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I want to install a 15-amp AC receptacle in my basement between the ceiling joists. The basement's not finished so there's plenty of room to play around. Should I be installing a "new work" new plastic outlet box or a steel one?

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Unless you're using armored (BX, or metal-sheathed) wire, you can use either type box, your preference really. If you use BX, you need to use a metal junction box.

There's a couple different types of boxes. The plastic ones with nails on them, and the metal ones with clips are designed to mount with their face 1/2" farther out than the front, which facilitates having them flush with 1/2" drywall when you finish it.

There's also a type of metal box that's more rounded, with several knock-outs in it (and many types of equivalent plastic boxes) which are not designed for going behind drywall, and look a bit better when you actually do see them (if you care).

Even if it's unfinished, you still need to put a cover plate on. The metal cover plates fit nicely over the rounded metal junction boxes, which makes them much less likely to crack if you hit them accidentally. Regular plastic covers can crack easily because they extend a far ways beyond the edge of the box.

It really comes down to personal preference on metal vs plastic, and then a question of if you will be drywalling, and intend this outlet to be available through the drywall. Also note, it's illegal to cover a junction with drywall: all boxes must be accessible. If you do put drywall up, you either have to remove this box, or have an outlet or blank faceplate on your ceiling.

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Thanks, that's very informative. I had planned to use just regular romex non-metallic wire. I kind of like the look and feel of the metal boxes so I might go with that. Tangential question: when would one use armored wire like you describe? I have some of it in my basement but it seem like only on old stuff. – Matthew Leingang Mar 7 '11 at 3:37
One would use armored cable back in 1950!!! – shirlock homes Mar 7 '11 at 7:09
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BTW, the rounded metal box greg described is commonly called a "handy" box. Perfect for you application. Don't forget to get some romex connectors to go in the knockouts! – shirlock homes Mar 7 '11 at 10:03
In a residential situation, you generally don't use BX. It's required in many commercial applications (in particular, you're not allowed to have exposed NM wire: so it must be in conduit or be BX), or if you're running through a wall with metal studs. I would personally use it if I had exposed wiring on a wall in a workshop or garage (though I'm equally likely to use NM in PVC conduit - depends on the situation, what's already there, and what I have on hand). In your situation, in the ceiling, I'd stick with NM. – gregmac Mar 7 '11 at 19:24
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With NM wire, I'd go new-work plastic myself, I find them easier to work with.

The metal boxes must be wired to the ground wire, and they need inserts for the knock-outs to keep the NM cable from possibly being cut by the sharp edges of the knock-out holes. With plastic, neither of these is a concern.

Also, remember that if you plan on finishing the ceiling, mount the box so it will be eventually flush with the drywall, not flush with the joists.

It's not related directly to your query, but don't forget to use a GFCI! It's required by NEC 2008 in an unfinished basement unless the outlet is dedicated to a security or fire alarm system. You may also want to go with a 20A to give you plenty of flexibility with how you're able to use it in the future (larger power tools, sump pump, etc.).

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Obviously, only use a 20A outlet if it is on a 20A breaker. – auujay Mar 7 '11 at 16:32
Thanks. This circuit I'm tapping into already has a GFCI receptacle on it. So I'm OK, right? – Matthew Leingang Mar 7 '11 at 18:17
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@Matthew Leingang: it is only protected by the GFCI if it is downstream from it. GFCI receptacles have "line" and "load" terminals, anything on 'load" is protected. Merely being on the same circuit (connected on the "line" side) is not enough. – gregmac Mar 7 '11 at 19:20
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IF you DO use a 20A breaker you need wire to match (12 gauge) vs 15A which uses lighter (14 gauge) wire AND 20A fixtures to match - which are labeled as such. Safety note: do NOT put the lights on the same circut as a power tool. – Mark Schultheiss Mar 7 '11 at 19:36
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@Mark Schultheiss - But I like the lights to dim when I run my table saw, it's mood lighting for getting down to business. – Tester101♦ Mar 8 '11 at 17:35
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