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Discuss Why are metal electrical boxes so small (compared to PVC?) in the American Professional Electrical Advice Forum area at ElectriciansForums.net

mjonis

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Hello all. First post. DIYer. Know enough to be dangerous (LOL).
It seems "standard" blue PVC boxes here in the US are 2.25" wide and depth can go up to 3" deep.

I've got an old house (built in 1955) with old BX armor shielded cable, and the old boxes are extremely difficult to put a dimmer or GFCI receptable in them. I tried hunting around and it seems if I want a 3" deep single gang box, it's only 2" wide. I can find a 2 1/8" wide box, but it's only like 2" deep.

Is there a reason that the metal boxes don't seem to be made as big as the blue PVC ones? (ie: 3.25" wide and 3" deep)?

Or is my Amazon/google-fu searching (single gang metal electrical box 2.25" wide) not sufficient and they are really made by someone somewhere?
 

Megawatt

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Hello all. First post. DIYer. Know enough to be dangerous (LOL).
It seems "standard" blue PVC boxes here in the US are 2.25" wide and depth can go up to 3" deep.

I've got an old house (built in 1955) with old BX armor shielded cable, and the old boxes are extremely difficult to put a dimmer or GFCI receptable in them. I tried hunting around and it seems if I want a 3" deep single gang box, it's only 2" wide. I can find a 2 1/8" wide box, but it's only like 2" deep.

Is there a reason that the metal boxes don't seem to be made as big as the blue PVC ones? (ie: 3.25" wide and 3" deep)?

Or is my Amazon/google-fu searching (single gang metal electrical box 2.25" wide) not sufficient and they are really made by someone somewhere?
Since you have a house that was built in 1955 that’s all they had to work with. You can turn off the power to every box you need bigger and the box will probably have a mail holding it in so just tear it out and use a blue plastic cut in box. You will have to trim out the sheet rock to make it fit
 
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mjonis

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Since you have a house that was built in 1955 that’s all they had to work with. You can turn off the power to every box you need bigger and the box will probably have a mail holding it in so just tear it out and use a blue plastic cut in box. You will have to trim out the sheet rock to make it fit

You can't (AFAIK) use BX metal/armor shielded cable with a plastic box (that's' what provides the ground as it were). Romex (NM-B) yes, but that would require re-wiring the entire house. Plus it doesn't answer the question (one would think nowadays with GFCI, etc. they'd make a bigger metal box).
 

Megawatt

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You can't (AFAIK) use BX metal/armor shielded cable with a plastic box (that's' what provides the ground as it were). Romex (NM-B) yes, but that would require re-wiring the entire house. Plus it doesn't answer the question (one would think nowadays with GFCI, etc. they'd make a bigger metal box).
They do make metal boxes that’s plenty deep enough to accommodate MC cable which you didn’t mention
 
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mjonis

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It’s not called bx cable it’s called MC or metal clad cable and check with your electrical parts house. That’s what you get when your working on an old house

They do make metal boxes that’s plenty deep enough to accommodate MC cable which you didn’t mention
If you read my post, it's not the DEPTH, it's the WIDTH that's the issue.

We're almost 75 years later, and new construction in some places requires metal shielded cable/boxes, so one would think that someone somewhere makes a metal box that's the same size as the PVC boxes. If they don't, fine, is there some technical reason that it's that way, or is it one of the great mysteries of life?
 

norcal

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The old cable was AC, not MC, they each have their own NEC articles, and installation requirements, one difference is AC requires the use of anti short bushings, & MC does not, & older AC cable without a bond strip is not suitable for grounding, in a fault the armor could be glowing red if it is relied on for a ground, as to metal boxes my preferred method is a 4X4 (1900) box, & a raised ring 1/8" deeper then the planned wall covering material, so for 1/2" drywall, use a 5/8" ring, & 5/8" use a 3/4" ring, using one the same depth will result in it being 1/8" shy of the finished surface, at trim out time there will not be any floating devices which is a pet peeve of mine when they are not nice & snug against the box & finished surface.
 

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