multiple service runs and grounding electrode conductor

[xguard]

We have a building with two service disconnects side by side. They are each fed from the same utility transformer but with separate runs. Both runs pass through the same current transformer box and share a meter.

Where I'm confused is in sizing the common portions of grounding electrode conductors. I'm wondering if there is a difference between this and a single run tapped off to two disconnects.

If each disconnect is supplied by it's own set of parallel (two per phase) 500 kcmil then the common grounding electrode conductors should be 3/0 and the taps off this 3/0 to each disconnect should be 2/0?

What about supply side bonding jumpers in a wire-way shared by both, on the line side of the disconnects?

Let me know if this isn't clear, I'm struggling some describing it.

 

[xguard]

We have a building with two service disconnects side by side. They are each fed from the same utility transformer but with separate runs. Both runs pass through the same current transformer box and share a meter.

Where I'm confused is in sizing the common portions of grounding electrode conductors. I'm wondering if there is a difference between this and a single run tapped off to two disconnects.

If each disconnect is supplied by it's own set of parallel (two per phase) 500 kcmil then the common grounding electrode conductors should be 3/0 and the taps off this 3/0 to each disconnect should be 2/0?

What about supply side bonding jumpers in a wire-way shared by both, on the line side of the disconnects?

Let me know if this isn't clear, I'm struggling some describing it.

No feedback???

 

[Strombea]

The electrode part is easy based on the simplicity of 250.66A and B if it's rods or concrete encased it never needs larger than #4 copper, but the tricky one is the shared wire way, I would try to avoid a shared wire way but I would guess that you would be using table 250.66 note 1. Refer to 250.64 D

please repost with your outcome, it's an interesting one.

- Adam

 

[synchro]

We have a building with two service disconnects side by side. They are each fed from the same utility transformer but with separate runs. Both runs pass through the same current transformer box and share a meter.
.........

What about supply side bonding jumpers in a wire-way shared by both, on the line side of the disconnects?

I'm assuming there's a neutral in each of the two separate sets of service conductors. I'm also assuming that their lengths are probably not well matched and that their currents can be completely different because they serve different loads, and so they'd have different voltage drops if they were not connected together. And because these two neutrals are connected at the transformer end, if they were not also connected after the service point then they'd be at different voltages. Therefore if they're each bonded with jumpers to the wireway, then a significant current may flow in the jumpers and through the wireway itself. It doesn't sound like a good situation. Given the setup, I think two alternatives would help but I'm not sure about code compliance, etc.:

1. You could connect the neutrals from the two sets of conductors on a separate (preferably isolated) neutral bar and then have a single bonding jumper to wireway. That way the unbalanced neutral current will flow through the bar and not the wireway.

2. If your loads allow it (for example mostly 3-phase delta loads on one set of conductors), and the two service runs A and B are in PVC conduits close to each other, then you could possibly use only the neutral conductor of run A, for example (so there's not a mismatch issue causing unwanted currents to flow). If the service conductors are overhead and well separated, or inside metal conduits, then this might not work well because of magnetic fields that would be induced because any unbalanced current on the phases of run B would have to flow through the neutral of run A. Code compliance may also be an issue.
On the other hand, this magnetic field issue may exist to some extent even if you use both neutrals because any mismatch in their resistance will cause currents to flow between the two neutrals, effectively creating a circulating current around the loop between the neutrals.

 

[Dennis Alwon]

The supply side bonding jumper can be sized based on T. 250.102(C). Please read the notes to the table. They are the same, or close to it, as T250.66 notes