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OK to Connect Flexible Metal Cable to Plastic J-Box?

Posted by tom_in_seattle (My Page) on
Thu, Jan 14, 10 at 16:03

I installed a metal junction box with receptacle in the firebox of our masonry fireplace. The new gas insert fan will use this receptacle.

I drilled a hole through the brick and ran flexible metal cable from the firebox through the brick and wall studs to a plastic junction box a few feet away (no drywall on the walls). I bored a hole in the side of the plastic box and connected the flexible metal cable via a fitting and lock nut.

Is this OK?


Follow-Up Postings:

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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"I bored a hole in the side of the plastic box and connected the flexible metal cable via a fitting and lock nut."

Did you use a clamp rated for the type of cable you are using (AC or MC)?

You may also have trouble picking up ground unless you used a cable with a ground conductor (MC).

Why not just use a metal box?


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

The cable I used was labeled "14/3 appliance cable" and is 6 ft long, with attached end connectors which thread onto the flexible metal conduit and have a threaded open end with a thin jam nut. The included wiring is black, white and green, solid 14 ga. I attached the green wire to the ground screw on the receptacle in the metal j-box... the other end will combine with the other two grounds in the plastic j-box, and then attach to the ground screw on that receptacle.

I suppose I could have used a metal box in the wall, but I had already installed a plastic one before we made the decision on the gas insert and it just seemed easier to bore a hole in the side and attach the (MC?) cable. All the other boxes in the house are plastic for use with NM cable.

I can post a picture later if that would help.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"The cable I used was labeled "14/3 appliance cable":"

That does not sound like an approved permanent wiring method.

Permanent armored wiring would be type AC or type MC.

Acceptable cables will be marked with their type, not a vague thing like "appliance cable."


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

OK, good point. The cable appears to be "similar" to MC but may not be actual MC. I may change the cable anyway since I'm thinking of putting the family room receptacles on their own breaker and upgrading the wiring to 12 ga, 20A.

The family room receptacle circuit currently is a branch off the garage and laundry room light circuit, which I was thinking of connecting to the new recessed and other lighting fixtures for the family room, then running a new home run for the receptacles.

Assuming I change out the cable to MC, can I still connect it to the plastic j-box as long as their is a separate ground wire for the receptacle in the firebox, or do your recommend I change that one j-box to a metal box?

Thanks for your help.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

I didn't think you could just drill a hole in a junction box. If the manufacturer doesn't say it's approved for a particular wiring method, then it's not approved.

Never saw a 14/3 metal clad "appliance cable." Unless it's approved as a permanent wiring method, you can't use it.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"I didn't think you could just drill a hole in a junction box. If the manufacturer doesn't say it's approved for a particular wiring method, then it's not approved."

I cannot use my slug buster set?

Holes are made in junction boxes, cut out boxes, etc. all the time.

Boxes are not approved for a wiring method unless they have integral clamps.
Plastic box tab 'clamps' are rated for NM and nothing else.
Metal boxes with internal clamps are specific because of the clamps.

If you use the correct external clamp you can use any approved wiring method.

I am far more concerned that a run ground conductor is going to be required to use any metallic method so ground can be picked up, and that an approved cable be used.

A metal box can be grounded by AC or metallic conduit, but a plastic box needs a run ground conductor.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Mike makes an excellent point about putting a hole in a box designed for NM cable. Duh, don't know why I thought I could do that :-)

I guess I'll just use a metal j-box that accepts NM wiring and also MC, then string a length of MC into the firebox.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"Mike makes an excellent point about putting a hole in a box designed for NM cable. Duh, don't know why I thought I could do that"

You can do that as long as you use the correct cable clamp to secure the cable to the box.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Thanks for the information brickeye.

I just picked up a 25 foot section of 12/2 MC cable with ground (THHN wire). Now I need to find the correct cable clamp to secure the MC to the box. I was going to get two of the "screw in" type connectors that thread internally into the MC cable and get secured to the box with a lock ring, but the big box store didn't have those except by the box of 100. Will check another.

Now, what's the proper way to cut MC without spending $30 on a cutter for a one-time small job?

Cheers.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"I suppose I could have used a metal box in the wall, but I had already installed a plastic one before we made the decision on the gas insert and it just seemed easier to bore a hole in the side and attach the (MC?) cable."

Jeez. A craptastic decision, IMHO. Parse the code as you will, but a metal box is about $2 and it's the Kosher way to do it, IMO.

Oh,it might pass. (Then again, it might flunk as not being "workmanlike".) But, still, it screams "AMATEUR".

