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Electrical guru's - How does this sound for my garage electrical overview

Doug427

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Jan 24, 2015
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60
Location
Houston
Just an overall view - I'm going to add a subpanel in my garage. Some call it attached, the town calls it detached. Either way, a general description to me is that it is a detached garage connected to the house by a covered but open walkway, if that makes any sense. The main panel for the house is in the garage. It's a 150 amp panel that is out of space. It has a small subpanel on it already that is 60 amp and also out of space.

I was thinking about just replacing the smaller subpanel, but the stove and oven is already on it, and I would rather have that separate from the garage panel. With my wife cooking with the high draw oven and stove, my compressor running in the garage, and perhaps some other load in the garage, I can see that starting to load up one subpanel.

Because of that, I'm going to add another 100 amp Square D panel immediately next to the main panel. I'll use a 100 amp breaker in the main panel box, using three lines of #1 THHN cable and a ground running through a 1 1/2" metal conduit into the subpanel hooked up thorough the 100 amp main breaker that is already installed in the new subpanel. I'll take out two breakers in the main box to make room, and put those two light duty household lighting circuits into the new subpanel.

There will be various lighting and electrical out let circuits in the garage, and I'll use 12/2 romex for all of them. As for the "special" electrical needs in the garage, here's what I plan:

A 18,000 BTU through the wall Air Conditioner for those brutally hot Houston summers. I can do a through the wall unit A/C install myself, where as a split system requires a pro install and is more pricey, neither of which sounds good to me. It'll only be used when I am out there working, so the increased efficiency means very little. I'll run that on 10/3 romex off of a 30 amp breaker in the new subpanel. This will be about a 40 foot run.

A 5HP 60 gallon compressor on it's own line. I'll use 8/3 line off of a 40 amp breaker. This will be about a 30 foot run.

A just in case, 220v outlet. To be used for whatever may pop up. It may well never get used, but better to tun one and have it if needed before the insulation and sheetrock go in. I'm thinking about another 8/3 line with a 40 amp breaker again. It'll be about a 35 foot run. What do you guys think, should I go bigger? Is there any equipment you can think of that is common in a car tinkerers garage that might need bigger? I'm not much of a welder, so I am not familiar with what a typical welding unit would need.

Anyone have any concerns or ideas about any of this?
 
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wyliesdiesels

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Aug 14, 2012
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Modesto, CA
First off, the 2 structures are connected and the branch circuits run between them. So its wired as an attached garage.

Second u only need #3 THHN cu for 100a. Use a #8 for the EGC or maybe u were planning on AL which would be #1 and #6 for the egc...

And third, why not remove the existing 60a subpanel and move those circuits into the new subpanel to make things less complicated?

Have u done load calcs to tell if youre really maxed on that panel?
 
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Doug427

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Jan 24, 2015
Messages
60
Location
Houston
I'll make the change you suggest to #3 THHN. I saw it usually uses #3 but figured going to #1 would be overkill in the direction of safety. I'll switch the plan to #3, I guess save a few dollars where I can. #8 ground is what I had planned. I forgot to mention that.

I've not done any load calcs. Just looking at margin of load, the stove and oven are on a 40 amp breaker. Assuming they're both on, I just guesstimated at a 40 amp load max. That possibly, worse case, leaves me with only 60 available in the subpanel. Assuming I have lets say a welder going on the extra outlet, the compressor running and all the lights on, etc, an available 60 amps is possibly getting a bit thin. I know it's a longshot, but since all these three panels will be within about 4 feet of each other on the same wall, the complexity of leaving the small one in place and adding a third shouldn't be too bad. Other than a bit of extra work, is there a downside to adding a third?
 

JoelR

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Dec 5, 2014
Messages
19
It might not hurt to get an ammeter on the feed wires for your main box to make sure its as balanced as possible. With so much high amperage stuff in that sub panel, i would guess your main box is pretty unbalanced
 
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wyliesdiesels

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Aug 14, 2012
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Modesto, CA
It might not hurt to get an ammeter on the feed wires for your main box to make sure its as balanced as possible. With so much high amperage stuff in that sub panel, i would guess your main box is pretty unbalanced

It sounds like the majority of his loads are 240v...
 
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Doug427

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Jan 24, 2015
Messages
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Location
Houston
Gents, I apologize for my lack of in depth electrical knowledge, but I have to admit that I have no idea what it means to check my electrical box for balance, nor how that is done.
 

zmaxmotorsports

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Jan 11, 2013
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South of omaha
Gents, I apologize for my lack of in depth electrical knowledge, but I have to admit that I have no idea what it means to check my electrical box for balance, nor how that is done.

Just make sure to alternate as youre installing breakers in your sub from side to side starting with the 2p's at the top and work your way down to make sure youre not loading 1 leg more than the other on the service.
 

rsnip988

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Apr 2, 2015
Messages
143
Location
Elon NC
Pictures of your situation are always useful, it helps when we can see what it looks like to give advice! ;o)

12/2 for all of the 20A outlets sound good, but may be a little overkill for lighting (IMO) I just used 14/2,saved some money and am WAY under on Amperage usage...

Instead of spending money on 4- 10/3 & 8/3 cables and running them 35-40ft, would it possibly be easier/cheaper to run your service feed to a new subpanel mounted in the garage? then short runs to the equipment locations? I'm thinking that having 3 heavy load panels in a 4 ft area would create a lot of heat... (but that's just my thinking and I am often wrong)

My Lincoln "Buzzbox" requires a 50A breaker for what that's worth on the 220V outlet question.

A lot of "car tinkerers" on here and a lot of them have gotten 2/4 post lifts for tinkering... I'm getting one as soon as i finish saving up for it. may want to check them out and place a dedicated outlet handy to where you might want one!
 
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