thinking that the above 2-2-2-4 USE wire would suffice all of the NEC requirements for this application but I just wanted to make sure it's not overkill before I pull the trigger. 



Thanks as always :smile:


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## CoolWill (Jan 5, 2019)

Sounds good if voltage drop isn't a factor.


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## HertzHound (Jan 22, 2019)

I didn't think type USE was allowed to be used indoors. The URD in your link technically isn't even in the code book, and is not allowed indoors.


I would look into XHHW if your going to go with aluminum. #2 aluminum is good for 90 amps at 75c. Since a 90 amp breaker is listed in 240.6(A), you really can't go next size higher.


Three #1 and a #8 XHHW should fit swimmingly in a 2" PVC


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## CoolWill (Jan 5, 2019)

HertzHound said:


> I didn't think type USE was allowed to be used indoors. The URD in your link technically isn't even in the code book, and is not allowed indoors.
> 
> 
> I would look into XHHW if your going to go with aluminum. #2 aluminum is good for 90 amps at 75c. Since a 90 amp breaker is listed in 240.6(A), you really can't go next size higher.
> ...


It's XLPE USE-2.


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## JoeSparky (Mar 25, 2010)

Last detached garage I did with 2-2-4-6 mobile home feeder - USE cable. 1-1/4" PVC the whole way panel to panel and underground. It was probably a wash over using SER in the basement (less labor) and splicing to the underground conductors (more labor and materials $$$)


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## VELOCI3 (Aug 15, 2019)

Copper. Detached building needs a ground rod with #6 too. 


Sent from my iPhone using Tapatalk


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## HertzHound (Jan 22, 2019)

The internet is loaded with information on whether USE can be used indoors. The NEC says under "uses not permitted" - Indoors. 



To each their own I guess. If there is some other marking on it I guess that makes it OK? If an inspector sees USE on it though, then what? Replace it? I took his post as a 100' run, and only 20' of it underground. So where is the other 80'?


https://www.ecmag.com/section/codes-standards/use-se


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## JoeSparky (Mar 25, 2010)

Quite a bit of the USE cable sold these days is also rated as RHH / RHW / RHW-2 or XHHW. All of which are permitted to be used indoors. The mobile home cable I referenced earlier in this thread was listed this way.


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## Dennis Alwon (May 9, 2009)

2 aluminum is only good for 90 amps not 100


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## JoeSparky (Mar 25, 2010)

Dennis Alwon said:


> 2 aluminum is only good for 90 amps not 100


The code is dumb. I can put a 100a service on a house with #2 AL. If I put a 100a sub in the same house, I have to use #1 AL?
:stupid:


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## CoolWill (Jan 5, 2019)

JoeSparky said:


> The code is dumb. I can put a 100a service on a house with #2 AL. If I put a 100a sub in the same house, I have to use #1 AL?
> :stupid:


But... Code! And... Arbitrary rules!


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