Grounding conductor in flexible metallic tubing?


Old 03-23-05, 05:49 PM
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Grounding conductor in flexible metallic tubing?

NEC 250-91 b lists flexible metallic tubing right along EMT, etc. as an acceptable ground without any additional ground conductor.

However, Exception No.1 seems to say that if flexible metallic tubing (Greenfield) is used in lengths over 6' or for more than 20 amps, a separate grounding conductor is required. Am I reading that right, or can I use 25' +/- lengths of 3/4" Greenfield with 3 #8's for a 40-amp oven service, without any additional grounding conductor ?

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Old 03-23-05, 06:25 PM
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You are reading it right.
You need a #10 ground run with the #8's. Why would you not want to run a ground?
Old 03-23-05, 06:45 PM
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Not that I don't want to run a ground, but I've almost never seen it done with metallic tubing.

Everything is required to be in conduit here (Chicago area) and EMT is always legal as a ground. I pretty much never use more than a foot or two of Greenfield, and since I don't use it much, I don't know the rules for longer runs of flexible metallic tubing.

In this case it's either use Greenfield or cut about 40' worth of slots into the wall and soffit to install conduit runs for a cooktop and double oven. Not a hard decision, and not hard to add a ground wire now before I've cut or pulled any wire at all.

Thank you !
Old 03-23-05, 07:20 PM
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Originally Posted by KVacek
...but I've almost never seen it done with metallic tubing.....
I find this surprising. I almost never see it not done.
I never like to rely on the integrity of a conduit system for my grounds, even though it is legal. I have seen way too many conduit runs/fittings get compromised. Once I leave the site who knows who will hang what on the conduit. Or what machine will run into it.
I think you get the idea.
Old 03-24-05, 12:40 PM
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You need a newer code edition!

There is no 250.91 in any recent NEC code.

Try 250.118 which describes which wires and raceways are permitted to serve as an EGC...this EGC is NOT a BONDING jumper!

Flexible Metallic Tubing [FMT] cannot be used in lengths exceeding 6 ft. It is NOT 'greenfield'..and cannot be mfg larger than trade size 3/4''

Greenfield CAN be installed in lengths exceeding 6ft.

Flexible Metallic Conduit [FMC] CAN be used in lengths exceeding 6 ft.It is also known as 'greenfield'...and can be mfg up thru size 4''

In circuits 20amps or less,and in lengths of 6ft or less....Both FMT and FMC are permitted to be the required circuit EGC .

Both are commonly used as 6ft fixture whips .

All greenfield runs [FMC] longer than 6ft require a EGC wire included ...[IF] the FMC and fittings are NOT listed for grounding.2002 NEC 250.118

You are correct ;Tf the product is listed for grounding,you can install 25ft of 3/4 " greenfield [FMC] with (3) 8 AWG THHN and use the FMC as a ground EGC... provided that both the FMC and also the fittings are listed for grounding.

3/4" FMC is good for (6) 8 AWG THHN....a copper 10AWG is ok for EGC up thru 60amps...and will fit easily.

If the FMC is ran enclosed ,PROTECTED within the the structure ,you may decide for your ownself whether to include the wire EGC.and I would agree with you....but see UL product standard 1 where you may discover that you may have a tough time finding any FMC listed for grounding in lengths over 6ft.

Exposed.....INCLUDE THE EGC..even if UL changes their mind!

NEC 2002
FMC art 348
FMT art 360
EGC art 250.118 1 thru 14
(5) is used for FMC and fittings that are listed for grounding.
UL product standard 1
found a couple UL links for you..
"FMC......Flexible metal conduit longer than six ft has not been judged to be suitable as a grounding means..."

FMT...Flexible metallic tubing is permitted to be used in lengths of six ft or less, in dry locations, in accessible locations when protected from physical damage or concealed, to contain branch circuit conductors at a maximum potential of 1000 V and when terminated in suitable fittings.

FMT...Flexible metallic tubing containing circuit conductors protected by overcurrent devices rated 20 A or less is suitable as a grounding means.

Last edited by driploop; 03-24-05 at 03:10 PM.

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