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Old 11-18-2008, 09:41 PM   #1
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Hello all, this is the first time I've posted on this site. I've read a ton of posts and basically got my whole wiring plan down. I just want to make sure that I've got it right before I buy all the stuff and start on this project.

Ok, in my garage I'm going to be using a welder 110v (20 amp) and I'd like to upgrade to a 240v compressor soon that requires 40 amps. The welder is rarely used but the compressor will be used frequently. In addition to that, I will have 12 flourescent lights and about 11 outlets. The outlets will rarely be used as most of my work is with air tools.

My wiring plan consists of running #6 awg individual copper wires rated for wet conditions (2 hots, ground, and neutral) 70' to my garage in buried plastic conduit over 18" deep (I have 3/4 there now but realize that I will likely have to dig up and bury larger). My dwelling panel is 100 amp and I was going to use a 60 amp breaker that will be hooked up to a 100 amp sub panel in the garage. I'm then going to hook up a seperate 8' copper ground pole that's in the ground, but will keep it on a seperate gound bar from the neutral bar. There is a diagram that Stubbie had posted that I'll be following.

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My question is does this sound right? Also, based on my load requirements, should the 60 amps be enough? I know there have been many posts on this subject, but in doing this wiring I'll feel much more confident that it's done correctly by asking these questions instead of just assuming that I figured this out right. Any help would be greatly appreciated!

Thanks for you time, Andy.

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Old 11-19-2008, 10:57 AM   #2
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Yes, you are on the correct path. Just remember you must use at minimum a 60 amp main breaker panel or disconnect at the garage. The drawing above should be followed exactly.

I just noticed the sub panel in the above drawing is a lug panel. This is compliant if you have no more than six breaker handles in the sub. If you have more than six breakers or plan to add more later, use a main breaker panel.


Last edited by J. V.; 11-19-2008 at 11:01 AM.
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Old 11-19-2008, 11:33 AM   #3
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I wouldn't pull up the 3/4" conduit, use it for something low voltage. Maybe phone lines, garage door opener wires...
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Old 11-20-2008, 10:31 AM   #4
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I wouldn't pull up the 3/4" conduit, use it for something low voltage. Maybe phone lines, garage door opener wires...
4 #6 wires in a 3/4" conduit is acceptable. Not over filled. Use the conduit you have in place.

Reference table C (1)

Last edited by J. V.; 11-20-2008 at 10:34 AM.
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Old 11-20-2008, 11:06 AM   #5
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You are probably going to need a sub-panel rated 100 amps in order to provide enough spaces for your circuits so simplest thing is a 100 amp main breaker panel. In your situation I would just put a 60 amp breaker in the house panel and use the 100 amp breaker in your sub-panel as your required disconnect. Only drawback to this you need to install the panel immediately after you enter the garage with the feeder. So if that is not what you want install a stand alone disconnect there then run your wiring from it to wherever you want your panel which then can be an mlo panel. The disconnect must be located at the nearest point of entry of the feeder...very close anyway. If you install a disconnect be sure it is rated 60 amps or more. Usually they come 30 , 60 and 100 amp rated.

For a feeder protected with a 60 amp breaker you only need a #10 ground wire and I recommend that it to be insulated not bare. If your using conduit do not pull cable use individual thwn wires. The neutral will be required to have white insulation unless the inspector will allow you to use black and then tape it white at the termination points.

Your future compressor is rather large at 40 amps (when running under load) but that is not uncommon for a maintenance shop . Since your house is only 100 amps be sure your not taxing that with this sub-panel install. You don't want your house main tripping. It would be very feasible to have a 50 amp load with that compressor running and lights and maybe a power tool. I'm just saying a 40 amp air compressor is a big boy so load management might be a consideration. If house demand is minimal while your working in the shop/garage you could consider upsising your breaker for the feeder to 70 amps provided your using 75C Thwn and house panel is rated for 75C connections. It would have to be very old not to be. In general I like to know what the calculated load is for the house. If the wife isn't in there cooking for the holidays while your working in the garage or you don't have a hot tub I don't see much issue with your plan.

Remember also that your compressor will cycle 24/7 occassionally from air loss in the lines/hoses unless you turn it off after use.

Good luck

Last edited by Stubbie; 11-20-2008 at 11:18 AM.
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Old 11-20-2008, 12:44 PM   #6
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Originally Posted by Stubbie View Post
For a feeder protected with a 60 amp breaker you only need a #10 ground wire and I recommend that it to be insulated not bare. If your using conduit do not pull cable use individual thwn wires. The neutral will be required to have white insulation unless the inspector will allow you to use black and then tape it white at the termination points.
I thought in an underground installation, the ground wire is Required to be insulated when run through the conduit. I don't think you have an "option" here on the ground wire...

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Remember also that your compressor will cycle 24/7 occassionally from air loss in the lines/hoses unless you turn it off after use.

