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When running NM cable, you're supposed to not make any bends sharper than 5 times the cable radius.*

When dressing wires to go into a box, though, you often curve them any which way or shape them into an accordion to leave a workable length while fitting them behind the device fixed to the front of the box.

Why be concerned about how sharply the wires are bent in one case but not the other?

(Regardless of the answer, Code demands conformance. This is purely curiosity.)

* Or something like that - I just remember the rule that for 14 or 12 gauge you want to go no sharper than the bend in a soda can. When I run thicker cable, I'll look up the details then.

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cable us larger, so needs a larger bend. A single wire is smaller, so needs a smaller bend. – Tester101 Sep 21 '12 at 20:32
Given this requirement, a spiral (as opposed to an accordion) and a deeper J-box would be two suggestions. – BMitch Sep 21 '12 at 23:06
@BMitch Good ideas. The spiral is definitely commonly encountered in wiring cabinets/breaker boxes. As far as I can tell, the min-radius requirement only applies to cables, not conductors. 300.19(C)(3) (supporting vertical cables/wire in a long run by winding them around insulating supports) even requires a deflection of >= 90 deg. – Jeremy W. Sherman Sep 21 '12 at 23:35
up vote 1 down vote accepted

the minimum bend radius can serve two purposes.

the first is to protect the inner "conductor". on fiber optic cable and very thin copper cables, going below the minimum bend radius can cause the "conductor" to break.

the second is to protect the sheathing. on heavier gauge wire such as electrical wiring, going below the minimum bend radius can cause the sheathing to break, exposing the inner conductors. when you get to the end of the wire (as in a switch box) a tight bend causes the sheath to retract from the exposed end instead of just breaking.

breaking the inner conductor is not as much of a concern with electrical wiring because, at these diameters, it generally takes repeated flexing to cause a failure in the conductor itself, and these types of cables are meant to be installed in fixed environments.

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Makes sense. I wonder if there's a paper trail as to when the bend radius table appeared in the NEC? – Jeremy W. Sherman Sep 24 '12 at 20:58

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