220 subpanel for compressor

[bchase]

I'm thinking about putting in a new subpanel for a compressor and maybe a few 110 outlets in the garage. Can I run a subpanel off an existing 30a breaker that i do not use on my main panel? It looks to me that i can simply reroute the existing 10-3 to a sub, then out to the needed connections. BTW, the comp. needs 220a circuit. So the way to wire the sub is ???
Thanks.

[imported_Ron]

Is the 30A spare breaker 1 pole or two? How far is the main panel from the new proposed sub? What size (HP, volts and amps) is the compressor?

[Anonymous]

The new sub will only be about 12-18 inches from the main. The existing 30a breaker is a two pole. The comp has a 3hp motor that runs 220v with about 17 amp max draw. from the sub i'll run an isolated 10-3 220 run to the compressor which is about 30 feet from the subpanel.

[imported_marako]

Is your main panel also in the garage? If so why not move the sub panel to the other side where you'll be working (since the compressor was 30' away you said). Also a 30 amp sub is kinda light. You'll need about 20 alone for the compressor. What if you have your 200 watts of lights one, and a large tool running when the compressor kicks on? I say replace the breaker with something like a 50 amp, put the sub where it makes sense and go from there.

If this is too much as you said will work.

[Mark_R]

Hmmm, food for discussion thought... if his compressor is truely 17A nameplate 'run' data, since that's over 50% motor load on a 30A breaker can he hang any additional load (outlets) on this circuit at all?

Or am I still in the kitchen?

[Anonymous]

I was actually thinking of changing to a 50a breaker just for the extra capacity. The location is kind of flexible so i may actually consider moving it closer to the compressor. So I'll assume from this point that running a subpanel off an existing breaker is allowable. So at the subpanel would it be better to hard wire the compressor directly or put a plug on the wall then wire a cord from the compressor? Also I'm curious how 220a appliances are wired direct. Obviously you'd have the 2 hots and a ground but what do you do with the neutral?

[imported_marako]

Technically for 240 circiuts you do not need a neutral. Other than a dryer and oven you only need to run 3 wires so a 12/3 w/grnd would work as the feed to your compressor. I however alway run cables with 3 wires and a ground. I hook the white up to the neutral bar at the panel, then simply cap it off and stuff it into the box on the far end. This gives me freedom down the road or if the code changes and requires.

If the compressor has a plug on it, then you that. If it has a wire with bar ends then "hard wire" it into a junction box near the compressor.

BTW you can run a subpanel off an existing breaker provided that breaker doens't do anything (as you described)

[Wgoodrich]

If the panel is only 2 feet from the main panel why install a sub panel? If the air compressor is plug in style 220 volt then no further disconnect form is required. Why not run from a 20 amp 220 volt breaker in the main panel to the air compressor 220 volt 20 amp receptacle with a dedicated 12/2wGrnd reidentifying the white as a hot conductor?

Seems easier and more prudent to run a 12/2 wground general lighting circuit to serve your lighitng needs and a dedicated 12/2wGrnd air compressor circuit and forget a sub panel. Don't understand why a sub panel is needed or desired in this occasion.

Mark R, A subpanel is allowed to serve forgetting the 50% rule concerning a branch circuit only. A sub panel is required to be sized by finding the largest load increased by 25% then adding the full load current of all other loads on that feeder serving that subpanel. If general lighting type receptacles are used off that subpanel easiest way to calculate unknown load would be use the 180 volt amp rating for duplex receptacles in commercial areas. This should cover you unknown loads.

The 50% rule only applies to branch circuit not feeders.

Hope this helps

Wg