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Old 10-19-2015, 08:34 PM   #1
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200 Amp panel - slots left but is it overloaded?


I want to add sub-panel put out in my garage, #6 copper back to a 50A breaker in my main panel. Need to run a welder, exhaust fan, and general lights/outlets.

I have plenty of slots left in my 40-space 200A main panel. However, if I add up every printed value on the switches of each breaker I come up with this:

"A" leg: 285 amps
"B" leg: 265 amps

If I move a 15A breaker from B to A then snap in the 50A into B, I will end up with a perfectly balanced panel -- but each leg will be 300 amps at 120V.

I got to this point because I have two 240V central air systems (air handler=15A, compressor=20A) along with a 7.5HP air compressor on a 240/50A circuit, along with all the other "regular" circuits for the 2200 sq/ft house.

So, I'm exceeding the 400A ceiling for this panel by a full 200A. I *know* all of this stuff will not be running at once, and even if the big ticket items are running they don't draw to more than around 60% of the full rating of their breakers.

My questions:

1. Is this OK?

2. If not, would an inspector fail this?

3. If it's a fail, what the heck do I do to fix this situation?

Not sure if it matters but we have *never* had a power problem in this house. The only time a breaker trips is if I do something stupid like cut an extension cord with my hedge trimmer .
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Old 10-19-2015, 08:40 PM   #2
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Adding breaker handles is meaningless. You do not have a problem. Add your new subpanel breaker as planned.
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Old 10-19-2015, 08:48 PM   #3
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Thanks for that assurance - but I do like to learn this stuff, so if adding the values of the breakers is meaningless then how do you know if the total potential load for your overall system is within the capacity of the panel/service?

Eventually I want to add an outbuilding for a woodworking shop. That could easily be another 100A sub-panel.

How do I know when I've crossed the line?
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Old 10-19-2015, 09:00 PM   #4
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http://www.electrical-knowhow.com/20...lculation.html
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Old 10-19-2015, 09:32 PM   #5
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Quote:
Originally Posted by chrisv2 View Post
Thanks for that assurance - but I do like to learn this stuff, so if adding the values of the breakers is meaningless then how do you know if the total potential load for your overall system is within the capacity of the panel/service?

Eventually I want to add an outbuilding for a woodworking shop. That could easily be another 100A sub-panel.

How do I know when I've crossed the line?
1-You crossed the line when your 200a main breaker trips.
2-You can add 4 receptacles each on their own 15a breaker, or all on one 15a breaker, the load is determined by what's plugged in, not the number of circuits or breaker handles.
3-If you add a woodworking shop, you won't be in the garage welding. "Non-coincidental loads" is the phrase of the day.
4-To get an idea of how close to capacity your panel is, look at your bill, your monthly Kwh usage, and reverse engineer. Example.

IF your Kwh rate is only .10 cents
And you are using 20 amps at 120v 24 hours a day
Your monthly electric bill would be about $173.00 a month.
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Old 10-19-2015, 11:02 PM   #6
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I like that you want to understand things. The cliff book explanation is that after around 30A @240v you're demand factor is 30% of your remaining usage. This excludes heating/cooling which is 100% the larger value. This also a very liberal load calc.
Counting handle ties is absurd as most contractors add too many branch circuits for general lighting (which includes 120v receptacle outlets).
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Old 10-20-2015, 08:58 AM   #7
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Thanks guys - this is immensely helpful. I'm going to look at the history of my electric bills/usage and use that worksheet - great stuff, thanks!

I was under the impression that an "overloaded" panel full of breakers that add up to some value higher than the service level of the house would cause the inspector to fail the panel.

I get how it works now...makes sense...
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Old 10-20-2015, 03:04 PM   #8
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Breakers servicing electric heat are the only ones that need be counted at anywhere near the rated number.
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Old 10-20-2015, 03:10 PM   #9
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I made a mistake and said 30% when it should have been 40%.
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