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Old 08-08-2011, 03:02 AM   #1
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Running a subpanel to unfinished basement

Hi all, I have experience working with electrical, wiring up unfinished basements but this will be my first time installing a subpanel. My main breaker is located on the far side of my attached garage (which is sheetrocked) and I am planning on running the power to my unfinished basement that I will need before framing it up (~100 feet). To minimize wire cost, I have decided to run a 40A circuit (8/3 wire) to the basement into a subpanel with probably 3 or 4 15-20A circuits running off of that. Due to the fact that the garage is already sheetrocked I was thinking of using conduit to run from the main panel across the back corner of the garage and down into the basement both for looks and for wire protection. What I'm wondering is if there are any special requirements for this application. Will NM-B wire work for this? Would the conduit need to be filled? Could I use metal conduit or would I need plastic? Is there a better way to do this install? Any information you can provide would be very appreciated, including anything you can think of that I may run into during this project. BTW, this installation will be in Minnesota if that has any bearing on the code.


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Old 08-08-2011, 08:01 AM   #2
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I would use individual THHN (black, red, white, green) conductors (not NM-B cable) in PVC conduit. For 40 Amps, the ground wire can be downsized to #10, which will save a few bucks.

Every 360 degrees of turns in your conduit requires a pull box (eg. pull elbow). More won't hurt.

According to the fill calculator, you need 3/4" conduit minimum for this run. You may want to upsize to 1".

I'd probably upsize your wires too, just in case you need more power in the basement. Won't cost you much to upsize your black, red, and white to #6 (your ground can still be #10 for 60 amps). If you upsize, the same conduit size is required (3/4").

I'd use stranded (vs. solid) wire - much more flexible and easier to pull.

Your voltage drop in either the 40A (#8) or the 60A (#6) scenarios over 100 is about 2.5% - should not be an issue.

Conduit fill calculator:
Voltage drop calculator:



Last edited by secutanudu; 08-08-2011 at 08:12 AM.
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Old 08-08-2011, 10:28 AM   #3
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If you are going to finish the new basement how would the look of cable enter this conversation. The cable will be behind the wall. Fishing the cable down into the main panel is no easier than pushing a conduit with connector down into the knock out.

At the top of this page is a search function. Use it. This question is very popular and has been revisited hundreds of time. Keep in mind your new basement is an attached structure as it is under the same roof as the rest of the house.
This means you only need a main lug panel, not a breaker panel. You also need no ground rod. But all the other sub panel rules still must be observed. Please read the other posts. You will learn plenty.
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Old 08-08-2011, 11:44 AM   #4
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I think his aesthetic concerns were in the garage, not the basement.

He could run conduit to a j-box in the other corner of the garage, then transition to 6-3 or 8-3 NM-B down to his subpanel.

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conduit , sub-panel

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