C.T. Cabinet Bond Vs Ground

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Brewcrew8

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We are installing a 3000A 480/277A Service onto a building. There is a 4/0 ground ring ran around the Transformer and Ct Cabinet pad and then to the Main disconnect. We are bringing that into the main disconnect outside for the service. I was under the assumption that we would run the 4/0 ground ring into the C.T. Cabinets as well... The inspector told us he wants a jumper from the neutral bus in the CT Cabinet to the Frame of the C.T. cabinet. That means we are using the 10 - 750MCM AL neutrals that go from the C.T. cabinet to the Service disconnect. Those 10 are bonded by the main bonding jumper to the 4/0 ground ring. Thoughts on why you would bond cabinet vs directly grounding it to the ground ring?
 
The CT cabinet should be bonded to the service neutral/grounded conductors.

Sometimes a jumper is used if the neutral bus is isolated from the cabinet.

You could have the GEC to the ground ring connected there also if you desired.

Here is a set up like that:

attachment.php
 
jumper said:
The CT cabinet should be bonded to the service neutral/grounded conductors.

Sometimes a jumper is used if the neutral bus is isolated from the cabinet.

You could have the GEC to the ground ring connected there also if you desired.

Here is a set up like that:

attachment.php
Click to expand...

Interesting. Are those lugs rated for two conductors?


SceneryDriver
 
Basically at the service, you have two options to bond something: Hit it with neutral "directly", or run a bonding jumper to the neutral in a different location. The former may create objectionable current, but no one usually cares.
 
Brewcrew8 said:
We are installing a 3000A 480/277A Service onto a building. There is a 4/0 ground ring ran around the Transformer and Ct Cabinet pad and then to the Main disconnect. We are bringing that into the main disconnect outside for the service. I was under the assumption that we would run the 4/0 ground ring into the C.T. Cabinets as well... The inspector told us he wants a jumper from the neutral bus in the CT Cabinet to the Frame of the C.T. cabinet. That means we are using the 10 - 750MCM AL neutrals that go from the C.T. cabinet to the Service disconnect. Those 10 are bonded by the main bonding jumper to the 4/0 ground ring. Thoughts on why you would bond cabinet vs directly grounding it to the ground ring?
Click to expand...

Remember 7500 MCM x 12.5 % = 937.5 MCM Main bonding jumper
 
Here. the POCO Blue Book specifies how bonding/grounding is to be done to and within CT cabinets.
 
The neutral bus in the CT cabinet is isolated from the cabinet. We are bonding the neutrals and GEC's using the main bonding jumper in the main service disconnect. Im just trying to figure out why he wants us to bond the CT Cabinet. At that point you will be using the 10-750 neutrals to carry the potential fault current from the CT cabinet to the main bonding jumper in the disconnect and out the GEC and ground ring. Is this to keep the cabinet at the same potential as everything else? Every other service I have seen they do the main bond in the main service disconnect. Then they just ground the CT cabinet with the GEC out to the ground ring. I just feel like if the cabinet were to have a Current Carrying Conductor or a lightning strike fault to it you would want it to go out on the GEC to the ground ring. Not clearing the fault back on the 10 neutrals to the main bonding jumper
 
If you did not bond the cabinet to the service neutral, how would it clear a fault condition?

Bonding it to the GES only would be illegal and dangerous.

A ground fault would have to go from the CT cabinet to the GES back to the disco to get to the tranny, energizing everything along the way. Not good.

Note: please hit the return key and separate large blocks of text. Us old farts have a hard time reading.:)
 
Brewcrew8 said:
The neutral bus in the CT cabinet is isolated from the cabinet. We are bonding the neutrals and GEC's using the main bonding jumper in the main service disconnect. Im just trying to figure out why he wants us to bond the CT Cabinet. At that point you will be using the 10-750 neutrals to carry the potential fault current from the CT cabinet to the main bonding jumper in the disconnect and out the GEC and ground ring. Is this to keep the cabinet at the same potential as everything else? Every other service I have seen they do the main bond in the main service disconnect. Then they just ground the CT cabinet with the GEC out to the ground ring. I just feel like if the cabinet were to have a Current Carrying Conductor or a lightning strike fault to it you would want it to go out on the GEC to the ground ring. Not clearing the fault back on the 10 neutrals to the main bonding jumper
Click to expand...

You can do this as long as you size the main bonding conductor from the service enclosure neutral based on 250.66 only if that is also sized no less than 12.5 % of MCM in your case 12.5 % of 7500 alum. or copper equivalent

It is actually a preferred method when metal conduit is used instead of PVC.

If the inspector insist that you bond the neutral at the CT cabinet location and will not except a bond coming from the service disconnect neutral to the CT cabinet, I would remove the bond from the CT cabinet to the service disconnect or I would up size it to 750 MCM

Brewcrew8 said:
Then they just ground the CT cabinet with the GEC out to the ground ring. I
Click to expand...

