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Old 03-18-2008, 11:23 AM   #1
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Wire Size and Conduit Fill


Good morning

I want to run a sub panel in my garage to power my welder and compressor and some misc convenience type circuits for my wood working tools.

My main panel (225 amp) is on the outside wall of the garage and the builder installed a sub panel on the inside of the garage, a few feet away. This panel is protected by a 90 amp breaker in the main panel and has breakers for most of the smaller circuits in the house.

I have already purchased a new 100 amp main breaker panel from Lowes. I have been told that I could feed this panel, with two #4's and two #6's in THHN copper - the total length of the run is 28' and I could protect it with a 100 amp breaker in the sub panel I am feeding it from.

Is this correct and what would be the minimum size of conduit that I would need. I would prefer to use the blue colored flexible ENT conduit or the gray pvc as my second choice.

Thanks for your help


Last edited by It's Me; 03-18-2008 at 11:26 AM.
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Old 03-18-2008, 01:18 PM   #2
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Wire Size and Conduit Fill


This will take a while to get you on the right road and cover all the bases so be patient. Questions first.

1.) What is the maximum branch breaker or per bus stab rating of your sub-panel. Should say on the door cover sheet?
2.) Are the walls open stud or finished drywall in other words what I'm getting at is are you surface mounting on the outside of drywall or is the feeder going to be inside the wall cavities thru drilled holes?

You were told wrong about the wire sizes. You may be able to use cable if you want just depends on how you answer 2 above. We eventually will get to all the other stuff.

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Old 03-18-2008, 02:11 PM   #3
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Wire Size and Conduit Fill


Thanks for the help Stubbie

Sub panel #1 (already installed by Builder)
Cutler Hammer
125 AMPS MAX
"Max Sum of Breakers Ratings per STAB:155A"

Sub Panel #2 (my new panel )
GE
"Combined marked rating of circuit breakers on any
one stab not to exceed 220A"

Walls will be drywalled when finished, currently open. Most of the run will be in an attic crawlspace. I just need to go up from sub panel #1, over in the crawl space and then down into sub panel #2.
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Old 03-18-2008, 04:57 PM   #4
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Wire Size and Conduit Fill


Excellent. You can install a 100 amp double pole breaker breaker in your sub-panel. Just be sure the one opposing it never exceeds 50 amps. So don't have a 60 amp double pole across from your 100 amp.
The 125 amp sub-panel should have grounds and neutrals separated on different bars with 4 wires coming from your main 225 amp panel (H-H-N- Grd). There should be no bonding jumper installed in the neutral bar.

Install a 100 amp branch breaker in sub-panel #1

You can use cable if you want you will need a SER aluminum 1-1-1 with ground. Or you can use a copper SER 3-3-3 with ground.

If running conduit you need thhn/thwn #3 with a #8 ground (4 Wires)

Forget the ENT use pvc or emt.

You will need 1 1/4 PVC or EMT

Use this diagram as a guide. Your work should be inspected by the local codes department.
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Wire Size and Conduit Fill-sub-panel-sub-panel.jpg  

Last edited by Stubbie; 03-18-2008 at 05:09 PM.
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Old 03-18-2008, 07:20 PM   #5
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Wire Size and Conduit Fill


WOW thats great Stubbie - I really appreciate your help.

One last question. I never really wanted the sub panel I am installing to be 100 amps. I have a 240v compressor that will require a 30 amp circuit, a 240v welder that will take a 20 amp circuit and then I planned on installing a couple of 20 amp circuits for convenience to power my wood working tools (table saw, joiner, planer, band saw, etc). It will only be me working in my garage. I was originally thinking of a 70 amp feed but was told a 100 amp breaker would be easier/cheaper to find... I have since located a source for a 70 amp breaker and it is the same price as the 100 amp...

If I just wanted my new sub panel to be 70 amps, could I use three #4 copper THHN and one #6 ground inside of 1" pvc? (Of course I would have a 70 amp breaker in sub panel #1)
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Old 03-18-2008, 07:37 PM   #6
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Wire Size and Conduit Fill


You actually only need a breaker in the 1st sub-panel the second one can be a main lug panel without main disconnect. Only one breaker to buy that way.
A 70 amp may not have enough spaces to have the number of circuits you want. The compressor and welder will take four full size spaces. It would probably need 6 spaces to do what you want.

Are you thinking about taking the 100 amp one back for exchange?

You would be fine with #4's for a 70 amp feeder and a minimum #8 copper for the ground.

1" will work.

The 1st sub-panel is full? Is that why you want the second?

If you put a 70 amp in the 1st sub you can use the #4's and #8 ground if copper. The 100 amp will be simply a disconnect for the second sub.

Last edited by Stubbie; 03-18-2008 at 07:41 PM.
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Old 03-18-2008, 08:06 PM   #7
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Wire Size and Conduit Fill


Yes, the first sub panel is just about full, and it already uses the mini breakers.

I plan on keeping the panel I bought. Actually I bought it quite some time ago, and I am just now getting around to installing it. It was fairly inexpensive and came with the 100 main breaker already installed/secured along with several other breakers. I will use the 100 amp breaker in this panel just to shut off the entire panel if needed.

Regarding the large wire size:

I don't like the idea of aluminum wire

When you get down into the larger copper wires and conduit, it starts getting harder to wrestle around with the stuff. I will basically be doing this myself and I don't have a whole lot of room to work with in the crawl space.

There is a pretty good price difference between the #4 THHN and #3 at Lowes. I want to do it right, but I really don't see me needing the full 100 amps.


If I can run the panel I have now with a 70 amp breaker in the first sub panel and use three #4's and a #6 in 1 inch conduit, that's what I will do.
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Old 03-18-2008, 08:10 PM   #8
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Wire Size and Conduit Fill


You are fine doing that and it is quite common to do so. You are real close to conduit fill at 1 inch but should be fine.
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Old 03-18-2008, 08:11 PM   #9
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Wire Size and Conduit Fill


No matter what size breaker panel you feed from this subpanel, it is going to be limited by the 90 amp from the main. If you truly need the 100 amps, you may have to feed it from the main panel and not the sub.
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Old 03-18-2008, 08:16 PM   #10
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Wire Size and Conduit Fill


Thank you guys very much for your help
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Old 03-18-2008, 08:23 PM   #11
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Wire Size and Conduit Fill


Jbfan does make a good point which I overlooked originally the 90 amp breaker in the main panel for the feeder to sub #1 is going to protect the 2nd panel at 90 amps. Placing a 100 amp in the 1st sub-panel to feed the 2nd one would not have served much purpose.

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