SSBJ Sizing for Customer Owned Pad Mounted Transformer

[pvgreeze]

Hello all, I'm reviewing a design for a client and I have a couple of questions regarding my interpretation of the NEC with respect to a customer owned transformer. This is a 2750kVA pad mounted transformer that is GndY - GndY. All equipment is outside; no "building or structure" is being serviced. My reading is of the relevant NEC codes is the following:

1. NEC 250.30(A)(2) requires a supply side bonding jumper (SSBJ) between the XFMR and the switchboard sized per NEC 250.102(C)
   • NEC 250.30(A)(1) Exception 2 does not apply as this is not a "building or structure" being fed from a separately derived system (SDS)
   • The transformer and the switchboard will be connected to the same ground ring/GEC system as required per NEC 250.50
2. The supply side bonding jumper needs to be sized based off the "Equivalent Area for Parallel Conductors"
   • Main Circuit Breaker is 2500A and being fed by 10 parallel sets of 350kcmil (aluminum)
   • Equivalent area is 3500kcmil (greater than 1750kcmil) - Note 1 says 12.5% (which is 437.5kcmil) and that the SSBJ shall not be required to be larger than the largest ungrounded conductor; thus, the SSBJ should be 350kcmil

The client has concerns about supplying an SSBJ between the XFMR and the switchboard. Is my reading on this correct? That the NEC requires a SSBJ between the GndY XFMR and the switchboard? The client recommended grounding at both the transformer and simply tying the switchboard to the ground ring. I'll try and find code references for this, but that seems pretty against the explicit requirement in NEC 250.30(A)(2). Any thoughts of feedback would be appreciated. Thanks!!

 

[don_resqcapt19]

In my opinion, you need a supply side bonding jumper between the transformer and the switchboard. This applies no matter of the system bonding jumper is at the transformer or in the switchboard. Assuming the 10 sets of 350kcmil, are in individual non-metallic raceways, you need a SSBJ in each raceway. It will be sized based on the size of the ungrounded conductors in the raceway, requiring 1/0 aluminum in each.

 

[pvgreeze]

In my opinion, you need a supply side bonding jumper between the transformer and the switchboard. This applies no matter of the system bonding jumper is at the transformer or in the switchboard. Assuming the 10 sets of 350kcmil, are in individual non-metallic raceways, you need a SSBJ in each raceway. It will be sized based on the size of the ungrounded conductors in the raceway, requiring 1/0 aluminum in each.

Thank you! Regarding the size of the size of the SSBJ in each raceway, that comes from 205.102(C)(2), correct? I suppose the "equivalent area for parallel conductors" would only be applicable in situations where the connections are being made in the same enclosure (e.g. Main Bonding Jumper).

 

[don_resqcapt19]

Thank you! Regarding the size of the size of the SSBJ in each raceway, that comes from 205.102(C)(2), correct? I suppose the "equivalent area for parallel conductors" would only be applicable in situations where the connections are being made in the same enclosure (e.g. Main Bonding Jumper).

Yes, where they are all in a cable tray or wireway, or where you have metallic raceways and you want to use a single SSBJ to connect to the grounding bushings on the raceways.