Subpanel Power Load Limit


Old 02-22-05, 11:55 AM
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Subpanel Power Load Limit

I need to install a subpanel in my home to allow for additional circuits (basement remodeling, power tools in expanded woodworking shop, etc.). From other threads I understand that I would use a 240V breaker in the main panel to feed the sub panel, and that the neutral/ground bridge in the sub-panel should not be connected and also that the sub-panel should not be grounded directly to a water pipe. I have a three questions that I didn't find answers to:
1. Main breaker in the primary panel is 200 amp; how large (power load) of a sub panel can I install? I live in DuPage COunty, Illinois; not sure but think the 1999 NEC is still our guideline. Second hand information seays subpanel cannot exceed 60% of main panel master breaker rating, but I don't know if htis is reliable information.
2. Do I install a master breaker in the sub-panel as well? In other words, wire from the 100 amp "feed" breaker in the main panel into a 100 amp "master" breaker in the subpanel?
3. I will be connecting the subpanel to the main panel using conduit, so I don't think I need to pull a separate ground wire; agree?
Thanks much!
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Old 02-22-05, 12:03 PM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
1. As large as you want (in theory anyway).

2. This depends on the size of the sub panel. With a large enough panel you need a main breaker. With a small panel you don;t need a main breaker.

3. You need to run four wires from your main panel to your sub panel. Two hot wires, a neutral and a ground.

What makes you think you need a sub panel? How new circuits do you plan to install? How many open spaces are in your main panel? How many breakers presently in your main panel, and are any of them tandem breakers?

Understand that a sub panel may be the best way to go, especially if the distance is not short from the main panel to the woodworking shop, but if you intend to put the sub panel right next to the main panel you should be certain that you really need it.
Old 02-22-05, 12:27 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
2. If the subpanel is in the same structure (which seems to be true here), then no main shutoff is required in the subpanel no matter what the size of the subpanel.

3. Although a grounding wire may not be required, in my opinion you'd be foolish not to pull one.
Old 02-22-05, 03:13 PM
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Thanks for the info; will pull the ground wire as well. My main panel is almost out of space (2-3 blanks left). I don't have any tandem breakers; strong preference not to use them as local inspectors (engaged when you buy/sell a house) won't approve these, so you end up putting the second panel in anyway - more work since you've added all that much more wiring in the interim.
Old 02-23-05, 01:17 PM
Join Date: Nov 2002
Location: SC
Posts: 171
Max Amp feed can very well depend on how large a breaker you can find to fit in your main panel. 100A may be the max 2 pole breaker. I've seen breakers larger than that that take up 4 spaces. As John said, no main is required in the sub because your breaker in the main panel feeding the sub protects the wires and subpanel and serves as a disconnecting means.
Old 02-23-05, 06:46 PM
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Thanks - 100 am it will be

Thanks much for the confirming info. In shopping around today I found a 100 am CH breaker and also a Siemens 100 amp load center, so I think my path forward is clear.

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