3/8" Flexible Metal Conduit

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jimc134

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chicago, il
Can I use 3/8" flexible metal conduit with two #12 THHN conductors (120V, 20A) to power a soup warmer in a kitchen cabinet? Length of FMC is less than six feet. Also, can you do this for a receptacle mounted under a kitchen cabinet?
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
3/8" FMC has very limited allowed uses in the NEC and when allowed it will have restrictions as to what you can use it for, not only does it have to follow what is required in 250.118 as was pointed out but also 348.20(A)(1)-(5) to which feeding a receptacle is not listed.

Also if we look at 348.60 if flexibility is required after installation then you have to install an EGC if this soup warmer is expected to be moved for cleaning which I think it will be, the problem can be if you use fittings that fit inside the 3/8" FMC then you are limited to just 2 #12 thhn/thwn conductors (table 348.22) which can be a problem installing an EGC so make sure you use connectors that go on the outside of the 3/8" flex so you can install an EGC.

Even with fixtures I have always installed an EGC, after seeing some 3/8" flex light up like a heater element from a short in a fixture it kind of scared me and I have always installed an EGC since then, but code does allow it to be used as an EGC on circuits up to 20 amps as long as flexibility is not expected after installation and listed connectors are used (250.118(5)a.)

This is about the best I can narrow the NEC requirements down of the limitations of using 3/8" flex in your installation, the end results will be up to your inspector.
 
peter d

peter d

Senior Member
Location
New England
hurk27 said:
3/8" FMC has very limited allowed uses in the NEC and when allowed it will have restrictions as to what you can use it for, not only does it have to follow what is required in 250.118 as was pointed out but also 348.20(A)(1)-(5) to which feeding a receptacle is not listed.
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:happyno:

Nowhere does the NEC forbid the use of 3/8" FMC to feed a receptacle.


Also if we look at 348.60 if flexibility is required after installation then you have to install an EGC if this soup warmer is expected to be moved for cleaning which I think it will be, the problem can be if you use fittings that fit inside the 3/8" FMC then you are limited to just 2 #12 thhn/thwn conductors (table 348.22) which can be a problem installing an EGC so make sure you use connectors that go on the outside of the 3/8" flex so you can install an EGC.
Click to expand...

Did you read the note under that table? An EGC may be added to the conductor count, so assuming you use an "inside" fitting, 3 #12's including the EGC are permissible.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
peter d said:
:happyno:

Nowhere does the NEC forbid the use of 3/8" FMC to feed a receptacle.
Click to expand...

348.20 clearly does not allow FMC smaller then 1/2" unless the use is listed in 348.20(A) (1) through (A)(5) and I don't see feeding a receptacle in it. so yes it does forbid it.

348.20 Size.
(A) Minimum. FMC less than metric designator 16 (trade
size 1⁄2) shall not be used unless permitted in 348.20(A)(1)
through (A)(5) for metric designator 12 (trade size 3⁄8).
Click to expand...

This is a case where because it is not listed it is not allowed because of the wording of 348.20(A)


peter d said:
Did you read the note under that table? An EGC may be added to the conductor count, so assuming you use an "inside" fitting, 3 #12's including the EGC are permissible.
Click to expand...

Good catch I did miss that little note, so you are correct an EGC can be added
 
peter d

peter d

Senior Member
Location
New England
hurk27 said:
348.20 clearly does not allow FMC smaller then 1/2" unless the use is listed in 348.20(A) (1) through (A)(5) and I don't see feeding a receptacle in it. so yes it does forbid it.
Click to expand...

I don't agree. The NEC is permissive, so if it's not expressly forbidden, then it's allowed.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
To be quit honest I don't see a problem with feeding a receptacle with 3/8" FMC and I have seen many manufactured office furniture systems use 3/8" FMC to feed receptacles and get them UL listed in doing so and it would be compliant as a listed assembly which is listed in 348.20, I have used it myself, but I do believe an inspector could tag it because of the wording in 348.20
 
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