Subpanel wiring help


Old 03-15-08, 03:16 PM
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Subpanel wiring help

I'm building approximately 900 sq ft to my house and I'm installing a subpanel less than 100 feet from my main panel. Subpanel consists of 9-20 amp circuits, 2-15 amp circuits and a 220 V 30 amp A/C circuit. I might be installing a 40 amp circuit for a steam shower. I want to run an NMC to my subpanel. don't know what breaker I should use or what wire size. The main house panel is at 125 amps, with 8-20 amp circuits, 1-30 amp 220V, and 1-220V 40 amp.
Thanks for your help
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Old 03-15-08, 03:52 PM
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Are your feeder wires coming from the power company to your main panel of sufficient ampacity to handle this much extra load? You may need new feeders and a new main panel.
Old 03-15-08, 03:53 PM
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You really need to do a load calculation for the sub panel AND for your service (main) panel.

Just for giggles I made an assumption that your eight existing 20 amp circuits were loaded to 50%, your existing 30 amp circuit was loaded to 80% and the existing 40 amp circuit was loaded to 80%. This adds up to 72 amps at 240 volts which would leave you 53 amps maximum for your proposed sub-panel.

NOTE that this is NOT the proper way to do a load calculation but it does give you some idea of what is involved. You need to know what the different loads are going to be for both panels. Most of the 240 volt loads will be to single devices and you can make some assumptions like knowing there is almost no possibility of a heating load and a cooling load being on simultaneously.
Old 03-15-08, 04:06 PM
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I'm in the process of having pge come out to verify my main panel. assuming that I upgrade my panel to a 200 amp service my subpanel would consist of circuits for two bedrooms living room and a half bath and a master bath and lighting circuits. main amperage would be jacuzzi tub which requires two 20amp 110v circuits drawing 12 amps on one and 7 amps on the other. the 220v 30 amp ac circuit drawing 20 amps. I want to leave room for possible steam bath that draws 30 amps. I know you can run 100 amp subpanel but if I don't need to that will save money on breaker and wire size.

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