Onwer's padmount transformer N-G bond question

[sparkyguy545]

Owner's (not utility) 1500 kVA padmount xfmr 12.47kV-480Y/277. Primary feed consists of phase conductors and EGC in PVC ductbank. Transformer pad will be new reinforced poured concrete with a frost-wall foundation on all four sides so that we can install CCE (Ufer) connected to xfmr ground lug (along with primary EGC). The current design has parallel secondary conductors consisting of phase and Neutral, but currently no EGC in PVC duct. Question: Wouldn't it be allowable to simply remove the factory X0-Gnd link in xfmr and install the equipment bonding jumper in the downstream gear only (switchboard)? (Recognizing it can be in one and only one place.) It would be much less expensive to remove a factory-link than to add an EBJ in each parallel raceway between xfmr and downstream gear. Further, the transformer frame would appear to be properly bonded through primary EGC and local Ufer CEE.

 

[augie47]

Unless you approach the situation as one structure feeding another and approach it simply as a customer SDS, 250.30 (A)(1) exception 2 would allow you to do both as I read it.

 

[don_resqcapt19]

The requirement for an EBJ between the transformer and the gear does not change when you move the system bonding jumper from the transformer to the gear. The noncurrent carrying parts of the transformer still have to have a connection to the secondary grounding system. The primary EGC is for primary faults and the secondary EBJ would be for secondary faults.

 

[sparkyguy545]

To clarify:

Unless absolutely necessary, additional conductors would require a Change Order and a lot of headaches. My goal is to confirm that I can provide an installation that meets NEC without adding additional conductors to the system.

I currently have PVC raceway with only phase and neutral conductors pulled. Since it is an SDS, 250.30A1 Exception 1 (as noted by Augie470) would appear to allow the installation of both the transformer bonding jumper and the one at the interior switchboard. Correct?

 

[don_resqcapt19]

...
I currently have PVC raceway with only phase and neutral conductors pulled. Since it is an SDS, 250.30A1 Exception 1 (as noted by Augie470) would appear to allow the installation of both the transformer bonding jumper and the one at the interior switchboard. Correct?

Yes, you can use the exception as long as there are no other current paths. Are there any other transformers that feed this same building?

 

[Cold Fusion]

... I currently have PVC raceway with only phase and neutral conductors pulled. Since it is an SDS, 250.30A1 Exception 1 (as noted by Augie470) would appear to allow the installation of both the transformer bonding jumper and the one at the interior switchboard. Correct?

I deal with a lot of transformers in this size range, all in industrial settings. And they all have a ground mat under and between the transformer pad and the building. The building steel is bonded to the ground mat. The transformer is bonded to the ground mat.

For your case, I'd want to see the construction drawings to verify no mat. I would not rely on a verbal. I'd probably even make a 4W DLRO measurement between the building and the transformer. The worst case is if you get ~1 ohm.

Just in case you missed my real feelings about this, I think this code section is in the running for the second most moronic. But as you noted, this is not about good design or reliability, it's about money and saving the dumb schmuck that did not specify 5W.

cf

 

[Cold Fusion]

The other thread on the size of the system bonding jumper reminded me of another issue with this (using the neutral for an EGC).

For transformers this size, (including the 2500kva in the other thread) there is usually a fair amount of control and instrumentation at the transformer - secondary CTs, Primary CTs and PTs, alarms for oil temp, oil level, winding temp, maybe even a control box heater and controls for a primary CB. Often those are fed with an MC cable, or have a 120VAC circuit going out to the transformer. And that often provides a grounding path.

You may wish to verify that doesn't exist either.

cf

 

[sparkyguy545]

It's new construction, no other transformers feeding the faciity, no other interconnections to/from the transformer, no powered accessories at transformer, all power metering (including all PT's & CT's) is done at the downstream swbd. inside the facilty fed form unit. Transformer Primary is 12.47 kV Delta (integral fusing), 480Y/277 Secondary. Transformer installation is new poured concrete pad with local GEC. Secondary extended directly to swbd inside the downstream facility through parallel conc-encased PVC ductbank. No other bells or whistles.