Electrical Question - Garage Subpanel

silver95taurus

silver95taurus

Forum Member
So, I'm adding a 100amp sub-panel in my garage to actually be able to use things in there without blowing the circuit every few minutes the compressor cycles. The 15amp that was existing wasn't done properly nor was it in conduit. I dug the 18" trench before everything started freezing, and I have 1.5" conduit. I already did the grounding rod in the ground the other day.

The question is, what wire do I need? I thought I needed 2-2-2 service wire, but that is incorrect. The inspector told be I need 4 separate wires going out there, but didn't have his book on hand to let me know what type. Trying to do as much this weekend as I can.

Thanks
 
I asked the guy at home depot aand he set me right up. I believe they have a licensed electrician on staff at all locations. If i get a chance ill go get the info off mine.
 
I'm not an electrician so I don't know if this changes anything but may be a good idea to make sure your wire is capable of running 220v. Just incase down the road you have a welder or compresssor that needs it.
 
What is the length of the run? That is needed to correctly size the wire for your amp load. And ill assume your running a 220v 100 amp panel. 110v 100 amp would be a waste to run.

Also you need two ground rods. One at the garage and one at the house. Can not just have the cold water supply ground when adding a subpanel.
 
Siegel1719 said:
What is the length of the run? That is needed to correctly size the wire for your amp load. And ill assume your running a 220v 100 amp panel. 110v 100 amp would be a waste to run.

Also you need two ground rods. One at the garage and one at the house. Can not just have the cold water supply ground when adding a subpanel.
Click to expand...

The run from my main box to the one in the garage will be roughly 60 feet of cable. The main thing I wanted is proper power for the garage. The PO had a 15amp indoor wire running 10" underground to the garage. not in conduit or anything. I'm just tired of constantly having to run back into the basement all the time.

In the garage I have 6 outlets, Garage door opener, 2 lights with 1 ballast each, 4 lights with 2 ballasts each. Then I will have a separate outlet for compressor and one for a small welder. I won't be doing a heavy duty welder, just one of those harbor freight units to play around with on small projects. Figure the subpanel would be the best way to wire to the garage. I believe I was just setting up for 110v 100amp. Since I was going to do a 100amp breaker in the panel.
 
LT1Pat said:
Why can't you use service wire? I would have thought 2-2-2 would have been fine.
Click to expand...

City inspector said its not meant to be buried? even tho I had it in conduit. 2-2-2 service wire is what the folks at Home Depot and Lowes both told me would work for the project. Its what I read on some sites too.
 
Service wire is not Wet rated so cant be buried. Wire must be thwn rated even in sealed conduit.

Wire sizes for 220v would be as follows
Copper
#4 for both hots
#4 for neutral
#8 for ground back to house panel

Aluminum
#2 for hots
#2 for neutral
#4 for ground

no idea how you could run a single 100amp 110v subpanel as im not an electrician and haven't heard of a 100amp 110v breaker to tie off your main panel. Also these sizes may need to be increased depending on the type of breakers. Some older breakers had some kind of heat rating that ment wire size had to be increased.

Also in your subpanel the neutrals and ground can NOT be combined like they can in the house panel. They must be separated and isolated.
 
Siegel1719 said:
Service wire is not Wet rated so cant be buried. Wire must be thwn rated even in sealed conduit.

Wire sizes for 220v would be as follows
Copper
#4 for both hots
#4 for neutral
#8 for ground back to house panel

Aluminum
#2 for hots
#2 for neutral
#4 for ground

no idea how you could run a single 100amp 110v subpanel as im not an electrician and haven't heard of a 100amp 110v breaker to tie off your main panel. Also these sizes may need to be increased depending on the type of breakers. Some older breakers had some kind of heat rating that ment wire size had to be increased.

Also in your subpanel the neutrals and ground can NOT be combined like they can in the house panel. They must be separated and isolated.
Click to expand...

I probably am doing a 220v setup then. I don't know too much about this, but was kinda forced into it with the addition of a winter motor swap project. I do know about the subpanel neutral and grounding screw thing.
 
I have a 200A main breaker main panel in the house, with a 100A subpanel feed to the garage, all in conduit - metal in the basement and transitioning to PVC underground (50' long, 18" deep). Both (2) hots and the neutral are 2 AWG copper THWN, and 4 AWG copper bare ground - 4 conductors total. You can go 3-3-3-4 copper if both breakers on each end are rated for 75* C connections. Or you could go 3-3-3-4 copper and just use 90A breaker. Although I think you will find that HD and Lowes carries 100A breakers, not 90A, and while they can order a 90A, it costs significantly more than a 100A. Same story with the 3 AWG vs. 2. The cost for 2 AWG @ HD is pretty much the same cost as 3 AWG, since 3AWG is seldom used. Therefore, short story, I would just use 2-2-2-4 copper and fuse it @ 100A and be done.

