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benjeremiah 01-17-2013 06:49 PM

correct wiring to subpanel in garage

I am working on connecting a subpanel in a new garage building... I am not going to be required to do an inspection of the electrical for this building (my township screwed up I suppose, I have a bldg. less than 1000 sq. ft... 988 to be precise, and they only issued a zoning permit which doesn't require separate electric and plumbing inspections). REGARDLESS of an inspection, I want to know that all my connections are positively correct before I switch on the new service to the garage... I would have an electrician who is part of my wife's family come over to take a look at my work, unfortunately he lives hours away.

In order to pave the way for my new outbuilding project, I did an upgrade to my home two years ago which included putting in a new main panel & meter, and a larger service wire to the outside line (made the line connections to it myself)... it was great to have a nice new 200 amp service panel to replace the crappy 60 amp fuse system that was here when I moved in. On that project, I learned a lot with the help of a wiring book by the author REX CAWLEY (Taunton press) and a few electricians I consulted with, and yes I had an inspection for that one and it passed on the first go round... unfortunately I had to return the book to it's owner after that here are my specific Q's:

1/ I have a square D QO 100 amp panel in the new garage... feed wire (4 AWG- 3 conductors and ground) is in an underground conduit, I am using a 100 amp breaker in my main panel at the house. I would like to confirm that the ground and neutral buses are separate in the subpanel? and the way to do that with the QO panel is you have to remove the ground bus from the bar it's attached to (next to the neutral bus) and attach it to the inside of the panel box?

2/ basically I want to confirm that I am grounding everything correctly: the subpanel feed ground will go to the ground bus (actually there are two of these, one has outlets for larger wires), and this will also be where the conductor for the 2 ground rods outside the garage will tie in? thereby all grounds will be connected to each other, bonded to the panel case and also back to the ground at the main panel in the house thru the feed wire ground?

3/ the neutral bus in the subpanel is to be isolated from the ground, which means that you do not install the green screw that would normally be put in to bond the neutral to the panel case?

I appreciate any and all feedback on the grounding Q's.

papachuck 01-17-2013 07:05 PM

Ground the pannel
you need to use A ground bar connected to the panel this is were you connect the outside ground wires. the neutral bar is where you hook your sub feed ground.

rrolleston 01-17-2013 07:05 PM

A quick question. Is that 4 gauge copper or aluminum?

And pictures would also really help us guide you so we can look at what is going on?

rrolleston 01-17-2013 07:07 PM

You are adding two ground rods to the sub panel that are separate from the main panel correct?

rrolleston 01-17-2013 07:14 PM

Also what type of wire are you using. Because 4 gauge can vary depending on the insulation type. And whether it's AL or CU

k_buz 01-17-2013 07:15 PM

#4 CU is not rated for 100A in this situation. You would either need to put the #4 on a 90A breaker, or install #3 CU.

papachuck 01-17-2013 07:20 PM


Originally Posted by k_buz (Post 1095977)
#4 CU is not rated for 100A in this situation. You would either need to put the #4 on a 90A breaker, or install #3 CU.

You are sure right there.

rrolleston 01-17-2013 07:21 PM


Originally Posted by k_buz (Post 1095977)
#4 CU is not rated for 100A in this situation. You would either need to put the #4 on a 90A breaker, or install #3 CU.

He must have been looking at the 90 C column when he selected the breaker but don't think that can be used here.

That's if he is using CU

Jim Port 01-17-2013 07:22 PM

You do not remove any of the neutral bars. You buy an auxilary ground bar and add it where the raised bumps in the back of the panel are.

benjeremiah 01-17-2013 07:26 PM


to answer your inquiries, RROLLESTON... I am using a 4 awg aluminum feeder, and using #6 bare copper to connect the two outside ground rods. and yes, the 2 copper ground rods referred to are going in outside the garage, I am using the same setup (8-10 ft. apart, connected with a continuous wire) as I have over at the house. I'll try to get a photo to put up... basically I wanted to know where the ground and neutral wires go in the new subpanel... neutral is on the neutral bus (there is a connector in place), and the ground? I thought the ground from the feeder should go to the ground bus, but perhaps I was wrong on this? thanks for the help... Ben

rrolleston 01-17-2013 07:30 PM

You will have to add a ground bar and then bring down the breaker in your main panel down to 70 amps for that size and type of wire.

Grounds go on the new ground bar you add and the neutrals go on the existing neutral bar.

benjeremiah 01-17-2013 07:58 PM

actually the feed wire is...
Sorry I was mistaken...

The feed wire I am using is 2 awg aluminum. it reads on the casing: SER TYPE XHHW2, and 3 CDR AWG 2 - 1 CDR AWG 4... it has been a while since I went to buy all the materials for the electrical installation, guess I had been thinking that it said "4 awg" on it somewhere but that's just the ground part. I guess this wire has a #4 ground and the hots and neutral are 2's. I hope that clears up the wire issue and negates your recommendation to lower the breaker to 70 amps, this is actually a woodworking shop and I'll need the 100 amps if possible. and I understand what the QO panel needs to be a subpanel, thanks to someone's response... I need to purchase a separate ground bus... just thought like other panels there would be a way to convert without buying something extra. I was trying to remove the buses needed for neutral (there are 4 small ones stacked next to each other) but instead I'll go out and buy the necessary separate ground bus to install in the box... thanks all. YOUr suggestions are appreciated.

benjeremiah 01-17-2013 08:01 PM

and a thank you to...
Thanks for the suggestion, JIM PORT. that's just what I needed to know... thank you.

rrolleston 01-17-2013 08:04 PM

SER is not rated for underground use. And the 2 gauge AL would have to be on a 90 amp breaker.

papachuck 01-17-2013 08:17 PM

slow down.

Originally Posted by papachuck (Post 1095968)
you need to use A ground bar connected to the panel this is were you connect the outside ground wires. the neutral bar is where you hook your sub feed ground.

I am assuming the Main panel has 2 ground rods and A #6 cu wire. the sub panel buss bar grounds are for service hook ups. that is why you prob have 2 big holes on each side of the panel if you use the correct wire size it should take up much of the hole. the small holes is for your circuits. if you don't run the correct 4 wire not size #of wires 2 for load 2 for ground wires you will have to use ground rod's in sub panel without bonding screw and hook that to A bare grounding bar to your panel box there is holes drilled in the panel for may have to purchase .

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