Uh, in my humble opinion, that is.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

He deoesn't have MC cable. I believe he has a flexible conduit with wires preinstalled in it (a whip for connecting air conditioners, etc..). Not that it matters.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"Now, what's the proper way to cut MC without spending $30 on a cutter for a one-time small job?"

With a hacksaw at 90 degrees to the armor spiral.

We did it that way for a ong time before the hand crank tyoe cutter was invented, and it was patented and expensive for many years.

Greenfield tubing is still cut with a hacksaw in many applications due to the limited size range of each crank cutter.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"He deoesn't have MC cable"

I do now (see my previous update).

As far as the plastic or metal box goes, I don't see any functional difference between them for this application. Cutting a hole in the plastic box may be amaturish... but I "am" an amateur :-) That said, I don't want any problem when it gets inspected so I may change to a metal box.

I picked up two of the "screw-in" fittings to connect the MC to the boxes. I assume those will be OK. If not, what is recommended.

Thanks.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

what screw in fittings are you talking about? If it's what I'm thinking of, they are for flex and not MC.


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Cable & Fitting Picture

Here's the cable and fitting I purchased. If the pictured fitting won't screw into the cable please recommend the proper type. When our new furnace was recently installed the electrician ran cable that looks exactly like in this picture, however where it mated to the metal box it used a cylindrical fitting that appears to screw-on over the outside of the cable since I don't see any other obvious attachment mechanism.

Hexus: please explain the difference between MC and Flex since they sound the same to me :-)


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

MC has wire already installed in it.
"Flex" is flexible spiral conduit (aluminum or steel) that does not have wire in it already and comes in various sizes.

Looking at the fitting in your picture I guess it is approved for use with MC (going off of what the package says). I've only used those types of screw in fittings with flex though and didn't think they were approved for MC.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Brickeyee,

If you use the correct external clamp you can use any approved wiring method.

A while back I asked about using a plastic box with MC and your response was that you hadn't seen a plastic box approved for metallic clad cable. So I could have just drilled out the box in question and used MC?


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

"A while back I asked about using a plastic box with MC and your response was that you hadn't seen a plastic box approved for metallic clad cable."

The plastic tab clamps in a plastic box are only approved for NM.

If you go to the trouble of making the correct size hole in the box and using an approved clamp you can use other methods, but you still have to watch for the grounding conductor.

If the cable does not have an actual grounding wire (and the bonding trip in AC does NOT count) then you have a way to connect to ground.

If you do not have a ground wire in the cable, you are going to have a problem attaching grounds in the box to just a cable clamp.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Even if the metallic cable has a grounding wire ran with it, the armor still has to contact a grounded connector. Sure couldn't do it with a plastic box at both ends, the armor would not be bonded. If the box at one end is grounded, it would ground the armor but I think but am not sure that the code enforces grounding of both ends for bonding purposes. Say the metallic cable was 100 ft long and the short happened at the end near the plastic box, the fault current would have to back track 100 ft to the metal box and then 100ft down the grounding wire. There may be a correct way to connect metallic to the plastic box but I believe you would need a bonding lock nut to bond the grounding wire to the cable armor.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Brickeyee,

Apparently there was some mis-communication about the use of plastic boxes with metallic clad cable. I knew that the clamps were rated for NM and presumed you were talking about the entire box.

Spencer,

Interesting question regarding bonding. Obviously we're talking about cable like type MC with a separate grounding wire or flex with a separate grounding wire. AC couldn't be used because there would be no way to ground the device ruling AC out immediately.

If there was a fault and the jacket became energized the current would have to travel down the jacket until it reached a metallic box. I don't understand your comment about 100 ft up and 100 ft back. Seems like it would be a straight shot to me but maybe I'm not seeing something.

If the code requires both ends to be bonded (I don't know) then a bonding lock nut seems to be the only way to accomplish that.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Definitely getting nick picky but I think the code does include this and I'm not sure how it is worded. I was using 100 ft as an example. It would be better for the fault 99 feet away to travel to the box 1ft away than to back track 99 ft. Seems it should be bonded at the 1st box that it leaves rather than relying on the equipment/ box down the line to take care of the bonding. Need a bonding expert to chime in on this.


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RE: OK to Connect Flexible Metal Cable to Plastic J-Box?

Since this run of MC is only 6 ft long bonding should be a non-issue, but it really depends what the code says for legality.

The easy way around all the 2nd guessing is to just change out the plastic j-box for a metal one. I think I can afford the extra buck fifty :-)

Good discussion.


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