I am assuming what you are calling a GEC bonds at the CT cabinet as it passes through to your ground ring
 
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Brewcrew8 said:
We are installing a 3000A 480/277A Service onto a building. There is a 4/0 ground ring ran around the Transformer and Ct Cabinet pad and then to the Main disconnect. We are bringing that into the main disconnect outside for the service. I was under the assumption that we would run the 4/0 ground ring into the C.T. Cabinets as well... The inspector told us he wants a jumper from the neutral bus in the CT Cabinet to the Frame of the C.T. cabinet. That means we are using the 10 - 750MCM AL neutrals that go from the C.T. cabinet to the Service disconnect. Those 10 are bonded by the main bonding jumper to the 4/0 ground ring. Thoughts on why you would bond cabinet vs directly grounding it to the ground ring?
Click to expand...

If your thought is float the neutral at the CT cabinet location. Bond the neutral at the service disconnect location, from the service disconnect location take a conductor based on 250.66 to a ground ring.

At the CT cabinet bond the cabinet enclosure to your ground ring keeping the natural floating and call it a day.

The problem I see is the bonding conductor that originates at the service disconnect natural bonds to the ground ring on its way to the CT cabinet enclosure is no longer just a GEC.

It would then be a main bonding jumper from the neutral to the CT cabinet enclosure.
Main bonding jumpers have to be un-spliced (continuous) and sized no less than 12.5 % of MCM of the un-grounded conductors for the service
 
It's called line-side grounding. Simple as that. Up to the first means of disconnect, the grounded conductor serves as the equipment grounding conductor. In laymen's terms, the CT cab needs to be grounded. Easiest way to do this is to bond the grounded conductor (in this case the neutral being the grounded conductor) to the cab.

Easy Peasy.
 
electrofelon said:
Basically at the service, you have two options to bond something: Hit it with neutral "directly", or run a bonding jumper to the neutral in a different location. The former may create objectionable current, but no one usually cares.
Click to expand...

In the utility metering world I lived in, we never connected the neutral to ground or the enclosure on a remote meter/CT setup (ditto for the GEC). The bonding of the neutral always happened at the service disconnect MBJ. We considered the CT primary conductors and buswork equivelant to the service drop. No grounds on the neutral until the service disconnect. We did require bonding from the CT enclosure to the service disc. enclosure with a bonding conductor or required bonding bushings on at least one end of metallic conduit or raceways ahead of the service disc. panel. If the meter/test switches were in a separate panel, they too were bonded, but not grounded. We didn't want to set up parallel paths back to the transformer because if there was a neutral connection problem, the ground would have to carry any imbalance or fault current. I assume that's why the neutral bus in a CT enclosure is usually installed on insulated standoffs. I'm sure there are many POCO's who differ but that's how I learned to do it. AHJ's really didn't care what we did ahead of the service disconnect.
 
meternerd said:
In the utility metering world I lived in, we never connected the neutral to ground or the enclosure on a remote meter/CT setup (ditto for the GEC). The bonding of the neutral always happened at the service disconnect MBJ. We considered the CT primary conductors and buswork equivelant to the service drop. No grounds on the neutral until the service disconnect. We did require bonding from the CT enclosure to the service disc. enclosure with a bonding conductor or required bonding bushings on at least one end of metallic conduit or raceways ahead of the service disc. panel. If the meter/test switches were in a separate panel, they too were bonded, but not grounded. We didn't want to set up parallel paths back to the transformer because if there was a neutral connection problem, the ground would have to carry any imbalance or fault current. I assume that's why the neutral bus in a CT enclosure is usually installed on insulated standoffs. I'm sure there are many POCO's who differ but that's how I learned to do it. AHJ's really didn't care what we did ahead of the service disconnect.
Click to expand...

Ok fair enough. Although that is not what I see most of the time. I Admit though I do like the idea of a single N-G bond. Every self contained meter I have seen is neural bonded, y'all just let that slide I assume?
 
electrofelon said:
Ok fair enough. Although that is not what I see most of the time. I Admit though I do like the idea of a single N-G bond. Every self contained meter I have seen is neural bonded, y'all just let that slide I assume?
Click to expand...

Yup. Self contained meters are usually in the same enclosure or very close to the service disconnect, so there wasn't really much of a parallel path.
 
Interesting, no neutral terminal block. Makes sense for single phase I guess. Now that I think of it, not sure I have ever seen the inside of a single phase ct cabinet. In upstate NY, the only time you would see that would be a 600 amp service. In Seattle, SCL doesn't allow 320 sockets for commercial, gotta be CT anything over 200.

Installed a 800A 3 phase CT last summer. It had a neutral terminal block but I cant remember if it was factory bonded or not.....
 
electrofelon said:
Interesting, no neutral terminal block. Makes sense for single phase I guess. Now that I think of it, not sure I have ever seen the inside of a single phase ct cabinet. In upstate NY, the only time you would see that would be a 600 amp service. In Seattle, SCL doesn't allow 320 sockets for commercial, gotta be CT anything over 200.

Installed a 800A 3 phase CT last summer. It had a neutral terminal block but I cant remember if it was factory bonded or not.....
Click to expand...

No neutral block, Isolated neutral block-same thing.

If nipples do not qualify for bonding, bushings and wire.

You are like me, used to non isolated neutral block.

All three set ups are legal.
 
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