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I have 3 THHN wires in EMT conduit surface mounted to one side of a wall, and I want to connect this to existing NM-B that runs to an box and outlet in the other side of the wall.

I plan on using a junction box to make the transition from THHN to NM-B. How do I make the connections from EMT to j-box? Can I simply use a knockout in the back of the j-box to have the NM-B make its exit and use the j-box as the cover for the hole into the wall?

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  • I'm presuming this EMT run and the junction box you propose are both surface-mounted, correct? – ThreePhaseEel Nov 10 '20 at 4:15
  • I've boiled the question down to the essential details and removed all the unnecessary details. If you believe that something critical is missing, please feel free to edit your post to roll back to the "wall-of-text" original, or, preferably, add in the critical facts you think are still needed. – FreeMan Nov 10 '20 at 11:44
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Keep in mind that everything has to be rated for the location; the boxes, fittings, conduit, wire, all have to be rated for the location in which they are being used. The question doesn't say this is an outside or wet location, so this answer presumes it's a dry location.

To connect EMT to a box through a knockout, you use a connector. If the conduit is attached snug to the wall with one-hole straps

one hole strap

you have a small challenge - the knockout in the box won't be quite snug to the wall. If you have the tool and are adept bending conduit, you can bend a small zig and zag into the conduit so it enters the box square, but that's probably not the case. So you'll want an offset connector, also called a box offset, which has that zig zag built in, so things mate up square.

box offset

If you use hangers to attach the EMT to the wall

emt hanger

they do not hold the conduit tight to the wall, and will not require an offset - things will meet square with a plain straight connector.

emt box connector

Hangers do make things simple, but they cost more than one hole straps, the attachment is not quite as strong, and they have sharp edges and bits that tend to catch on things, and one hole straps look a little nicer.

The NM cable that you're going to fish in the wall must be clamped where it enters the boxes. You can use a fitting like this one to clamp NM cable in a knockout in the back of the surface mount box:

arlington NM94x white button

You'll want to drill the wall a little bigger than the outside diameter of that button so the box sets flush on the wall.

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    I was 100% with you until the very last sentence. Can you explain that one? What button? What hole in the wall? – FreeMan Nov 10 '20 at 14:39
  • @freeman - button is just a nickname for those push-in cable clamps. – batsplatsterson Nov 10 '20 at 15:08
  • @freeman - the boxes are on opposite sides of the same wall; the new box is surface mounted. The cable will be fished in the space between the studs inside the wall. It will enter the surface mount box from the back. So the hole drilled in the wall will have to be bigger than the knockout - need a little room for the flanges on the button to expand. – batsplatsterson Nov 10 '20 at 15:11
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    Ah... you you're saying to drill a hole behind the surface mount box big enough to clear the button so the cable can exit through the back of the box and into the wall. Gotcha. I was looking at the pics above that sentence and it just wasn't registering with me. Now I'm 100% with you to the bitter end. ;) – FreeMan Nov 10 '20 at 15:13

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