Bus Duct with no Ground

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tmillard

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Just ran into a strange problem the other day. We are replacing a 1200A panel that is fed with bus duct from a switchboard. After the contractor opened up the duct, we realized there was no ground bus bar (or neutral bus bar either). So we called the manufacture, Sq D, and they noted that it wasn't uncommon for older bus duct not to have a ground.

So do I have to replace the bus duct with new? replace with conduit/wire? or add a ground wire? I am familiar 300.3(B) where all circuit conductors must be grouped together, so its probably not a simple as adding a ground wire.

Any thoughts would be appreciated!

Tom
 

jim dungar

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Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Up until the early-mid 80's, most bus ducts were manufactured without a 'listed' grounding bus. This has nothing to do with wether the system is grounded or not. The concept was similar to not pulling a grounding conductor in PVC conduit, and instead relying on the building steel and other incidental contacts for the bonding.
 

wireguy8169

Senior Member
Location
Southern Maine
Equipment Ground?

Equipment Ground?

Could you give more info on the existing system what your keeping and whats being changed? You could look at 250.80 250.92. Have you been told you need to do something? I am not saying this to get out of it or to "get ir dun", I am just wondering if its not already safe?
For instance is it connected to the grounding electrode system some how (building steel for instance). I am think that it would be treated like a conduit. As long as its connected to the system via approved method(s) then it should be good to go, you will just have to make sure the new panel is up to code. If the duct is not attached to something then you may need to research an approved method to do it again it may already be done.
 

tmillard

Member
Bus Duct with no Ground

The bus duct originally ended in a large pull box where it was tapped and one feed fed an electric boiler and another feed fed a chiller. Since both the boiler and chiller were removed before our original survey, it is unclear how they were originally grounded.

The electrical contractor issued and RFI and noted there was no ground and they wanted a written letter from the engineer (me) stating it was acceptable as is. This is what led me to post the question.

The Sq D rep stated the metal bus duct housing was not acceptable as a ground since all the parts were painted. I would really like to try to avoid replacing the bus duct if possible.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The Sq D rep stated the metal bus duct housing was not acceptable as a ground since all the parts were painted. I would really like to try to avoid replacing the bus duct if possible.

Any reason you can't add an egc inside the bus duct housing?

It seems to me I have seen people route wires in some now and then.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Could there be a reason its not grounded? Does it fall under any of 250.21?
Even if the system was an ungrounded system as permitted in 250.21, that does not change the requirement to provide an equipment grounding conductor for the circuits.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Even if the system was an ungrounded system as permitted in 250.21, that does not change the requirement to provide an equipment grounding conductor for the circuits.


And that requirement has been around for a long time. How did this kind of product provide an EGC back when it was installed?

Can you get special permission to route the EGC along the outside of the bus housing?
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
And that requirement has been around for a long time. How did this kind of product provide an EGC back when it was installed?
Yes, the rule is old and that is a very good question.

Was the enclosure permitted as the grounding conductor back then? I would expect that there would be enough cross sectional area in the enclosure to be an EGC, but the connections between the sections may be the issue. Maybe you could provide a bonding connection of some type at each joint.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Yes, the rule is old and that is a very good question.

Was the enclosure permitted as the grounding conductor back then? I would expect that there would be enough cross sectional area in the enclosure to be an EGC, but the connections between the sections may be the issue. Maybe you could provide a bonding connection of some type at each joint.

I wonder if drilling and tapping through the flanges and installing a 1/4-20 bolt would be considered adequate.

Or maybe removing a single bolt that is existing at each flange, grinding the paint off and reinstalling it with a star washer on both sides?
 

jdsmith

Senior Member
Location
Ohio
Any reason you can't add an egc inside the bus duct housing?

It seems to me I have seen people route wires in some now and then.

I would propose the same thing assuming this is a low voltage system. We have run #6 neutral wires from the transformer neutral bushing through the bus duct up into the back of the switchgear where the neutral grounding resistor was located on a high resistance grounded system. In our case it was only a #6 neutral, but I don't see why a ground for a 1200A bus duct couldn't be pulled inside low voltage bus as long as there was space.
 
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