GRC LOCKNUTS

[jimhibberd]

I'VE ADMITTEDLY GOTTEN A LITTLE RUSTY ON THE CODE BUT I THOUGHT THE CODE SPECIFICALLY REQUIRED TWO LOCKNUTS WHERE A THREADED CONDUIT ENTERS A BOX, ONE ON EACH SIDE OF THE KNOCKOUT. I'VE LOOKED BUT HAVEN'T FOUND WHAT I REMEMBER SEEING. CAN YOU HELP ME OUT?

 

[don_resqcapt19]

Two lock nuts are not always required. You can use a metal bushing and an exterior lock nut.

 

[infinity]

Two lock nuts are not always required. You can use a metal bushing and an exterior lock nut.

Yup, it had been done that way for 75 years or longer on conduits installed in decks. The key is that the bushing is metallic. If the conductor are #4 or larger then the bushing must be insulated or have an identified insulating material that is securely fastened in place.

300.4(G) Insulated Fittings. Where raceways contain 4 AWG
or larger insulated circuit conductors, and these conductors
enter a cabinet, a box, an enclosure, or a raceway, the con-
ductors shall be protected by an identified fitting providing
a smoothly rounded insulating surface, unless the conduc-
tors are separated from the fitting or raceway by identified
insulating material that is securely fastened in place.
Exception: Where threaded hubs or bosses that are an
integral part of a cabinet, box, enclosure, or raceway pro-
vide a smoothly rounded or flared entry for conductors.
Conduit bushings constructed wholly of insulating ma-
terial shall not be used to secure a fitting or raceway. The
insulating fitting or insulating material shall have a tem-
perature rating not less than the insulation temperature rat-
ing of the installed conductors.

 

[GoldDigger]

Yup, it had been done that way for 75 years or longer on conduits installed in decks. The key is that the bushing is metallic. If the conductor are #4 or larger then the bushing must be insulated or have an identified insulating material that is securely fastened in place.

Conduit bushings constructed wholly of insulating ma-
terial shall not be used to secure a fitting or raceway.

Does that mean that an entirely plastic bushing can be used, as long at there is something else to secure a conduit to the box?

 

[infinity]

Does that mean that an entirely plastic bushing can be used, as long at there is something else to secure a conduit to the box?

I'm guessing that a locknut would be required behind the plastic bushing.

 

[augie47]

Also take care on circuits over 250 volt to ground. See 250.97 for specifics. Where locknuts alone are acceptable, two are required.

 

[don_resqcapt19]

Also take care on circuits over 250 volt to ground. See 250.97 for specifics. Where locknuts alone are acceptable, two are required.

Some of the metal bushings are designed with teeth like lock nuts have so they can serve both purposes.

 

[augie47]

Some of the metal bushings are designed with teeth like lock nuts have so they can serve both purposes.

don:
Do you think that would fall under condition (4) ?

(1) Threadless couplings and connectors for cables with
metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate
metal conduit, one inside and one outside of boxes and
cabinets
(3) Fittings with shoulders that seat firmly against the box
or cabinet, such as electrical metallic tubing connectors,
flexible metal conduit connectors, and cable connectors,
with one locknut on the inside of boxes and
cabinets
(4) Listed fittings

 

[don_resqcapt19]

don:
Do you think that would fall under condition (4) ?

(1) Threadless couplings and connectors for cables with
metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate
metal conduit, one inside and one outside of boxes and
cabinets
(3) Fittings with shoulders that seat firmly against the box
or cabinet, such as electrical metallic tubing connectors,
flexible metal conduit connectors, and cable connectors,
with one locknut on the inside of boxes and
cabinets
(4) Listed fittings

I would accept it as list item 2.

 

[Volta]

I would accept it as list item 2.

Hmm, I would be hesitant to accept such as a worker may tighten such a toothed bushing until it feels tight, but without the teeth fully gripping the enclosure interior. Rather, it may easily bottom out on the end of the conduit instead. I would feel that such a bushing would not qualify for either the precise wording of 250.97 Ex.(2) or necessarily the intent of firm contact.

 

[don_resqcapt19]

Hmm, I would be hesitant to accept such as a worker may tighten such a toothed bushing until it feels tight, but without the teeth fully gripping the enclosure interior. Rather, it may easily bottom out on the end of the conduit instead. I would feel that such a bushing would not qualify for either the precise wording of 250.97 Ex.(2) or necessarily the intent of firm contact.

Poor workman ship lets the same thing happen with two lock nuts. Yes, it probably doesn't meet the letter of the rule, but I would accept it.