ground bushing vs. plastic bushing

[misterbill1972]

I have a MSB going to a 400A stand up section. This is in a 4" EMT conduit, ss connectors, 600 MCM wire, 3/0 ground. This is a straight run with no stops. There is no concentric K.O. I have P.B. on both ends. I was told one end needs to have a G.B. I cannot find this in code, but it does ring a bell.

 

[raider1]

After the service disconnecting means, there are very few places that you are required to have a bonding bushing. Take a look at 250.97 for bonding over 250 volts.

Chris

 

[barbeer]

What is an MSB? Does 250.92 apply? if so MEG em!

 

[augie47]

Your reference would be 250.92(B) if it's "service conductors", which, from your description, I believe it is not.
or 250.96 and 250.97 if either are applicable to your particular installation.

 

[infinity]

I'm assuming that you're going from a MSB (main switchboard) to a 400 separate section. Do you have an OCPD ahead of the 400 amp feeder or a tap off of the line side of the service disconnect?

 

[misterbill1972]

It is a 400A breaker feeding a 400A main breaker stand up panel

 

[Smart $]

As per 250.97, if over 250 Volts to ground, bonding per 250.92(B), except (B)(1), is required. Many make the mistake of overlooking 250.92(B)(2), where it says, "...threaded bosses on enclosures where made up wrenchtight."

If your run uses [listed for grounding] ss connectors with threaded bosses, no bonding bushing is required based on voltage over 250 to ground.

 

[infinity]

It is a 400A breaker feeding a 400A main breaker stand up panel

Then no bonding bushing are required. The EMT connectors are suitable for bonding in this setup.

 

[stickboy1375]

I have a MSB going to a 400A stand up section. This is in a 4" EMT conduit, ss connectors, 600 MCM wire, 3/0 ground. This is a straight run with no stops. There is no concentric K.O. I have P.B. on both ends. I was told one end needs to have a G.B. I cannot find this in code, but it does ring a bell.

I hate acronyms... :grin:

 

[raider1]

I hate acronyms... :grin:

" Seeing as how the V.P. is such a V.I.P. shouldn't we keep the P.C. on the Q.T. 'cause if it leaks to the V.C. he could become a M.I.A. and then we'd all be put out on K.P. "

(Adrian Cronauer: Good morning vietnam)

Chris

 

[Mike01]

600Kcmil's

600Kcmil's

Wow 600Kcils run for a 400A panel board why not 500?s? Make sure you can terminate the 600?s in the panel board a lot of times the lugs are only suitable for up to 500?s and even if you replace the lugs for 600?s I would be willing to bet in a standard panel you would violate the bending radius required for the cable size. You might want to check the manufacturers literature most have two columns one indicating the ?actual lug size? and the one adjacent to it indicating the ?required UL wire bending space per NEC table 373-6. if you are using mechanical lugs you will probably be ok but if you use compression lugs it might be a problem with copper conductors.

 

[infinity]

You might want to check the manufacturers literature most have two columns one indicating the ?actual lug size? and the one adjacent to it indicating the ?required UL wire bending space per NEC table 373-6.

What code year are you quoting?

 

[Smart $]

Wow 600Kcils run for a 400A panel board why not 500’s?

...and just how is it that you see 500's as havng a 400A feeder ampacity (i.e. assuming you are not using the next standard size exception for breaker rating, as there is no indication in the OP or follow-up)?

 

[crossman]

As per 250.97, if over 250 Volts to ground, bonding per 250.92(B), except (B)(1), is required. Many make the mistake of overlooking 250.92(B)(2), where it says, "...threaded bosses on enclosures where made up wrenchtight."

If your run uses[listed for grounding] ss connectors with threaded bosses, no bonding bushing is required based on voltage over 250 to ground.

I agree that the installation is okay without grounding bushings, but disagree on your reference and reasoning.

IMHO, in 250.92(B)(2), the "threaded bosses" are not referring to seperate connectors which are attached to the enclosure by a locknut such as a set-screw connector. Threaded bosses are an integral part of the enclosure and contain female threads in which a threaded conduit is screwed into.

I have never seen a set-screw connector with a threaded boss. The threads on the connector are not a "boss". As I said, a "boss" is part of the enclosure and is typically cast when the entire box is cast, for example, a cast-iron FS box has a "casted on" boss with female threads to which a threaded conduit is screwed into.

The actual reference which applies is 250.92(B)(3) which is for threadless connectors and couplings. The "threadless" is not referring to the threads for the locknut, the "threadless" is referring to where the conduit enters the connector, such as a set screw connector.

 

[Smart $]

I agree that the installation is okay without grounding bushings, but disagree on your reference and reasoning.

IMHO, in 250.92(B)(2), the "threaded bosses" are not referring to seperate connectors which are attached to the enclosure by a locknut such as a set-screw connector. Threaded bosses are an integral part of the enclosure and contain female threads in which a threaded conduit is screwed into.

I have never seen a set-screw connector with a threaded boss. The threads on the connector are not a "boss". As I said, a "boss" is part of the enclosure and is typically cast when the entire box is cast, for example, a cast-iron FS box has a "casted on" boss with female threads to which a threaded conduit is screwed into.

I'll concede to your refutal of 250.92(B)(3). However, I must note it is a matter of interpretation. What you are calling a "boss", I call a "hub", integral if part of the enclosure. I believe you are also associating female threads with your concept of a boss. The NEC does not define the term "boss", and I further believe it to be a carry-over term from pipefitter terminology. For the sake of a better understanding of the term "boss" consider the following...

Webster's Revised Unabridged Dictionary (1913)

Boss
(v. t.) To ornament with bosses; to stud.

(n.) The enlarged part of a shaft, on which a wheel is keyed, or at the end, where it is coupled to another.

(n.) Any protuberant part; a round, swelling part or body; a knoblike process; as, a boss of wood.

(n.) A wooden vessel for the mortar used in tiling or masonry, hung by a hook from the laths, or from the rounds of a ladder.

(n.) A swage or die used for shaping metals.

(n.) A protuberant ornament on any work, either of different material from that of the work or of the same, as upon a buckler or bridle; a stud; a knob; the central projection of a shield. See Umbilicus.

(n.) A projecting ornament placed at the intersection of the ribs of ceilings, whether vaulted or flat, and in other situations.

(n.) A master workman or superintendent; a director or manager; a political dictator.

(n.) A head or reservoir of water.

Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter.

The actual reference which applies is 250.92(B)(3) which is for threadless connectors and couplings. The "threadless" is not referring to the threads for the locknut, the "threadless" is referring to where the conduit enters the connector, such as a set screw connector.

Moreover, your reference to 250.92(B)(3) can also be contested on the basis of interpretation. Typical "threadless" couplings and connectors are used on raceways (RMC, IMC) which typically use threaded connectors and couplings. EMT connectors do not qualify in this regard.


***

So perhaps we must all concede to 250.92(B)(4) and whether a set-screw EMT connector is listed for grounding.

 

[crossman]

Smart, thanks for your commentary.

My opinion follows: (of course, your opinion is just as good as mine:smile: )

Boss = integral part of the enclosure with female threads.

Hub = a seperate fitting which attaches to the box and has female threads

Threadless fitting = set screw or compression fitting, could be for RMC, IMC, or EMT and perhaps others.