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Old 01-30-2011, 06:47 PM   #1
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sizing new subpanel


I have 100A service. My main panel is full, but I want to run four new circuits:

2 x 20A circuits for bathroom GFCIs (biggest load - hairdryer on each)
1 x 20A circuit for bedroom outlets (biggest load - multiple window ACs)
1 x 20A circuit for finished basement (biggest load - fridge+TV)


The more significant loads in my main panel are:

2 pole 15A -central air handler
2 pole 30A - central air condenser
2 pole 30A - feed to garage fuse box
2 pole 30A - dryer

I was thinking of adding an 80A subpanel adjacent to the main panel to accomodate my new circuits. I have two 15A breakers that I can remove from the main panel to install the new 2-pole breaker for the subpanel. Does this make sense? Could I get away with a 60A breaker, which is cheaper, since the four circuits would unlikely draw the maximum 80A?

I was actually surprised the house had 100A service, considering it has central air, but we never had any power problems last summer. I'd like to upgrade to 200A service at some point, but I understand that it's a big expense.

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Old 01-30-2011, 06:52 PM   #2
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sizing new subpanel


A 80 amp sub is more than sufficient for your current needs. 60 amp would be enough.

Multiple AC on a single 20 amp circuit does not sound good to me. One AC per circuit is all I would do if I was installing new circuits.

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Old 01-30-2011, 07:32 PM   #3
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The window AC I have requires 5.5A continuous. I guess I should probably limit to two.
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Old 01-30-2011, 08:04 PM   #4
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That's a small unit. Most of the ones I am used to seeing are 10-12 amp range. Some even need 20 amp circuits.
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Old 01-30-2011, 11:18 PM   #5
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sizing new subpanel


what about using tandem breakers?
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Old 01-31-2011, 04:36 AM   #6
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sizing new subpanel


yea its a 6000 BTU energy star unit. Last summer I had to plug it in to an existing 2-wire outlet using an adapter, but I'd feel more comfortable using newer wiring to power the AC. The house is 60+ yrs old and I don't know the condition of the wiring everywhere.

I thought about tandem breakers, since there are a few already in the main panel, But, the panel itself is not rated for tandems. What's confusing is that an electrician had serviced the panel before we purchased the house, but must've not had a problem with them being there. Is there aw ay to rationalize that I could add new tandems in the main panel?

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