SDS raceway grounding

[gwz2]

Four SDS senerios:

All four senerios have 4 - 600 Kcmil conductors in 4 raceways between transformer enclosure and the OCPD is some 30 feet away.

Transformer is outside and the OCPD is in an adjacent building.

1) Bonding Jumper ( BJ ) 250.30(A)(1) is at the transformer Xo location. The 4 raceways have a metal (RMC) 90? at each end of a RNC run.

2) BJ is at the OCPD. The 4 raceways have a metal (RMC) 90? at each end of a RNC run.

3) BJ is at the transformer Xo location. The four runs of conduit are all RMC.

4) BJ is at the OCPD location. The four conduit runs are all RMC.

Question(s),

What size conduit bonding conductor and how to accomplish the proper grounding of the metal portions of the raceways in each senerio above ?

Use 2002 NEC substantations.

 

[don_resqcapt19]

Re: SDS raceway grounding

In case (1) and (2) a bonding jumper is permitted at both locations by Exception #1 to 250.30(A)(1), so a bonding jumper is not required in the raceways.
In cases (3) an bonding jumper is required to connect the grounded conductor at the transformer to the EGCs in the OCPD. This conductor must be per 250.30(A)(1) which sends you to 250.28. 250.28(A) requires that the bonding jumper be " a wire, bus, screw, or similar suitable conductor". This section rules out the use of the conduit as the bonding jumper. The only question is the required size the bonding jumper. If you use Table 250.66 and the 600 kcmil in each raceway you would have to use a 1/0 bonding jumper. If you use 12.5% of the total area, you would need the next size larger than 75kcmil. That would be a #1. 310.4 does not apply to conductors other than phase, neutral, or grounded circuit conductors, so it is my opinion that a #1 in each raceway would be code compliant.
In case (4) we don't need a bonding jumper to connect the EGCs to the grounded conductor, but we do need an equipment bonding conductor for the transformer case. 250.102(A) does not list rigid as an equipment bonding conductor, so again, I think that a #1 bonding jumper is required in each raceway.
Don

 

[gwz2]

Re: SDS raceway grounding

Thanks Don for the quick reply.

Left out an important detail. The outdoor transformer is pad mounted and the conduit RMC 90?s do not have metal to metal contact between the 90?s and the transformer enclosure.

How does one actually determine that there is not a parallel path of Exception No. 1 to 250.30(A)(1) such as an old buried water line being in contact with the transformer pad re-inforcing mesh in an old industrial premise ?

Glenn

 

[pierre]

Re: SDS raceway grounding

Don
For # 4:
Would not the RMC be an effective EGC [250.118(2)]for the connection between the transformer and the panel? That being said, a bonding jumper would not be required. Also refering to 250.96(A)

Pierre

 

[don_resqcapt19]

Re: SDS raceway grounding

Pierre,
Case 4 requires an equipment bonding jumper and not an equipment grounding conductor. I find no permission in the code to use rigid conduit as an equipment bonding jumper.
Don

 

[don_resqcapt19]

Re: SDS raceway grounding

Glenn,
If there is not metal to metal contact, a #1 bonding jumper would be required to the metal 90s.
As far as the parallel path, that path would have to be between the transformer and the building. If this is a new installation, you would find the other paths, when you installed the transformer pad. If you don't find a physical path while installing the pad, you are permitted to bond the grounded conductor at both locations. If it is an old pad, you could take some resistance readings between the pad and the building. As long as the value is not less than the resistance of the earth between the two points, you are ok.
Don