Good luck
Anyone found a solution to this? I am constanly having to make sure the air compressor is off when leaving the garage, otherwise it turns on when no one is using it and its a loud compressor.
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Old 11-20-2008, 01:00 PM   #7
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Anyone found a solution to this? I am constanly having to make sure the air compressor is off when leaving the garage, otherwise it turns on when no one is using it and its a loud compressor.
The solution is to find the leaks and stop them.
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Old 11-20-2008, 02:07 PM   #8
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I thought in an underground installation, the ground wire is Required to be insulated when run through the conduit. I don't think you have an "option" here on the ground wire...
It can't be bare aluminum

Think of this.. if I had an underground run of RMC (galvanized) listed for direct burial ran to an outbuilding and I only wanted to run 3 wires... but I am required to run an equipment ground by code. What would I be allowed to use for my equipment ground?
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Old 11-24-2008, 09:42 AM   #9
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Hey all, thanks for the tips. Yes, I am planning on running a sub panel in the garage rated at 100 amps. It's going to be right inside the door, which works out great b/c that where my feed from the house comes in. If I use a 70 amp breaker, would I have to use a heaver wire than 3 #6s (with a #10 ground, insulated b/c being in the ground, I could see bare wire corroding after time)? As for power usage, the compressor will be mostly run on the weekends and when I leave the garage I'll shut it off every time. Also, I do all the cooking so unless the wife is out in the garage sanding on a car, I should be fine there! The only thing I could see is that if the electric dryer is running at the same time as the compressor. However, if we make an effort to do laundry during the week, we should be fine.
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Old 11-24-2008, 09:55 AM   #10
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You need a #8 copper ground if you decide to use a 70 amp breaker.
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Old 10-15-2009, 09:07 PM   #11
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Ok, I finally am running power out to my sub panel in the garage. I noticed something when I wired the garage up and hooked up the current wiring to the subpanel. The main breaker in the sub panel has 2 wires that go into it. It seems like on my panel, one wire powers one side of the box and the other wire powers the other side of the box. Is this normal? If I'm wiring up my compressor, does enough power flow through one side of the panel where it will power my compressor with no problem? I thought that was the main reason why I had to run 2 hot #6s. A quick update, the compressor I ended up going with runs 23 amps. This is great that I have more power out there b/c I have a welder and also want to put some baseboard heaters in.
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Old 10-16-2009, 12:54 PM   #12
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The bus bars in the panel look after which phases are used in each breaker. They alternate down each side. Depending on which brand of panel you bought you have to make sure that your 240V breakers are in a spot that picks up 2 opposite phases. At my old house I wired up a dryer and was only getting 110V to it because of this, an electrician friend informed me of the error of my ways, I moved the breaker down one spot and everything worked fine.

If you buy a good panel (Cutler-Hammer) it is impossible to make this mistake.

I am actually doing exactly the same thing as you are. I just finished building my garage, ran #6 underground cable (black stuff) from my newly installed 200A panel (which feeds the original 100A house panel as a sub-panel) to the garage with a 60A breaker into a 100A entrance rated Cutler Hammer service panel in the garage. 30A for the Millermatic DVI2 welder and the rest for lighting and receptacles.

Rough in inspection went good, needed to ground the receptacle boxes and pig-tail to the receptacles, install 1x4 wood runners for the cable in the ceiling and pour concrete on the cable where it runs over bedrock and I can't get 18" of depth. Final inspection Monday.

It's funny some of the differences between the Canadian electrical code and the US.

-In Canada the ground for the sub-panel has to be carried back to the main panel and use the common ground with the main, this eliminates differential grounding if there is a ground fault with the either panel.

-Also it looks like your Neutral is bonded to ground in the sub-panel, up here that bond is removed in the sub-panel.

Last edited by Salem747; 10-16-2009 at 01:08 PM.
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Old 10-16-2009, 09:45 PM   #13
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I see now, that makes total sense. I have a GE box and yeah, with a double pole breaker pretty impossible to screw it up. Just didn't look quite close enough I guess. Thanks for the help, I really appreciate it!
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Old 10-16-2009, 11:42 PM   #14
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-In Canada the ground for the sub-panel has to be carried back to the main panel and use the common ground with the main, this eliminates differential grounding if there is a ground fault with the either panel.

-Also it looks like your Neutral is bonded to ground in the sub-panel, up here that bond is removed in the sub-panel.

Unforetally no it is not legit with four conductor feeder but if it was three conductor feeder for subpanel yeah it is legit but most states are allready adopt the most recent code now and many local area is getting strict with subpanel regulations now.

{ In France we don't have three conductor subpanel feed like that it have to be either four or five conductor set up depending if use single or tri phase }


Merci.

Marc
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Old 10-19-2009, 02:32 PM   #15
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Inspector was by today and gave me final approval, just have to backfill my trench and attach cover plates once the interior walls are in. One more step complete.

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