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Old 02-02-2010, 02:04 PM   #1
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How to connect new BX or Romex to old K&T boxes?


What is the correct way to connect MC or romex into an old knob & tube box? The boxes are a lot like these: http://electrical.hardwarestore.com/...ox-239517.aspx , including the dual holes with clamps. I would just ditch the clamps and use regular bx or pinch connectors but the raw holes are too small (~0.7"). Using the built in clamps to pinch in romex or armored cable seems like less than a good idea as the raw holes are, well, raw. Didn't stop them from pinching in the K&T wires but that was the 40's.

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Old 02-02-2010, 04:43 PM   #2
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How to connect new BX or Romex to old K&T boxes?


Why aren't you replacing the box when you are at this?

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Old 02-02-2010, 04:55 PM   #3
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How to connect new BX or Romex to old K&T boxes?


From ""Codecheck Electrical":



- http://www.thehomeinspector.com/Insp...TCodeCheck.jpg
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Old 02-02-2010, 05:44 PM   #4
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How to connect new BX or Romex to old K&T boxes?


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Originally Posted by sdsester View Post
Why aren't you replacing the box when you are at this?
So I don't have to knock out a bunch of wall. The boxes are in good shape. I can fish the cables to the boxes relatively easily. It's a one story house with access down into the walls from the attic. That's the way the wires are currently run.

My original plan was to pre-attach the connector to the cable, fish it down to the box, and screw on the nut from inside the box. Then I realized that the knockouts weren't big enough for the standard 1/2" connectors.

This house is from the early 40's and has all sorts of hybrid things going on. The electrical is in conduit from the basement to a box in a middle wall where changes to K&T that goes up into the attic and back down throughout the house. It looks as though it was built that way. I have knocked out the wall where the conduit>K&T conversion box is and will make that conduit all the way to the attic. From there I will run romex or MC, but would prefer MC.

The walls are plaster over perforated drywall. I guess that's easier to deal with than plaster and lath but still, not so much fun to fix. I will if I have to but I'd really rather use the old boxes if there's a way.
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Old 02-02-2010, 06:22 PM   #5
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How to connect new BX or Romex to old K&T boxes?


Sounds like you are trying to use the old boxes that were made to utilize clamps to hold the cables in the box.

You may find out that the older boxes do not have enough cubic inch capacity for all the new wires and devices. Older versions of the code only counted one allowance for the devices. Todays code calls for two allowances along with the one allowance for the grounds.
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Old 02-02-2010, 08:48 PM   #6
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How to connect new BX or Romex to old K&T boxes?


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Sounds like you are trying to use the old boxes that were made to utilize clamps to hold the cables in the box.
Yep. Well I didn't necassarily intend to use the clamps but the boxes, yes.

I can think of ways to use the clamps that would be perfectly safe, such as using MC with some sort of "long sleeve" red hat insert, and then not cranking the crap out of the clamps, but I don't suppose that passes muster.

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You may find out that the older boxes do not have enough cubic inch capacity for all the new wires and devices. Older versions of the code only counted one allowance for the devices. Todays code calls for two allowances along with the one allowance for the grounds.
These boxes are 2Wx3Hx2.5D. There will only be one outlet or switch in each and one cable run to each. Plenty of room for that. Would I really be required to replace them when doing a K&T replacement just for their size?

It would be a huge amount of work to replace all the boxes and repair the walls compared to maybe a day or less of work fishing new cables down.

[EDIT]:I originally thought the clamps pressed the cables against the raw opening (knockout). I took one of the clamps out and it has a back plate that it squeezes the cables against, not the edge of the opening. (I can post pictures of what I'm dealing with if that would help. Again, the boxes are similer to the ones in that link I posted.) Couldn't I just use armored cable with a red hat then?

Last edited by Ptron; 02-02-2010 at 08:59 PM. Reason: new information
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Old 02-03-2010, 05:54 AM   #7
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How to connect new BX or Romex to old K&T boxes?


Sorry, I misread OP's question as being how to attach modern cable to a K&T system.

When extensive rewiring of K&T systems is undertaken, most AHJs in my area would require that new cabling be attached to boxes, and with connectors, listed for use with the cable.
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Old 02-03-2010, 06:58 AM   #8
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How to connect new BX or Romex to old K&T boxes?


Proper clamps need to be used in the boxes to secure the cable and to ensure the grounding path.
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Old 02-03-2010, 08:44 AM   #9
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How to connect new BX or Romex to old K&T boxes?


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Sorry, I misread OP's question as being how to attach modern cable to a K&T system.

The K&T will be completely replaced by modern cable. I just want to know how to reuse the original boxes. Certainly, this must be done form time to time?

Quote:
When extensive rewiring of K&T systems is undertaken, most AHJs in my area would require that new cabling be attached to boxes, and with connectors, listed for use with the cable.

Heh, heh. Well, I have connectors that are correct for the cable, they just don't play nice with the smaller holes in the box.

The clamps built into the boxes would probably work perfectly. They even have a couple ribs on the plates that would grip the cable, (And they're conveniently spring loaded) I just don't know if this is allowed.

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Proper clamps need to be used in the boxes to secure the cable and to ensure the grounding path.

Yes, this is my intention. Using the built in clamps would provide a ground connection between the box and cable if it were MC but I would still be using a ground wire anyway.
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Old 02-03-2010, 08:55 AM   #10
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How to connect new BX or Romex to old K&T boxes?


The clamps are allowed to connect the cable to the box. There are different clamps for different cable types. Some are for NM cables, others are for metallic cables like MC or AC.
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Old 02-04-2010, 12:10 PM   #11
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How to connect new BX or Romex to old K&T boxes?


Thanks very much for the help. Just to be clear, I'm talking about using the clamps that are built into the boxes, not the ones you buy seperately that fit in the knockouts and attatch with locknuts.

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