Conduit hardware

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npattison

Member
Location
Washington
The question is how many 1" lock nuts are required (per NEC) to connect two metallic enclosures via 1" rigid conduit, 6" in length?
I've typically seen a total of 2 locknuts; one per the inside of each enclosure. One inspector has required 4; one on the inside and one on the outside of each enclosure. Can anyone shed some light on this? Each metallic enclosure is bolted down and will not experience the outdoor elements.
 
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earlydean

Senior Member
Re: Conduit hardware

I've looked in the Handbook....no answer. But, the pictures in Article 344 show locknuts on the outside of all enclosures.
250.118 identifies rigid metal conduit as an equipment grounding conductor. 250.120 requires all fittings to be made tight using suitable tools.
250.4(A)(3) requires rigid conduit to be connected together in such a way as to provide an effective ground-fault path.
250.4(A)(5) requires there to be a permanent, low impediance circuit capable of carrying any fault current likely to be imposed upon it.

I've looked in the UL White book.....the only place I could see, that even discussed locknuts was CONDUIT FITTINGS (DWTT), under sealing locknuts opposing locknuts are required, only where one or both of them are sealing locknuts to ensure watertightness.

Apparently, it is not specifically required, but is the most economical and commonly applied way applied to achieve the required grounding and bonding.

Earl
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Re: Conduit hardware

I have always used 4 and never saw it done any other way. If you consider concentric KO's and sheet metal flexing you begin to see why two lock nuts on each end tightened against each other with the sheet metal in between is the only reliable way to insure that ground continuity is maintained.
 
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Guest

Guest
Re: Conduit hardware

Using only two is going to bow the panel sides if the locknuts are tightened up securely.

If they are not tightened securely then the bond will suffer.

The best way to prevent panel bowing; get proper tightness; and proper bonding is to use four locknuts. The locknut teeth are supposed to bite into the metal. They can't bite in if they're not tight.

With two nuts total even if they did bite in with thermal flexing (hot/cold cycles) the bite would be lost over time.

Four is good.

Two is cheesey.
 
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gregoryelectricinc

Senior Member
Re: Conduit hardware

I agree with Wayne...4 is good. In my area, no way would an inspector sign off on the 2 locknut method without at least installing bond bushings on both ends of the conduit (system must be intrinsically safe). Just my 2 pennys.
Jim
 
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