Main bonding jumper and equipment ground

[george nicholson]

I have an existing entrance gutter that feeds 4 existing disconnects. It's all code from prior installation -- this work is not in question.

I am adding a 5th fused swritch. The service entrance neutral is bonded to the ground system inside the gutter. There are separate ground terminals and neutral terminals in the gutter. I can NOT run a ground thru the utility companies CT meter can.

Here is what I want to do but I am told its incorrect:

1) add a 400A power company CT meter can connected to the gutter with rigid. The 3 phase conductorsand 1 neutral only are allowed to travel thru the utility can. No bonding or grounding in the CT meter can. Its all connecetd with rigid pipe.

2) Feed out of the CT can thru rigid into a 400A fused switch with the neutral and 3 phase conductors. (750XHHW alum)

2) add a ground lug to the fused switch with green screw tapped to switch can and jump from it to the neutral lug with 2/0 copper.

3) leave the switch with PVC and run 100' to a gutter that has 2 MB load centers with the 3 phase conductors, the neutral, and a green #4 EGC. The green gEGC is #4 copper and its connected to the ground terminal on the switch can as its origin.

4) At the metal gutter where the 2ea 200A MB load centers are located the green #4 EGC is terminated at a ground terminal tapped to the gutter with a green screw.

5) The MB load centers are connecetd with emt to the metal gutter. The 3 phase conductors and neutral are connecetd to the load center and the neutral is NOT bonded to the can or ground at this end.

6) There is a separate ground terminal in the load center tapped to the load center can from factory and its is used for required equipment grounds leaving the load center but is not bonded to any conduits or the terminal strip in the gutter.

What is wrong with this install. Assume the existing entrance gutter and all work inside it are OK.

This was turned down but we are told only that the bonding and grounding is inadequate at the CT and fused switch? I was told the fused disconnect should have neutral and ground bonded as if it was the first point of entry.

Any suggestions?

 

[tryinghard]

?

2) Feed out of the CT can thru rigid into a 400A fused switch with the neutral and 3 phase conductors. (750XHHW alum)

You?ll actually be serving rather than feeding the 400A fused switch.

2) add a ground lug to the fused switch with green screw tapped to switch can and jump from it to the neutral lug with 2/0 copper.

This is your bonding jumper and it is correctly sized, 250.28 & Table 250.66, it is what allows fault current (shorts) to travel from frames to/through the neutral (grounded conductor) to the source usually XO of a transformer.

3) leave the switch with PVC and run 100' to a gutter that has 2 MB load centers with the 3 phase conductors, the neutral, and a green #4 EGC. The green gEGC is #4 copper and its connected to the ground terminal on the switch can as its origin.

I usually stay with the same circuit material but I believe you can use copper EGC if you choose, but you need to size it from Table 250.122 as at least #1 AL or #3 CU, #4 CU is too small.

4) At the metal gutter where the 2ea 200A MB load centers are located the green #4 EGC is terminated at a ground terminal tapped to the gutter with a green screw.

Again upsize this EGC to at least a #1 AL or #3 CU (#2 CU is most common), and you could use landing lugs or a ground bus to install #6 CU EGC?s to each 200A MB load center.

5) The MB load centers are connecetd with emt to the metal gutter. The 3 phase conductors and neutral are connecetd to the load center and the neutral is NOT bonded to the can or ground at this end.

6) There is a separate ground terminal in the load center tapped to the load center can from factory and its is used for required equipment grounds leaving the load center but is not bonded to any conduits or the terminal strip in the gutter.

If the load centers outgoing conduits are metallic they at least have to mechanically bond through their tight fittings or you must use bond bushings, see 250.96. You are correct in isolating the neutral and not bonding it past the service disconnect.

What is wrong with this install. Assume the existing entrance gutter and all work inside it are OK.

Sounds like the #4 CU EGC from service disconnect to downstream gutter, upsize this according to Table 250.122 to #3 CU or larger.

?I was told the fused disconnect should have neutral and ground bonded as if it was the first point of entry?

If this is the 400A service disconnect then this is correct because the bus gutter ahead of it is service. Do not bond downstream past the 400A fused disconnect though.

 

[george nicholson]

The inspector wants the ground conductor carried from the service gutter with the current carrying conductors into the switch.

We can not do this due to utility not allowing this ground to pass thru their meter device.

I say we complete the equipment ground requirement by bonding the neutral to the switch can and ground buss within the switch then the ground that travels with the pvc starts with the ground terminal in the switch? There is no external ground brought into the switch its derived thru the grounded neutral that comes from the service gutter.

 

[tryinghard]

This problem sounds like it is between the inspector and the serving utility company. This installation will need to be done to the serving utility company?s engineering; you should verify your application to their engineering documents and maybe even post them on site for the inspector.

The use of bus gutter is the alternative to switchgear distribution and the grounding with bonding requirements are the same, 250.24(A)(5) & 250.142(B).

 

[tryinghard]

The inspector wants the ground conductor carried from the service gutter with the current carrying conductors into the switch...

Maybe the inspector is talking about bonding the service as in 250.92. Often pictures help:

 

[george nicholson]

Thanks. 250.92A is how we have it wired now. I am going to show the code again.