adding a subpanel: a few questions please

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Old 09-11-04, 03:00 PM
SparkyJoe
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adding a subpanel: a few questions please

I'd like a small subpanel for some additional household outlets, and I happen to have quite a bit of 10/3 romex left over from another job.

I believe #10 cable handles 30 amps, so at the main panel, I would attach the 10/3 cable to a dual pole 30 amp breaker, right? Something I've never figured out is if a dual pole 30 amp breaker handles 30 amps total, or 30 on each wire for a total of sixty.

Thanks!
 

Last edited by SparkyJoe; 09-11-04 at 03:11 PM.
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Old 09-11-04, 04:10 PM
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Yes, you would attach the black and red wires of 10/3 cable to a 30 amp breaker in the main panel. Each wire can handle 30 amps. You have 30 amps at 240 volts,

However, using a piece of 10/3 because it is left over is being penny foolish. If I were installing a sub panel I would go at least 60 amps. With 30 amps you will need to be very careful what you use on those circuits. With 60 amps you will have enough for most uses, plus some to spare should you need other circuits in the future.
 
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Old 09-11-04, 04:40 PM
SparkyJoe
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Thanks for the reply racraft.

I agree about which wire to use, but when I said it was left over, I wasn't telling the whole story. In fact the run of 10/3 is in place but suddenly not needed due to my noob error. I thought I was going to use it on a 20 amp breaker for an appliance, but that was wrong. Now it's there, hooked up at the main panel, and pointless unless I use it for something else. Rather than removing it (feeding it into the main panel sucked!) I figured I could use it for a small sub panel and avoid having completely wasted the time and expense of running that cable in the first place. So it's a recovery move rather than a choice from scratch!

I just got back from the store and have a 70 amp panel (the smallest they had in stock) with two 15 amp breakers. That should be okay, right?

Thanks again!
 
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Old 09-11-04, 06:35 PM
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I hope that you also bought a grounding bar kit for that sub panel.
 
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Old 09-11-04, 06:37 PM
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Don't use this cable unless it has three insulated conductors plus a grounding wire. If it only has three wires total, don't use it. Even so, whether or not this is a good idea depends in part on how old that old cable is.

Something I've never figured out is if a dual pole 30 amp breaker handles 30 amps total, or 30 on each wire for a total of sixty
Neither. It handles 30 amps total, and 30 amps on each wire. They are the same 30 amps.
 
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Old 09-12-04, 09:38 AM
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I don’t see the point of installing the 2 breakers sub-panel. It will just increase the cost of the work by adding a panel and 2 15 breakers. If you will only be installing two 15 amp circuits, why not wire them as a shared neutral circuit? (Spliting the “black” and “red” wires into two separate circuits, while sharing the “white” wire). Protect with a dual pole 15 amp breaker at the main panel. The panel buys you nothing, except a local disconnect point. (There are lots of posts on how to install a shared neutral circuit in this forum)

If you do wish to make use of the 10/3 wire with a sub-panel, I would recommend installing a 4 breaker panel, protected by a dual pole 30 amp breaker at the main panel. This would give you up to 4 15 amp circuits, making the whole thing a little more worthwhile.
 
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Old 09-12-04, 11:19 AM
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I agree. Using the same 10/3 (with ground!!!) cable, a subpanel will provide twice the available power (7200 watts) than a 15-amp multiwire circuit (3600 watts). Using a subpanel will allow you to fully load four 15-amp circuits, if you have a subpanel with space for more breakers. If your subpanel only has room for two breakers, it doesn't offer any advantages.
 
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