[allnight]

i have a shed that had a window ac in it.It was fed with 10-2 wire with a 30 amp 220 volt breaker from the main panel from the house. I want to put in a sub panel ,the sub panel i have is a 125 amp 2 main lug with neutral bar and ground bar can i use the 10-2 wire to wire the sub panel? Ill only be hooking up a 115v ac and a overhead light and one outlet.

 

[Jim Port]

If the AC is 240 volts you do not have a neutral in the cable. You can reconfigure the cable for 120 volt but every other breaker tab will be dead.

 

[seharper]

You can do a subpanel easily enough, with only one kink.

You will only be able to run 120V @ 30A. So when your feeder comes in, you will need to separate ground to the ground bar, neutral to the neutral bar, and you'll have 1 hot left. You'll have to split the hot to both "hot" lugs. At that point 240V loads won't work and you must not use multi-wire branch circuits (MWBC aka shared neutral).

Check the main panel and make sure the white wire is on the neutral bar. If it's on one side of that 30A supply breaker, move it to the neutral bar. It is OK to use only one side of the 30A 2-pole breaker.

in fact 30A 1-pole breakers aren't worth buying because they're used by almost nothing.

You can use that subpanel you have, but since this is an outbuilding, you need a "main disconnect switch". You can go get an air conditioner disconnect or a pricey 30A rated switch *if you really want to*, but the easier way is to install a "main breaker" in the panel. And the easiest way to do that on a main-lug panel is to install a 2-pole breaker in the top left, bolt it down, and backfeed it. The value of the "main breaker" does not matter since it's just a disconnect.

Like I say, you pigtail 2 wires off your hot wire, and split them to the "main breaker". If your breaker is listed to have 2 wires per lug, you can just land on one of them and have a jumper to the other one.

Then you use it just like normal.

By the way, my whole 1000sf cottage runs on 30A/120V. So it's perfectly usable.

 

[Jim Port]

Feeding both legs of the panel could overload the neutral. You would be creating a mwbc.

 

[seharper]

You would be creating a mwbc.

This is a new panel that cannot support 240V loads or MWBCs, and none will be installed.

So I really don't see how that's possible. Can you describe a scenario where that would happen?

 

[Jim Port]

If they feed both hot lugs like you suggested the feed now could have neutral current from 2 circuits on the same leg flowing on the undersized neutral.

 

[AllanJ]

Given the 10-2 feed bringing 30 amps at 120 volts to the shed you have a subpanel where the hot conductor of the feed was pigtailed to both of the top (master) breaker.

A maximum of 30 amps can come in on the feed hot conductor to be split up among the breakers on both sides of the panel. So a maximum of 30 amps can go back on the feed neutral which is also 10 gauge and wil not overload given a proper breaker at the supra panel.

The only caution is, as mentioned earlier, not to wire up any of the branch circuits as a multiwire branch circuit namely one 12-3 with red and black to different (any two) breakers in that panel and sharing the white neutral.