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# Determing maximum amps for existing sub panel

#1
03-27-11, 07:46 PM
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Join Date: Mar 2011
Location: USA
Posts: 5
Determing maximum amps for existing sub panel

I am trying to add another 120v circuit in a sub panel in my garage. I want to make sure the sub panel can handle the new circuit. In the main panel, I see a 50A double pole breaker that supplies the sub panel. All circuits in the sub panel are 120v.

What is the max amps that the panel can handle? Since the breaker is 50A @ 240v, I calculate 100A @ 120v. Is this correct?

#2
03-27-11, 09:10 PM
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Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,399
Not quite. If you have a fifty ampere 240 volt feeder to the sub-panel then the ultimate short term (less than three hour) rating is 12,000 watts and that can be further subdivided as a maximum of 6,000 watts on each "leg" of pf the sub-panel. 6,000 watts at 120 volts is fifty amperes so each "leg" of the sub-panel can support a maximum of fifty amperes. The problem (if there is one) comes when the "load" exceeds that fifty amperes on either leg.

So, the answer to whether or not the panel can support 100 amperes at 120 volts is a qualified yes, it can IF the load is equally divided among the two legs. It CANNOT support 75 amperes on one leg while supporting 25 amperes on the other leg.

Does this help?

#3
03-28-11, 02:48 PM
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Join Date: Mar 2011
Location: USA
Posts: 5
That helps tremendously Furd - thanks for your reply on this. Just to clarify, what I need to do from this point is to calculate the total load of each sub panel "leg' and make sure each does not exceed 50A (100A @ 120v). Then add the circuit to the "leg" that has the least load of the two (assuming is does not exceed 100A @ 120v)

Also one thing to note: I am calculating the loads using the amps listed on the appliance sticker

#4
03-28-11, 03:03 PM
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Sounds like a good plan. Some appliances listed amps are the maximum possible it can draw and some are continuous, so also make note of that and your typical usage pattern as you add it up. An example would be a heater with high and low setting -- the nameplate would list amps for the high setting, but if you only it on the low setting you might want to consider that in your calculation.

#5
03-28-11, 03:42 PM
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Join Date: Mar 2006
Location: Wet side of Washington state.
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Remember that it is a maximum of fifty amperes per leg. Also, to add to what Ibpooks wrote, if you have both a window (or portable) air conditioner and an electric heater you do not add both because the chances of both being used at the same time is nil.

#6
03-28-11, 04:38 PM
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Join Date: Mar 2011
Location: USA
Posts: 5
Thanks for the help all. I am using the nameplate amperage as a safe bet here so that I do not underestimate the load. I bought one those load meters that one plugs between the appliance and the receptacle to measure actual load where I can.

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