Bonding Conduits

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Brandon Loyd

Senior Member
Correct me if I am wrong, but if a metel conduit enters a metal enclosure through a concentric knockout or reducing bushings, doesn't code require you to use a ground/bond bushing and/or a Meyers hub? What about a 4s box with concentric knockouts, or are those not considered concentric? What about a piece of MC landed to a metal enclosure through a concentric knockout?
 
It is not so easy.

Lets start with the main code section

250.97 Bonding for Over 250 Volts. For circuits of over
250 volts to ground, the electrical continuity of metal raceways
and cables with metal sheaths that contain any conductor
other than service conductors shall be ensured by
one or more of the methods specified for services in
250.92(B), except for (B)(1).

Exception: Where oversized, concentric, or eccentric knockouts
are not encountered, or where a box or enclosure with
concentric or eccentric knockouts is listed to provide a
reliable bonding connection, the following methods shall be
permitted:

(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
Click to expand...
 
I didn't realize there were concentric knockout listed as being a good bond. Would 4s and 5s box be considered the type listed as a good bond? This wouldn't resolve an issue where a MC is landed in a non-listed concentric.
 
Brandon Loyd said:
I didn't realize there were concentric knockout listed as being a good bond. Would 4s and 5s box be considered the type listed as a good bond? This wouldn't resolve an issue where a MC is landed in a non-listed concentric.
Click to expand...

I think you will find that the 4 and 5 boxes you are using are in fact listed for grounding, they meet the "eccentric knockouts is listed to provide a reliable bonding connection" requirement.

Do you know the model and brand boxes you are using?
 
Do the same rules apply for reducing washers? Am I correct in saying you can use either a bond bushing or a myers hub to meet bonding requirements?
 
Brandon Loyd said:
Do the same rules apply for reducing washers? Am I correct in saying you can use either a bond bushing or a myers hub to meet bonding requirements?
Click to expand...

Reducing washer are in fact listed for grounding. You could blow out all the concentric KOs, install reducing washers and call it a day.

As far a a Myers hub, if it is in the right size KO you are fine, if it is in concentrics I don't think so unless it is a bonding type hub with a lug for a bond wire.

But as Infinity asked, what voltage? it makes a difference.
 
That hard to believe reducing bushing create a better bond than concentric knockouts. I have always been taught Myers Hubs could be used in leui of bond bushings. How are the rules different for 120/208&240 systems vs. 277/480 systems? I was unaware there were different rules.
 
How would you know if a concentric knockout was listed for grounding if the panel or j-box is old and/or has no lableing?
 
Brandon Loyd said:
How would you know if a concentric knockout was listed for grounding if the panel or j-box is old and/or has no lableing?
Click to expand...

If the box did not have any labeling and I could not find it or did not want to take the time to find it I would just go with the worst case scenario. Not sure of your exact reason for asking if this is an actual installation or a test or quiz question, but if its 250V or higher and you do not know just use a bonding bushing or or hub that you can bond.
 
We perform Infrared and Ultrasonic inspections. We also look for code violations and electrical safety hazards during our inspections. I am trying to generalize this bonding issue as much as possible, so I can easily explain it to others. So if I am following right so far, some concentric ko are listed for grounding and some aren't. 250v and higher, if I can't find labeling that says it's listed for grounding, I must assume this is a code/safety problem that should be addressed.

What if I am dealing with 240v or less? How do the rules differ?
 
With 250 volts or less to ground, unless it is a service conduit, there is seldom a requirement for
bonding other than standard terminations to enclosures. (Loosely joined raceways, etc would be the exception where a bonding jumper would be required)
 
Brandon Loyd said:
That hard to believe reducing bushing create a better bond than concentric knockouts. I have always been taught Myers Hubs could be used in leui of bond bushings. How are the rules different for 120/208&240 systems vs. 277/480 systems? I was unaware there were different rules.
Click to expand...

As pointed out, see 250.97. Concentric KOs are not intended to withstand the heat generated by a 277 V ground fault. With a 1 ohm ground fault, 120 volts (using P= I squared R) generates about 15,000 watts for heat, but at 277 its 75,000 watts.

A meyers hub is not listed as a bonding bushing, its listed as a fitting. Meyers hubs have a grounding lock ring that is available.

If you can, get a copy of Mike Holts DVD on Grounding vs Bonding...
 
REDEYE123

REDEYE123

If you look at the lable on the cardboard box that the 4 S boxes come in it will say that "no additional grounding is required for voltage over 250"
 
iwire said:
Reducing washer are in fact listed for grounding. You could blow out all the concentric KOs, install reducing washers and call it a day.

As far a a Myers hub, if it is in the right size KO you are fine, if it is in concentrics I don't think so unless it is a bonding type hub with a lug for a bond wire.

But as Infinity asked, what voltage? it makes a difference.
Click to expand...

The code references using fittings designed to make paint removal unnecassary on enclosures. Locknuts work well for this because they bite into the metal. If you were to use reducing bushings, the locknuts wouldn't be able to make contact with the metal behind the paint. Do they use conductive paints and enamels on enclosures to eliminate this problem?
 
Brandon Loyd said:
How would you know if a concentric knockout was listed for grounding if the panel or j-box is old and/or has no lableing?
Click to expand...
As far as I know there are no panels so listed. Not sure about junction boxes, there may be so. The only thing I have seen listed for that is 4" and 4 11/16" boxes.
 
Raco makes a 4S box with concentric KOs listed for bonding > 250 Volts, its called a TKO, and its obvious as the KOs take some effort to remove. Contrast that to a panel with KOs you can push out by hand. The listing information would be on the carton the 4S boxes came in.
 
Brandon Loyd said:
The code references using fittings designed to make paint removal unnecassary on enclosures. Locknuts work well for this because they bite into the metal. If you were to use reducing bushings, the locknuts wouldn't be able to make contact with the metal behind the paint. Do they use conductive paints and enamels on enclosures to eliminate this problem?
Click to expand...

You are bringing up the wording in 250.12

250.12 Clean Surfaces. Nonconductive coatings (such as
paint, lacquer, and enamel) on equipment to be grounded shall
be removed from threads and other contact surfaces to ensure
good electrical continuity or be connected by means of fittings
designed so as to make such removal unnecessary.
Click to expand...


In my opinion we would have to remove the paint to use reducing washers as a fault path as they are not designed so as to make such removal unnecessary.
 
iwire said:
You are bringing up the wording in 250.12




In my opinion we would have to remove the paint to use reducing washers as a fault path as they are not designed so as to make such removal unnecessary.
Click to expand...

Paint removal would make sense to me, or just install a bond bushing. As far a looking at the packaging to find out if it is listed or not, this wouldn't work for me. I am inspecting previous installations, and would need to be able to identify that installation as being a violation or not. Does any know how to tell just by looking at the KO?
 
Brandon Loyd said:
Paint removal would make sense to me, or just install a bond bushing. As far a looking at the packaging to find out if it is listed or not, this wouldn't work for me. I am inspecting previous installations, and would need to be able to identify that installation as being a violation or not. Does any know how to tell just by looking at the KO?
Click to expand...

Did this work pass inspection when first installed ? If so, what justification is there to inspect

to todays code ? There are lots of homes without GFCI, AFCI, smoke dectors, etc, etc, are
[
we expected to bring these prior installations up to code ? What a nightmere !
 
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