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I will be mounting a 4”x4” electrical box at the back of an under the sink cabinet. A wire for a garbage disposal will be fed through a hole directly in the back of the box. Another wire will be fed through the side hole from the adjacent dishwasher, and I intend to put a connector there. Do wires feeding from the wall directly into the cabinet and through the back of the box require a cable connector? I will also be wiring another box for 240v wall oven using #8 cable the same way - thinking same rule would apply. Would it?

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    +1 for asking and not just doing! – FreeMan Dec 4 '20 at 15:08
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    @FreeMan thank you! Yeah, there is a balance between asking too many “stupid” questions and not enough. I try to strike that balance and hope that others will have same questions. When it comes to electrical, I’d rather make sure. – David Dec 4 '20 at 15:16
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Cables require a cable clamp - the clamp can be external to the box (which requires installing the clamp and cable before mounting the box in the case described, unless you have access to the back of the box when mounted) or internal to the box (where it adds to the count for box fill), but you can't just stuff the cable into the box through a bare hole and call it good.

There are two functions of a cable clamp - one is strain relief - if the cable is pulled on, the clamp takes that strain, and the electrical connections of the conductors do not. The other is preventing damage to the insulation from contact with the sharp edges of the hole in the box.

Wires in conduit do not require clamps. Cables require clamps.

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    Thanks, that’s easy enough, so basically feed through the clamp, tighten and mount the box. – David Dec 4 '20 at 15:13
  • Couldn't you put a grommet instead of a clamp? Not saying a clamp would be wrong, just that a grommet would be sufficient. – Ben Dec 5 '20 at 1:15
  • A grommet does not provide strain relief. – Ecnerwal Dec 5 '20 at 1:26
  • The simplest form of a clamp is something that doesn't allow the cable to be pulled out. The classic version was a knot (probably not acceptable by code -- although we don't know where the OP lives [using imperial and wire gauge kinda narrows it down] -- and not even feasible with thicker cables). But a simple metal ring deformed onto the cable to stop the pulling does the same. And, of course there are grommets that also provide strain relief. I know nothing of the local regulations but if that's acceptable, I guess that would be the cleanest solution. – Gábor Dec 5 '20 at 9:43
  • The one I had in mind is more aptly called strain relief bushing, though... – Gábor Dec 5 '20 at 9:45
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Any cable going into a box needs clamp of some sort.

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