For a 2 bedroom, 1 bath addition, a contractor wants to add a 100A subpanel to my garage that we can then branch to feed the two bedrooms and 1 bathroom. The house has an existing 200A main that we will tap into. The subpanel will be about 45-50 ft away from the 200A panel. This distance is about 12 feet underground in 1 ” plastic conduit and the rest in 3/4 ” aluminum flex.
Questions:
For a 2 bed and 1 bath (shower only, no jazuzzi tub) addition, would you put a 100A or 50A subpanel.
What size wire would you use to feed the subpanel ?
Can you give me the website address for the wire calc that you use.
thanks
Replies
A pure load calc would say you only need 2 or 3 circuits. A 20a 120v for the bathroom and 3va per square foot for the bedrooms.
One 20a circuit would cover 800 square feet meeting minimum code. A simple 2/4 slot panel fed at 20 or 30a /240v might be plenty ... unless you are thinking about adding more loads. Bear in mind you need GFCI for the bathroom and AFCI for the bedroom. That means 2 or maybe 3 full size breakers
If you go 100a you need three #2s or #3s if you can find it (table 310.16) and a #8 for the ground (250.122)
That will fit in a 1", just barely, if you use #3s. If you have to use #2s you need 1 1/4 pipe. I would not try to pull either of them through anything smaller than 1 1/2"
Working backward from the 3/4" FMC you will be limited to three #6s and a #10 ground which gets you a 60a panel, probably plenty for what you say. Getting wire in FMC is hard if you don't have a fish tape and pull it in. Use plenty of lube.
50A is three #8s and a 10, easier to pull. This will all be based on the size of the breaker you feed the outbound circuit with. You could still use a 100a panel at the far end but it would be limited to the size of the feeder.
30a would be 4 #10s.
That is from the NEC. You can see it at
http://www.nfpa.org/codes-and-standards/document-information-pages?mode=code&code=70&DocNum=70
.
Greg
thanks a lot for the info and link. Do you know where in NFPA. 70 it says what the wire size limits are for 3/4 FMC? I'm looking in 2011 version.
Kirk
Table C.3 in annex C in the back is for Flexible Metal Conduit (Greenfield)
You will be looking at the rows for THHN/THWN if you use the standard wire for things like this.
It says you can get a max of 4 #6 in a 3/4". The fact that one can be a #10 does not change the answer.
If you want to do the math you can use the dimensions in the tables of chapter 9 and you have to stay at or below 40% fill. I have an electrical calculator program that does all of this.
You wrote that a contractor wants to install this 100 amp panel, yet you are doing a lot of research on wire size, need, etc. This tells me that you don't trust this contractor... perhaps he's trying to pad the work required and his bottom line. So, in my mind, you are doing right by your research. Now, get another contractor.
I'm not an electrician but have installed conduit in more than a few places for my own work. Even I know not to try to pull large wires through small pipe. Conduit is cheap, labor is not.
Reeeeelllaaaaaaaxxxxxxx. I'm simply trying to understand what he is doing.