Subpanel Wiring Questions

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09-07-2007, 11:07 PM
Jake Helmboldt

Subpanel Wiring Questions

I'm about to install a subpanel in the garage and had some questions (I realize some of this depends on local code, but in general):

1. In a garage (masonry w/ exposed walls, joists) does the wiring need to be in conduit, including that leading to the panel? If so, is PVC acceptable?
2. Is NM acceptable in conduit, or does it need to be outdoor cable?
3. If the sub has a 100A main breaker (to be used as the cutoff in the garage), can I use a 60A breaker in the main panel, thereby only needing to run 6/3 instead of the much more difficult run of 2/3, ande simply limiting my load center to the 60A?

thanks, Jake
09-08-2007, 12:01 AM
Alan Tolchinsky

Hi Jake,

I'll answer some of your questions. Note I am not an electrician but just did a subpanel and circuits in my garage and had it inspected.

You need conduit if you're doing it on the wall instead of inside the wall. Conduit protects the wiring and can act as a ground if it's EMT. EMT (electro metallic tubing) is what I used and is very cheap and easy to use.

No you shouldn't use regular wire (Romex) inside conduit. You need THHN wire inside conduit for heat issues concerning wire in conduit.

This type of installation is pretty straight forward and can be learned from books. Judging from your questions ( no offense here) you need to do more research on this and maybe think of letting a pro do it for you. If you have it inspected (highly recommended) ask your inspector how he wants it done. This will save you a lot of "redoing" of your work.

Good luck. Alan
09-08-2007, 12:02 AM
paul wiser

questions about your garage

how far from the main house panel is your garage? if it is a separate building you will have to run uf cable out to the garage. if your subpanel has a 100 amp breaker, you should use #2 to feed it. you would use a 100 breaker in the house panel .if you want to you can put a 70 amp breaker panel before your subpanel with a 60 amp breaker in it to shut off the power w/o running to the house each time.then you can put a 60 amp breaker in the house panel. if the garage is attached you can use 6/3 romex to feed the panel. you should use conduit for running the interior wiring on a masonry wall
09-08-2007, 12:13 AM
Jake Helmboldt

Total distance from main to sub is about 30', but w/ bends, etc probably 50' of conductors. It is considered attached, but will have to go underground from the basement to the garage via conduit. I've spoken with the local inspector and he has clarified this issue for me.

I have three 6 awg conductors (the existing, poorly run wiring from the previous owner) which is why I was considering using that (and putting it in conduit) for this rather than buying and wrestling with the #2. That is what led me to ask about using a 60A in the main w/ the 100A as a disconnect at the sub.
09-08-2007, 1:14 AM
Rick Christopherson

Quote:

Originally Posted by Jake Helmboldt

It is considered attached, ...

I have three 6 awg conductors ...

The first problem you are going to run into is that you must have 4 wires between the main panel and the subpanel. Because this is an attached structure, you must use the existing grounding system of the main load center, and that means you need to carry the ground wire from the house. At the subpanel, you can drive an additional ground rod of you wish, but it must still be tied back into the original ground rod of the house.

As for feeding out of the main panel with a 60 amp breaker, yes, you can do this. The 100 amp main breaker in the subpanel would serve as the system disconnect as you described. It does not need to match the wire gauge because the breaker at the main panel is performing this function.

Romex is a heck of a lot easier to deal with, so unless you are in Chicago, or some other odd city that prohibits its use, you can use romex. As was already stated, you can't use romex if it is surface mounted and exposed.

Oh, even if you did have to run new conductor to the garage, 60 amp would be just fine. The only time you would need more than that is if you were going to have welding equipment.
09-08-2007, 9:21 AM
Kent Fitzgerald

Quote:

Originally Posted by Jake Helmboldt

3. If the sub has a 100A main breaker (to be used as the cutoff in the garage), can I use a 60A breaker in the main panel, thereby only needing to run 6/3 instead of the much more difficult run of 2/3, ande simply limiting my load center to the 60A?

You could, but my preference would be to put a separate disconnect box in the garage and use a main lug panel (no main breaker) for the sub. This will allow you to work on a completely dead subpanel when you're adding branch circuits... plus it's pretty cool to have a big gray ON-OFF switch in your shop.
09-08-2007, 2:49 PM
Chris Friesen

Quote:

Originally Posted by Kent Fitzgerald

You could, but my preference would be to put a separate disconnect box in the garage and use a main lug panel (no main breaker) for the sub. This will allow you to work on a completely dead subpanel when you're adding branch circuits...

In my panel the main breaker section is separate from the branch circuit section...flip the main breaker in the subpanel and the branch circuit section is totally dead.

I'll give you the part about the big switch in the shop though...
09-08-2007, 9:40 PM
Jake Helmboldt

Quote:

Originally Posted by Rick Christopherson

The first problem you are going to run into is that you must have 4 wires between the main panel and the subpanel. Because this is an attached structure.

Rick, I'm glad you mentioned that. I was aware of that and had drafted a previous post that got an error and got lost when I tried to post it. One of my other questions had been, if I am able to use the existing 3 individual conductors 6 awg w/ 60A breaker, what gauge ground would I need to run with them?

thanks, Jake
09-09-2007, 12:27 AM
Michael Lutz

Jake,

You can use 6 awg grounding connector or a smaller one provided you do the calculation based on the NEC.

For the subpanel I put in a 100 A main disconnect box, mainly because it was less expensive than many of the subpannel boxes I saw. I connected the subpanel to a 70 Amp breaker from the house panel and ran #4 AWG wires. The panels sit right next to each other.

If the wiring is exposed, use conduit. If it is enclosed use, NM or romex cable. Provided your local code allows such use.

Mike
09-09-2007, 6:21 PM
Rick Christopherson

What I don't understand is why these postings get moved to the oblivion of the off-topic forum? We're talking about wiring up a workshop so you can use it, not about what you had for lunch! This information is just as pertinent to woodworking equipment as which tool to buy.

I am glad you got an answer, because if I wasn't bored today, I wouldn't have known you asked a follow-up question.