Rigid metal conduit

[wyboy]

Where is it required in the NEC that Rigid metal conduit have a locknut on the inside and outside of the enclosure. I know it is required, but I can?t find it in the white book or the NEC

 

[hurk27]

Where is it required in the NEC that Rigid metal conduit have a locknut on the inside and outside of the enclosure. I know it is required, but I can?t find it in the white book or the NEC

The only place in the NEC where if specifically states this is 250.97(2) for voltages over 250, and a few places in article 550, but how else are you to secure RMC to a box?

 

[jumper]

The only place in the NEC where if specifically states this is 250.97(2) for voltages over 250, and a few places in article 550, but how else are you to secure RMC to a box?

Threaded hub.

 

[augie47]

The only place in the NEC where if specifically states this is 250.97(2) for voltages over 250, and a few places in article 550, but how else are you to secure RMC to a box?

A locknut on the outside with a malleable bushing on the inside was acceptable practice around here for years.
That said, I've seen shorts develop on some old "rigid" installs and it looks like someone lit sparklers at j boxes all the way back to the panel.

 

[tkb]

Where is it required in the NEC that Rigid metal conduit have a locknut on the inside and outside of the enclosure. I know it is required, but I can’t find it in the white book or the NEC

You can use a single locknut and a metal bushing and not double locknuts, except:
250.97 execption 2

300.4(G) handbook commentary shows that double locknuts are needed if you use a plastic bushing.

501.30(A), 502.30(A), 503.30(A), 505.25(A) & 506.25(A) refers to "locknut-bushing" and "double locknut" methods.

Double locknuts are specifically required per 550.16(F), 551.47(B) & 552.48(B)

Handbook Index,
Locknuts, double, required
Hazardous (classified) locations, 501.30(A)
Mobile homes, 550.15(F)
Over 250 volts to ground, 250.97 Ex.
Recreational vehicles, 551.47(B)

 

[infinity]

A wholly metal bushing and one locknut is permitted if the conductors are #6 and smaller. #4 and larger would require a bushing that is insulated.

 

[tkb]

A wholly metal bushing and one locknut is permitted if the conductors are #6 and smaller. #4 and larger would require a bushing that is insulated.

I agree and double locknuts would be required unless you had a metal bushing that was also insulated.
Double locknuts and a plastic bushing would cost less than the bushing.

http://www.emersonindustrial.com/en...ted-bush-imc/SpecGrade-Insulated-Bush-IMC.pdf

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 conductors shall be protected by an identified fitting providing a smoothly rounded insulating surface, unless the conductors 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 provide a smoothly rounded or flared entry for conductors.

Conduit bushings constructed wholly of insulating material shall not be used to secure a fitting or raceway. The insulating fitting or insulating material shall have a temperature rating not less than the insulation temperature rating of the installed conductors.

It's funny that it doesn't say specifically that you can used the single locknut and metal bushing method, but it eludes to it.

 

[Ponchik]

So, by reading these posts i understand that RMC connecting to a box or cabinet doesn't require (2) locknuts? one inside and one outside????
How will the conduit be secured to the box?

am i understanding this correctly? so if i install one locknut on the inside of the box it is OK?

 

[K8MHZ]

The only place in the NEC where if specifically states this is 250.97(2) for voltages over 250, and a few places in article 550, but how else are you to secure RMC to a box?

Make sure about three threads stick into the box, no more. Then secure said RMC so that no more threads can push into the box. Apply locknut on inside of box and tighten until box starts to bend.

I have actually seen this done.

 

[K8MHZ]

So, by reading these posts i understand that RMC connecting to a box or cabinet doesn't require (2) locknuts? one inside and one outside????
How will the conduit be secured to the box?

am i understanding this correctly? so if i install one locknut on the inside of the box it is OK?

It depends on your definition of 'OK'. To me, it's not OK, no matter if the NEC addresses it or not. Remember, if you follow the NEC to the letter you will be no more than the worst electrician allowed by law.

 

[tkb]

So, by reading these posts i understand that RMC connecting to a box or cabinet doesn't require (2) locknuts? one inside and one outside????
How will the conduit be secured to the box?

am i understanding this correctly? so if i install one locknut on the inside of the box it is OK?

To secure RMC to a box with one locknut, the locknut wouls have to be on the outside and a metal bushing on the inside.
The locknut on the outside would have to be tightend after the metal bushing is screwed on the end of the RMC so there is contact between the box, bushing and locknut.

 

[rbalex]

...
501.30(A), 502.30(A), 503.30(A), 505.25(A) & 506.25(A) refers to "locknut-bushing" and "double locknut" methods...

You might want to look at the various hazardous location applications mentioned where "... locknut-bushing and double-locknut types of contacts shall not be depended on for bonding purposes, but bonding jumpers with proper fittings or other approved means of bonding shall be used." [Underline mine] Obviously it doesn't prohibit it, but you need to be prepared for a much more rigorus bonding method.

 

[tkb]

You might want to look at the various hazardous location applications mentioned where "... locknut-bushing and double-locknut types of contacts shall not be depended on for bonding purposes, but bonding jumpers with proper fittings or other approved means of bonding shall be used." [Underline mine] Obviously it doesn't prohibit it, but you need to be prepared for a much more rigorus bonding method.

I agree that there may be additional bonding required for hazardous locations, but I was just trying to point out that for systems under 250 volt to ground, the single locknut-bushing method is not prohibited by code and acceptable for bonding purposes for RMC and IMC.

 

[realolman]

.... Remember, if you follow the NEC to the letter you will be no more than the worst electrician allowed by law.

that's excellent

 

[infinity]

To secure RMC to a box with one locknut, the locknut wouls have to be on the outside and a metal bushing on the inside.
The locknut on the outside would have to be tightend after the metal bushing is screwed on the end of the RMC so there is contact between the box, bushing and locknut.

I agree that there may be additional bonding required for hazardous locations, but I was just trying to point out that for systems under 250 volt to ground, the single locknut-bushing method is not prohibited by code and acceptable for bonding purposes for RMC and IMC.

I agree. This has been SOP for deck boxes for 100 years.

 

[K8MHZ]

that's excellent

I have to give credit where credit is due.

I heard that from a friend, fellow ham and chief electrical inspector for Muskegon. I have used it more than once.....and got into some interesting debates right here on MH about it.