Hi all,
I'm part of the design team for a new high rise. It has 480V distribution stepping down to 208/120 in each electrical closet. There are 2-3 transformers per closet, so we're talking about a sizable number of transformers. The building is approaching completion and is planning to open in a few months.
Well, inspections have rolled around, and the AHJ found all transformer secondary SSBJs were sized as EGCs. In some cases the sizes were correct purely by coincidence, but for most transformers the SSBJ is exactly one size too small. The client is now looking for the quickest code-compliant solution.
Key points:
- This project is under the 2011 NEC.
- N-G bond is at the transformer via manufacturer-provided jumper.
- SSBJ is connected from the transformer ground terminal bus to the panelboard ground terminal bus.
- Secondary protection is the MCB of the downstream panel. Secondary feeder is run in 4-5 feet of FMC followed by 10-15 feet of EMT. There are three 90-degree bends: the FMC termination at the transformer, plus two 90s for the overhead transition. I've attached a rough sketch.
- Typical wire fill in secondary conduit is between 20% and 28%.
Some questions:
1. Is it possible to reliably pull a new ground wire in the same conduit without damaging the insulation on the existing bundle? Or would you rather pull the whole feeder out and re-bundle? The contractor has already looked at pulling out the whole feeder and found it would take 2-3 months at ~4-6 hours per transformer.
2. 250.30(A)(2) allows nonflexible metal raceway to be used as a SSBJ, but we have FMC for the final few feet of the run. Is it NEC-compliant to jumper over the FMC portion using a grounding clamp and correctly-sized wire outside the conduit, provided the jumper is less than 6 feet? I had trouble finding anything for or against this.
3. The AHJ briefly considered allowing the SSBJs to remain as-is, although I don't know if that went anywhere. What are the dangers realistically posed by an undersized SSBJ? My understanding is that in the event of a fault between the transformer and the secondary OCP, the SSBJ needs to survive long enough for the primary OCP to trip, since the secondary isn't seeing the fault. And if the fault is past the secondary OCP, then the SSBJ is just a continuation of the EGC path, so a larger SSBJ wouldn't appear to matter. Has anyone experienced an issue stemming from an undersized SSBJ?
And maybe the most important question - in my shoes (or the contractor's), how would you approach this?
Thanks.
I'm part of the design team for a new high rise. It has 480V distribution stepping down to 208/120 in each electrical closet. There are 2-3 transformers per closet, so we're talking about a sizable number of transformers. The building is approaching completion and is planning to open in a few months.
Well, inspections have rolled around, and the AHJ found all transformer secondary SSBJs were sized as EGCs. In some cases the sizes were correct purely by coincidence, but for most transformers the SSBJ is exactly one size too small. The client is now looking for the quickest code-compliant solution.
Key points:
- This project is under the 2011 NEC.
- N-G bond is at the transformer via manufacturer-provided jumper.
- SSBJ is connected from the transformer ground terminal bus to the panelboard ground terminal bus.
- Secondary protection is the MCB of the downstream panel. Secondary feeder is run in 4-5 feet of FMC followed by 10-15 feet of EMT. There are three 90-degree bends: the FMC termination at the transformer, plus two 90s for the overhead transition. I've attached a rough sketch.
- Typical wire fill in secondary conduit is between 20% and 28%.
Some questions:
1. Is it possible to reliably pull a new ground wire in the same conduit without damaging the insulation on the existing bundle? Or would you rather pull the whole feeder out and re-bundle? The contractor has already looked at pulling out the whole feeder and found it would take 2-3 months at ~4-6 hours per transformer.
2. 250.30(A)(2) allows nonflexible metal raceway to be used as a SSBJ, but we have FMC for the final few feet of the run. Is it NEC-compliant to jumper over the FMC portion using a grounding clamp and correctly-sized wire outside the conduit, provided the jumper is less than 6 feet? I had trouble finding anything for or against this.
3. The AHJ briefly considered allowing the SSBJs to remain as-is, although I don't know if that went anywhere. What are the dangers realistically posed by an undersized SSBJ? My understanding is that in the event of a fault between the transformer and the secondary OCP, the SSBJ needs to survive long enough for the primary OCP to trip, since the secondary isn't seeing the fault. And if the fault is past the secondary OCP, then the SSBJ is just a continuation of the EGC path, so a larger SSBJ wouldn't appear to matter. Has anyone experienced an issue stemming from an undersized SSBJ?
And maybe the most important question - in my shoes (or the contractor's), how would you approach this?
Thanks.
