This is a question that has been driving me CRAZY and I’m not physically located on the job site so I will explain this the best way possible. Anytime I have ever installed a transformer regardless if it’s step up/step down, I’ve always understood that you must bond to building steel (or a water pipe electrode) per 250.52(A)/250.30(A)(7). So from the best i can tell from the 2 transformers I terminated, the transformer is fed directly from an MSB in an electrical room and it comes underground around 300 feet away, stubs up outve the concrete and flexes into the transformer, and then from the transformer into another panel. This is 480/277 to 208/120. In the particular transformers I did, the primary wires consisted of 3 hots and a ground, and the secondary wires consisted of 3 hots, a neutral, and a ground. So the ground from the primary and secondary both land on a set of lugs bonded to the enclosure, and then the system bonding jumper is installed from there to the neutral on the secondary side. But there is no grounding electrode conductor to building steel or anything whatsoever inside the transformer or secondary panel. I can’t remember off the top of my head what size conductors were all involved, but if I remember correctly 250.30(A)(3) requires that you MUST install a grounding electrode conductor to the neutral terminal of a separately derived system. Our project manager is the one telling us this is code, and I just want to know where in that code book should I be looking because I feel like it isn’t right. But some more important info I believe is that there is a ground ring that is ran around the entire structure of the building and throughout the premises it’s cad welded to various columns, and I recall seeing somewhere that a ground ring would change the requirements but we aren’t tied to it at the transformer so I wouldn’t think it matters. Also their are several separately derived systems (transformers) throughout the building and they’re all the same way. I seen somewhere if you have multiple separately derived systems you could bond all of the neutral points to a common grounding electrode conductor in an accessible location. I was wondering if the tap rules in 250.66 would apply to this if this was all done inside the MSBs. If anyone could give me some direction or a reference to give a look it would be much appreciated, I’m still a relatively young journeyman and I thought this might just be a case of something over my head. Thanks in advance!
Grounding and Bonding a Separately Derived System
- Thread starter Sparky558
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Simple answer: each secondary neutral should be bonded; in this case, to the nearest building steel would be good.
Also, the incoming building primary feeders should be bonded to electrodes.
Also, the incoming building primary feeders should be bonded to electrodes.
I firmly agree with you, but it’s not my call. Just looking to see what everyone else thought or if there was something I was missing. And the incoming primary feeders are bonded to electrodes which tie into to the ground ring, everything looks good as gold except for the transformers out in the plant (in my opinion). Just to make matters worse, the specs and details call for the ground to building steel connection at each one. But somehow this has slipped by
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