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Old 10-01-2010, 01:56 PM   #1
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New Switch Wiring, Old Box


I replaced a light and the switch loop wiring to the switch box. The old wire was knob and tube so the switch box has two knockouts removed. The new NM cable uses only one of the holes. Is it required to plug the unused hole and if so what methods can be used? The knockout is the same size as those for NM cable on modern metal boxes so they're too small for 3/8" knockout plugs.

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Old 10-01-2010, 02:51 PM   #2
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New Switch Wiring, Old Box


It is required to plug the hole. It is also required to have a proper clamp for the cable. If the box doesn't have proper clamps then replace it. If it has regular round knockout holes then you can install a clamp and get a plug that will fit the knockout.

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Old 10-01-2010, 03:16 PM   #3
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New Switch Wiring, Old Box


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It is required to plug the hole. It is also required to have a proper clamp for the cable. If the box doesn't have proper clamps then replace it. If it has regular round knockout holes then you can install a clamp and get a plug that will fit the knockout.
It has proper NM clamps. I'd rather just plug the hole if I can. I have to cut a big hole in the plaster to replace the box it as it has a stud-face mounting bracket. I've replaced one before and it's a pita.
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Old 10-01-2010, 05:43 PM   #4
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New Switch Wiring, Old Box


The NM clamp from the box should almost seal off the unused KO.
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Old 10-01-2010, 09:23 PM   #5
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New Switch Wiring, Old Box


I think you should be fine. Most of the K&T boxes I have seen were just holes in the box with wire with loom over them coming through. NO clamps.
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Old 10-02-2010, 08:57 AM   #6
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New Switch Wiring, Old Box


Thanks folks. Sounds good.

This house was built in the late 40's so I'm guessing it was the later days of K&T. The boxes are just like the modern metal ones, with clamps. The wiring is in pretty good shape too.

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