georgeswe
Senior Member
- Location
- The Republic of Southern California
I recently went on a service call to a medical office in a building that was build approx 7 yrs. ago. My customer has been in the building for about 5 yrs. The reason for the trip is that they have had some sensitive equipment burning out PC boards and have had a computer power supply fail recently. They said they have had other equipment failing over the years. They have had the medical equipment techs out but never bothered to call an Electrician.
The building has approx. 20 tenants. The main Service is a 277/480 3ph 4 wire gear and from there the distribution feeds to 6 electrical rooms (3) per floor. In these electrical rooms there is a 277/480 panel (for lighting loads mainly) and in the case of my customer a 150 KVA Transformer feeding a 400 amp panel that has a 200 amp main feeding a second panel. All these panels are shared by all the tenenats in their immediate are. My customer did not want to have any issues with people turing off breakers to their suite inadvetantly so they installed a 100 amp 3 ph main to a seperte panel in their suite.
I found the circuit that was feeding the equipment that had the trouble and opened up J boxes and made sure that the terminations were good. I was expecting to find a loose neutral. they are using Multi-Wireire branch circuiting and I was aauming that a loose neutral would be causing the problem, However what I found was that there was a 9 volt reading from Ground to Neutral at the equipment. I chased it back to the panel my customers suite. then I went back to the 400 amp panel and When I opened up the transformer this is what I saw
There is a LB in the side of the transformer that has a 3/0 ground it is bonded to the transformer then it feeds to the ground buss in the 400 amp panel. The secondary feed off the transformer is run in parallel and the XO is not bonded to ground and feeds to a isolated neutral buss in the 400 amp panel. I'm afraid that this is the same situation in all the other (5) electric rooms as well. I want to properly reference the correct NEC code. I am looking at 250.20(B)(2) and 250.20(B)(3). I would also like to ask if this situation could have cause the equipment failures that they are having. I spoke to a few other tenants and they have also mentioning that they replace their fixture lamps out a lot.(thought it could be unrelated). I know that regardless if this is the cause this needs to be rectified quickly, since it is a safety issue first and foremost.
The building has approx. 20 tenants. The main Service is a 277/480 3ph 4 wire gear and from there the distribution feeds to 6 electrical rooms (3) per floor. In these electrical rooms there is a 277/480 panel (for lighting loads mainly) and in the case of my customer a 150 KVA Transformer feeding a 400 amp panel that has a 200 amp main feeding a second panel. All these panels are shared by all the tenenats in their immediate are. My customer did not want to have any issues with people turing off breakers to their suite inadvetantly so they installed a 100 amp 3 ph main to a seperte panel in their suite.
I found the circuit that was feeding the equipment that had the trouble and opened up J boxes and made sure that the terminations were good. I was expecting to find a loose neutral. they are using Multi-Wireire branch circuiting and I was aauming that a loose neutral would be causing the problem, However what I found was that there was a 9 volt reading from Ground to Neutral at the equipment. I chased it back to the panel my customers suite. then I went back to the 400 amp panel and When I opened up the transformer this is what I saw
There is a LB in the side of the transformer that has a 3/0 ground it is bonded to the transformer then it feeds to the ground buss in the 400 amp panel. The secondary feed off the transformer is run in parallel and the XO is not bonded to ground and feeds to a isolated neutral buss in the 400 amp panel. I'm afraid that this is the same situation in all the other (5) electric rooms as well. I want to properly reference the correct NEC code. I am looking at 250.20(B)(2) and 250.20(B)(3). I would also like to ask if this situation could have cause the equipment failures that they are having. I spoke to a few other tenants and they have also mentioning that they replace their fixture lamps out a lot.(thought it could be unrelated). I know that regardless if this is the cause this needs to be rectified quickly, since it is a safety issue first and foremost.
