Nec 342.46

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erickench

erickench

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Brooklyn, NY
I look at the above NEC section and I compare it with NEC 300.4(F). I see two different rules for the same situation. In one case the rule says that RMC entering an enclosure must be provided with a bushing regardless of the size of the conductor. But in the other case the rule says that a bushing must be provided if the conductor is No. 4 AWG or larger. Now what makes these two situations different? The only thing I can say is that NEC 300.4(F) should be written so that the rule apply's to all conduit except RMC and IMC.
 
There is nothing wrong with it as it stands, we simply meet the most restrictive rule.

When we run RMC its bushings all around, when we run EMT we don't have worry about bushings until we run into 300.4(F).
 
342.46 Bushings.
Where a conduit enters a box, fitting, or other enclosure, a bushing shall be provided to protect the wire from abrasion unless the design of the box, fitting, or enclosure is such as to afford equivalent protection.
FPN: See 300.4(G) for the protection of conductors 4 AWG and larger at bushings.
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300.4(G) Insulated Fittings. Where raceways contain 4 AWG or larger insulated circuit conductors and these conductors enter a cabinet, box, enclosure, or raceway, the conductors shall be protected by a substantial fitting providing a smoothly rounded insulating surface, unless the conductors are separated from the fitting or raceway by substantial 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.
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300.4 would apply to other raceways like EMT for #4 and larger conductors which don't require bushings for smaller conductors. 342.46 is more restrictive. Not sure I'm properly following the question?
 
Nec 300.4(f)

Nec 300.4(f)

Okay I tried to edit it but I ran out of time. I'll rephrase. NEC 300.4(F) state's that a raceway containing conductor size no. 4 AWG or larger entering an enclosure is to be provided with a rounded insulating surface. But after the exception the same section goes on to say that a bushing shall not be used to secure a raceway. Therefore the rounded insulating surface could not be a bushing because it would be in conflict with NEC 342.46 where it states that an RMC entering an enclosure would have to be provided with a bushing regardless of the size of the conductor. If we were to assume that this rounded insulating surface is a bushing then there would be two different rules for the same situation.
 
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Thought about this some more. For RMC with conductors smaller than #4 a metal bushing with one locknut on the outside would comply. When you went up to #4 and larger conductors two locknuts and a plastic bushing would comply.
 
Okay I just read a little bit about Electrical Bushings on WIKIPAEDIA. It seems they all contain insulating material. They do make metal bushings but I don't think they are suitable for electrical.
 
The reason for the bushing on RMC and IMC regardless of conductor size vs. 300.4 is due to the availability of field threading...is what I have been told as far as sharp edges. etc...unless someone already mentioned that or that is incorrect.
 
Yup, metal bushings were the norm on concrete slab construction back in the day. Typically seen on 1/2" and 3/4" conduit encased in the concrete slab with "mud" boxes. Metal bushing on the inside and locknut on the outside. Saved one locknut at every termination.
 
erickench said:
Okay I just read a little bit about Electrical Bushings on WIKIPAEDIA. It seems they all contain insulating material. They do make metal bushings but I don't think they are suitable for electrical.
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No, not all pipe bushing are made with insulating material.

I can still buy steel conduit bushings.


See the top of the first page of this pdf

http://www.o-zgedney.com/PDF/OZG_DB_MalleableIronConduitBushings[1].pdf





They are not a bad idea when running 1/2" and 3/4" conduits into stamped steel boxes. By using the steel bushing I can eliminate the lock nut inside the box and still have my bushing. The reason is that when bringing two conduits into the corners of a typical 4" square box the conduits can hit each other inside the box so you want to keep the pipes out of the box as much as possible.
 
infinity said:
Yup, metal bushings were the norm on concrete slab construction back in the day. Typically seen on 1/2" and 3/4" conduit encased in the concrete slab with "mud" boxes. Metal bushing on the inside and locknut on the outside. Saved one locknut at every termination.
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and no equipment grounds pulled :D
those were the "good days". I have seen ground faults at one end of a long run with many boxes and sparks would rain from several boxes at the loose locknut.
 
The EMT box connectors have smooth openings to satify #6 or smaller wires.
With RMC you secure the conduit with locknuts on each side of the box. Yes they do make
metal as well as plastic bushings.
 
I think the two differ by means of running threads on threaded pipe verses threaded fittings, I know if I were placing a 1/2 inch threaded rigid pipe into a box with a lock nut on each side, I would place a plastic bushing on it.
If it were a 1/2 EMT fitting, I wouldn't be to concerned with placing a plastic bushing on it.

My interpretation is, bush all running threads on every size rigid pipe unless you purchased a compression fitting for rigid pipe..; for all other different electrical pipes apply 300.4 (G)
 
Okay we'll just interpret the rule as applying to all conduit other than RMC and IMC. RMC and IMC must always have a bushing for insulated conductors entering an enclosure regardless of size. The other types of conduits will be provided with bushings if the insulated conductor is no. 4 or larger.:)
 
That's the way I was taught back in the late seventies by every inspector that demanded that I did it that way. Rigid pipe has sharper threads that are cut in, where is fittings are manufactured in a molding process..
 
David C. Adkins said:
... the two differ by means of running threads on threaded pipe
verses threaded fittings ...
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David,

Yes. Good observation.

Rigid threads will cut/eat cable insulation,
thus the NEC ruling for Rigid threads,
all sizes conduit and all sizes cable.

Other conduits will not cut smaller cable
because the cable is more flexible and folds around.
#4 and larger is gradually stiffer and will be / can be pressed very tightly against the edge of a conduit connector,
thus the NEC rules that a bushing is necessary.
 
erickench said:
just wish they would clarify the section in the code.
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Eric,

You are a P.E. and I am just a college guy with a license.
Most of the guys have a lot of experience that I don't have.
Over the code versions since 1965
I have noticed many subtle changes, and clarifications,
and a gradual and careful re-evaluation of electrical fire safety.

To most experienced electricians, these things make sense.
They work with the material, stub their toes on it, bend it, measure it,
and usually have a feeling for the intent of the NEC.

I just keep feeling around for the physics of the issue,
and feeling around for the logical 'intent' of the code.
Usually I come out on top, I think.

But I still read these threads with interest and curiosity
as to the thoughts of experienced electricians and engineers.
 
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