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I have a 12 gauge stranded wire extension cord, but I could not find a switch that I needed to connect in the middle. So, I used a regular wall switch and box. My question is, should I simply connect the ground wires back together and not connect them to the box, or should I connect them to the box too? What is the safest option?

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If the box is metal, it should be connected to the ground conductor in the cord.

Imagine this scenario: the hot wire comes loose inside this box, and makes contact with the box itself (which is not connected to ground). You go to turn on the switch and touch the box while standing on the ground. Because the box is energized by the hot wire, and you're a path to ground, you get a shock (or worse).

The safety ground wire is designed to reduce this risk by ensuring any conductive surfaces are firmly bonded to ground. In the scenario above, if the box were grounded, a short would happen as soon as the hot wire contacted the box. This would flip your circuit breaker, so you'd know something was wrong. Way better than having the box sitting there energized -- an invisible danger just waiting for someone to touch it!

So: ground the box and ensure the ground conductor continues on past the switch. And make sure you're using a properly grounded outlet to feed this cord.

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  • If you must do this, in addition to properly grounding of the box, you should also use the proper fittings for the cord going into and out of the box, i.e., "listed for the purpose" of flexible cords, as compared with, say, Type NM-B "Romex" cables. Realize that your fire insurance will not be amused if your handiwork results in a hazard.
    – Upnorth
    Oct 6, 2017 at 18:05
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    I might point out that OSHA does not like this type of modification to extension cords. The NEC doesn't say you can't do it ,but 400.9 Splices and 400.10 Pull at Joints and Terminals help you to understand what you need to do. I would suggest if you must add this switch, you also add a GFCI male adapter on the end of it for protection. Stay Safe.
    – Retired Master Electrician
    Oct 6, 2017 at 19:50
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    One might also use two shorter extension cords and put an in-line 12/3 GFCI between them. I just got some 20A units for under $10 with a locking end.
    – Upnorth
    Oct 7, 2017 at 2:38
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Thank you all very much for your help! Before I heard back from you all I changed the entire assembly into a plastic box with a plastic faceplate. But your comments here have confirmed to me that getting rid of the metal box was the best thing.

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