You could also go with Aluminum to save some $, although I prefer CU. You'd have to look up the ampacity tables but it's probably something like 1 AWG AL, although I would probably just pull 1/0 if you can fit it. SEC is not code for underground burial, even in conduit.

You need 2 ground rods at the house panel, and 1 at the garage subpanel. Water & gas pipes must be bonded (normally done @ the water heater), and obviously the cold water pipe must be bonded on both side of the meter by 1 continuous connection to the panel.

You need to terminate grounds separate from the neutrals in the sub-panel.
You need to properly deox any AL being used.
I much prefer Square D QO panels.

I'd also suggest thinking about any switch circuits you may want between the house & garage... & pulling any 12-3 or 14-3 that is needed. e.g. I have my outdoor & indoor garage lights on separate 3-way switch circuits (switch in the garage and house), and the furnace in my garage can be turned on/off by a switch in my house - god forbid I have to walk outside to a cold garage. :) But 1.5" is going to be somewhat limiting if you wanted to add switch circuits.
 
Last edited:
Is aluminum rated to run underground thru pvc? I just did this exact thing last weekend but my garage is attached to the house. I only had to go 20ft from my basement thru the bond into my garage but used mobile home wire 2-2-2-4 and it was only 1.49ft. Lowes sells a GE box that comes with the 100a main and 4 20a breakers for like $60. Just a heads up if you are trying to do it on a budget
 
I am wanting to put 100 amp 220v in my garage also. My house panel only has a 100 amp main breaker though. That panel is feeding the garage. What needs to be done?
 
greatwhiteLSC said:
I am wanting to put 100 amp 220v in my garage also. My house panel only has a 100 amp main breaker though. That panel is feeding the garage. What needs to be done?
Click to expand...

I would get the house service upgraded to 200 amp first before you run your sub panel.

--Joe
 
soap said:
I would get the house service upgraded to 200 amp first before you run your sub panel.

--Joe
Click to expand...

You don't need to do this, If you call for and upgrade to 200 amps you have to pay for the leads pole to house. (this is per several friends and several other i know who work for DTE)

You can upgrade the inside to 200 amps, size everything correctly etc.. If you think about it you will never see a condition where you pull over 100-120 amps unless you have everything on in the house and I mean everything, then go out in the garage and try to run a compressor or heavy load item... only then will you get close to being 150 amps. The lines to your current 100 amp panel are over rate by 25% (125 amp continuous capable) If you switch to 200 amps and start having problems (lights flickering in the house) You call DTE tell them your lights flicker in the house when using higher power items and they will come up and fix the issue (by upgrading you to a 200 map service for FREE)
 
2-2-2-4 direct burial at that distance. Save the conduit for communications cabling or a water line someday.
 
It's been a couple years since I upgraded my service, but there was no cost charged by DTE to me to upgrade the drop from 100A to 200A other than buying the larger outdoor meter can. (The 200A meter is 'free', but the metal box it goes in is larger for 200A vs. 100A services. There was also no charge for the new drop itself from the pole. I think that meter can was actually free too when I did mine, but DTE started charging for them w/n the past year or two. Regardless, i wouldn't worry about the DTE costs... should be 0 or very close to 0... especially compared to the materials cost & labor.)

That said, DTE will not upgrade your drop until you have an APPROVED final inspection (permit). So if you're going to DIY, which can be fine, just make sure you know the codes and the proper way to do things... otherwise, it may all be for naught. The city inspector is not going to teach you...

While you may debate the need for a 200A in some homes, often what is more needed that comes along with the new 200A panel is more breaker/circuit spaces. Many older 100A panels are 20 spaces or less... and were designed before people had as many computers, home theater, new kitchen wiring codes, tankless water heaters, the desire for a whole house surge suppressor, etc. In my small 1,100 sq foot home, I'm using 30 spaces in my main panel and another 10 in my sub panel.

Also, lastly, and while I am not a licensed electrician, I would double check with the city what else would need to be upgraded if you do upgrade the main service to the house. 2 ground rods, etc. are obvious and shouldn't be a big deal. However, current NEC code in many cities now require all bedroom receptacles to be AFCI protected in the panel, smokes on each level on the same branch circuit and signal wired together, etc. I *believe* you would not need to do any of this so long as you are only upgrading the main service and not touching any branch circuits, but it may be worth double checking. I would also consider adding in a whole house surge suppressor in the new main panel while you're at it.
 
You must log in or register to reply here.
Back
Top