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I am mounting a bathroom vent onto a piece of plywood. I took a 3 prong extension cord and cut the end off and wired it into the vent. The other end of the cord will be plugged in to a receptacle. I am wiring a switch into the extension cord to control the vent on/off. My question is, is it safe to mount this switch onto the piece of plywood with a metal box? It just seems like I might have to worry about touching the box and getting shocked.

Thanks for your advice.

  • Why are you opposed to hardwiring this vent fan? – ThreePhaseEel Jun 5 at 0:58
  • I'm not. I simply honestly didn't know about the rules and regulations mentioned below. I am not an electrician; I'm just trying to get the stink out of my downstairs bathroom. – Milton McDonald Jun 5 at 9:48
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  1. Extension cord cabling is not designed for permanent installations (someone will quote you the NEC reference, I'm sure).
  2. Is the receptacle properly grounded?
  3. If you attach the ground to the metal box or to the switch's ground connector (which is connected to the mounting yoke, which should make metal-to-metal contact with the box) the box itself will also be grounded and you shouldn't have a problem.
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  • The receptacle is properly grounded and the switch has a green screw. So should I just cut the green wire in the extension cord and connect both sides to the green screw? – Milton McDonald Jun 4 at 13:50
  • Yes, but note item #1 in my response. – FreeMan Jun 4 at 13:51
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    Item #1 noted. Thanks! – Milton McDonald Jun 4 at 13:52
  • The problem SO, SJO, or other extension cord types are not listed for this use , this is a case where you are going around code and it is not allowed if you need surface mount MC cable or conduit methods will be needed. – Ed Beal Jun 4 at 13:55
  • Thanks for this info. I am happy to do this the right way. You say MC cable is satisfactory for my purpose? – Milton McDonald Jun 4 at 13:59
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I've always wondered why people are hostile to metal boxes. I guess you think a metal box will shock you and a plastic box will not? That seems like good motivation to get your grounding right.

What a metal box does is a fantastic job of containing both meltdowns from wiring faults, and also assures a breaker trip if a hot wire gets loose in the box (or neutral wire, if GFCI protected). Plastic fails spectacularly at both, and will actually augment a fire, since plastic is, after all, made of petroleum! Yeah, they add borate flame retardant to the plastic, but that only makes it self-extinguish if the external heat source is removed. As long as it's fed by anything else burning, it is exothermic. Not what you want next to plywood!

It seems important to you that this be cord-and-plug connected. I don't fully understand your application, but let's review a few rules.

First, you cannot cord-and-plug connect something that is concealed. The classic case is fluorescent troffers in drop ceilings. There are several other places you cannot.

However, there are also several places/reasons you can. These are covered in NEC 400.7 and 400.8, which you can read about here.

However, fixed in-wall wiring does not play well with plugs. So putting an allowed in-wall wiring method on a plug is a non-starter. You would need to hardwire it, unless your case falls under a flexible cord exception.

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  • Actually, I'm not hostile. But when it comes to safety, I figure there's no dumb question. – Milton McDonald Jun 4 at 18:20

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