Another sub-panel in garage question...

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  #1  
Old 04-17-14, 09:21 AM
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Another sub-panel in garage question...

I want to put a 100A sub-panel out in the garage approx 50 wire-feet away from my service panel.

200A service panel; I used the "Mike Holt" spreadsheet for my house (without sub-panel) and it says I have 125A of demand right now.

100A sub-panel will source:
  • 7.5A 230V air compressor 31A motor plate, on a 50A breaker
  • Small welder 115V on a 20A breaker
  • 6-8 general 115V outlets
  • 10 flourscent light fixtures with 3 tubes each
I know the 100A panel is overkill but wanted to leave room for future.

Does this plan make sense:
  • #3/4 copper from the service panel to the sub-panel
  • #6/3 copper from the sub-panel 50A breaker, to the compressor
  • local disconnect switch at compressor, since it's in a different room, underneath the garage
Questions:

1. Someone told me I need a dedicated earth ground at the sub-panel. Is that true, and does it mean I simply run #6 bare stranded copper from the sub-panel to a copper rod into the earth, outside the house?

2. The service panel will have a 100A breaker in it, to source the sub-panel. Do I also need a "main breaker" in the sub-panel? It seems odd to me if not, but it also seems odd to have another 100A breaker at the sub-panel when there is already one in the service panel.

3. For the 6-8 general outlets, I want them to be able to run fairly significant loads - bench grinder, maybe a 2HP tablesaw, heat gun, etc. I'm not really sure yet what would be expected of them. Do I put each on a dedicated 20A breaker? Or put them in pairs on a 20A breaker? Not sure how to determine this.

Maybe the 100A sub is not overkill, after all? I really want to avoid having to put a separate service entrance to the garage.
 
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Old 04-17-14, 10:05 AM
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Someone told me I need a dedicated earth ground at the sub-panel. Is that true, and does it mean I simply run #6 bare stranded copper from the sub-panel to a copper rod into the earth, outside the house?
Yes, a separate ground rod for a detached structure.
The service panel will have a 100A breaker in it, to source the sub-panel. Do I also need a "main breaker" in the sub-panel?
Code requires a disconnect if you have space in the panel for more than six circuits. That disconnect must be rated 100 amps or more in your case because yiu have a 100a feed. The easiest cheapest way is to use a main breaker panel. The main breaker in the subpanel is used solely as a disconnect switch not a breaker.
For the 6-8 general outlets, I want them to be able to run fairly significant loads - bench grinder, maybe a 2HP tablesaw, heat gun, etc. I'm not really sure yet what would be expected of them. Do I put each on a dedicated 20A breaker? Or put them in pairs on a 20A breaker?
As rule of thumb I'd suggest only devices that draw more then 12 amps on a dedicated circuit. Except lights the rest can share. Lights should be on their own circuit so you aren't in the dark if something trips a breaker.
 
  #3  
Old 04-17-14, 10:29 AM
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Thanks Ray - the garage is attached to the house, but if it's safer to run a dedicated ground from the new sub-panel I will do that; it won't be too much extra cost.

The fellow at my local electrical supply company said he never sells #3-3wg copper (doesn't even stock it) -- and suggested I use #2 aluminum instead.

Right - lights on their own circuit; I will make sure of that.

On the outlets, maybe I will make a couple of them on their own 20A circuit and the rest I will put on a single 20A.

Thanks again!
 
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Old 04-17-14, 10:36 AM
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the garage is attached to the house.
Than no ground rod or GEC is needed, just an EGC wire. No disconnect either so no main breaker panel. Just a main lug only.
said he never sells #3-3wg copper (doesn't even stock it) -- and suggested I use #2 aluminum instead
Only if both sets of terminals are rated for 90. Better to use #1 (individual conductors in conduit) or 1/0 cable. but I'll let the pros weigh in on that.
 
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Old 04-17-14, 10:42 AM
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If the garage is attached to the house you do not need a ground rod, nor does the 100 amp panel need a main breaker. A main lug only (MLO) is fine and will likely save you some money.

Any cable is limited to the 60 degree column. You can use aluminum cable, however #2 will "only" get you 80 amps. This would likely be fine for you but if you want the full 100 amps you will need to step up to 1/0 aluminum. (BTW - #3 copper cable will only get you 90 amps) If you run conduit and run THHN or XHHW you can do #3 THHN or #1 XHHW for 100 amps.

Just for a note: When talking cable like SER or NM-b it is 3/3 or 6/3 w/Ground. Unless your compressor needs a neutral (most do not) you would only need 6/2 w/Ground.
 
  #6  
Old 04-17-14, 11:38 AM
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OK, I understand. 1/0 AL for 100A.

I can't really do conduit since I want to pass this wire through the floor joists. I'll have to go over to the supply house and buy a short length to see how easily I can manipulate 1/0 ...afraid it might be too tough to get into my service panel...

Compressor does not need neutral.
 
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Old 04-17-14, 12:18 PM
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IF you can't do conduit then it will need to be a cable. Do not just use individual 1/0 conductors. I had a guy do that and wanted me to hook it up. So you would need a cable 1/0-1/0-1/0-2 SER. According to my supplier it is about 1 1/8" diameter cable.

Just for thought:
1-1-1-3 is good for 90 amps and is 1.08"
2-2-2-4 is good for 80 amps and is .96"
 
  #8  
Old 04-17-14, 12:42 PM
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Yessir, no conduit so it will be a cable, I totally understand.

I'm going to need to actually look at these cables and bring a 4-5' length home and try myself to see how hard it will be to manipulate into my service panel.

Another option is I simply run the compressor back to the service panel on its own 50A breaker, and then do a 50A sub-panel MLO out in the garage, fed from a separate 50A breaker.

Then I can do all of this on 6/2-wg copper (for the compressor) and 6/3-wg (for the sub-panel).

Would that be ok?
 
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Old 04-17-14, 12:52 PM
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Yes. So long as it is an attached garage you don't even need a subpanel. You can run individual cables or a combination of individual and subpanel as you asked about.
 
  #10  
Old 04-17-14, 12:58 PM
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ok great - yeah I would like a subpanel out there only because I may need to add circuits in the future and the extra panel would give me some flexibility.

I'd also like to put wiring "races" on the walls in the garage, for the outlets. I'm not sure what these things are called but we had them in a lab I worked in years ago -- they were long grey metal boxes on the wall and you could move outlets around if you needed to. If anyone knows what they are called or where to find them, I'd like to find out...thx.
 
  #11  
Old 04-18-14, 12:01 AM
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I'd also like to put wiring "races" on the walls in the garage, for the outlets. I'm not sure what these things are called but we had them in a lab I worked in years ago -- they were long grey metal boxes on the wall and you could move outlets around if you needed to. If anyone knows what they are called or where to find them, I'd like to find out...thx.
Sounds like you are asking about bus duct. Once you discover the price you will decide that you don't need it.
 
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Old 04-18-14, 03:26 AM
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You could install GFI protected plugmold from Wiremold. It has receptacles built-in and is like a plug strip.
 
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