240V to 120 conversion(s)

October 26, 2008 7:55 AM

I need to convert a double 30 amp breaker that feeds a 240V dryer outlet into a subpanel for several 120 lines...

I'm finishing my basement. The original plan allowed for a washer /dryer down there, long since relocated upstairs, so I have an unused dedicated 20amp circuit (washing machine) and an unused double 30 amp circuit for the dryer. There are a couple other 15 amp circuits that feed stuff down there that are pretty much accounted for.

My hope, and I will be utilizing the services of an electrician (but would like to be prepared for the conversation I'm going to have as much as possible), is that I can turn the dryer outlet into some combination of circuits, i.e. a subpanel, that would give me at least 2 extra 20 amp, or 2 extra 30 amp, circuits down there for 120V wiring which I could then use to feed the bed, bath and whatever other stuff I need (like a small 120V space heater).

I ran 2 courses of 12G romex yesterday from just at the 240V box into the eventual bathroom and bedroom. Tell me if I've just wasted a ton of time and money.

Thanks.

posted by docpops to home & garden (9 comments total)

Not an electrician, but sounds right to me.
posted by gjc at 8:04 AM on October 26, 2008

Oh, duh, I forgot the science of it. In residential electricity, 120v is one side of the circuit, and "common" is the other side. The difference between the two is 120v. Where 240v is just two 120v legs out of phase with each other, so the difference between the two is 240v. So it should work just fine.
posted by gjc at 8:07 AM on October 26, 2008

Actually, this is *really* good planning. As gjc says, 240 is just a couple 120's out of phase with each other - different legs of power. If you can swing it, running alternating outlets from each leg will mean fewer things on each leg overall - as opposed to loading up one leg only. Then you're less likely to have lights dim, and other such symptoms of dirty/sub-optimal power design.
posted by notsnot at 9:29 AM on October 26, 2008

(also, you could have run one course of 12/3 instead of two courses of 12/2 - on the 12/2, you've got one course running 120a, neutral and ground, and the other is 120b, n, and g. On 12/3, you'd have 120a, 120b, n, g.)
posted by notsnot at 9:32 AM on October 26, 2008

Thanks so far. So, in my simplistic sense of things, can I ask the electrician to put in a panel with, say, a 30 and 2 15's, or 2 30's? The only things down there that will be potential big loads are an AV system, which I figure will be fine on a separate 15, and the bedroom, which will be a small 20 inch flat panel, a 1000W cadet heater, and a couple 65 watt cans and a bedside light or two. I figured I'd ask for a 30 amp for the bedroom, but wasn't sure what made the most sense.

Notsnot, I should have asked this earlier. Romex is wicked expensive. No wonder the tweakers are ripping it out of the walls.
posted by docpops at 9:41 AM on October 26, 2008

The 12 gauge romex you've installed is only rated for 25 amps, so I'd stick with a standard 20 amp circuit for the bedroom, with an arc fault circuit interrupter. That should be sufficient.

The electrician should be able to tell you how much juice you can afford for the subpanel, but two 20A circuits seems like a good idea.
posted by Knappster at 11:22 AM on October 26, 2008

It'd be unusual to install 30 amp circuits for regular outlets - in fact, I'm pretty sure that you can't even get a standard 120v outlet that's rated at 30 amps.

As knappster correctly stated, your 12 gauge is only good up to 25 amps.

Depending on local codes you can usually have up to 15 "devices" per circuit (i.e. 10 lights + five receptacles).

I find that folks tend to overthink the wiring thing and go overboard on number of circuits/separate breakers for everything. In a normal residential setting simpler is usually fine (and cheaper as you have found)

The device that draws the most in your scenario (1000w heater) is going to be pulling under 10 amps (more like 8) at any given time, leaving you plenty of room for your other devices..
posted by davey_darling at 3:39 PM on October 26, 2008

Thanks for all the good input.
posted by docpops at 7:48 AM on October 27, 2008

The 30 amp circuit was converted to a subpanel with 2 20 amp lines on 12G wire, with one circuit to a bedroom running lights (4), outlets, and a small 120V 1000W cadet heater which will likely rarely get used.

The other 20 amp line is for a few outlets in the rec room where a treadmill is used along with 6 overhead can lights.

So far I've run the heater, treadmill (at 8 mph), run all the bedroom and rec room lights, and no tripping.
posted by docpops at 8:43 AM on November 26, 2008