ThaddeusW
Member
- Location
- New York, NY
We are installing a new 480V machine in our facility with 120/208Y service. Our electrician installed a 75KVA 208Δ to 277/480Y transformer to power the machine. Machine has a 90A disconnect and no 277V line to neutral loads, it's all line-line. The transformer is fed by a 200A breaker from a sub panel which is next to and fed from the main switchboard by a 250A breaker. In my past life I was an electrician (I do industrial automation now) and I took a look at his initial wiring which was incorrect. Originally he had the transformer feeder EGC hooked to the secondary neutral bar, no secondary neutral bond to the transformer case, and no neutral connection to the building ground (GEC). I told him to move the feeder EGC to the case ground lug, hook a bonding jumper from another case lug to the secondary's neutral bar, and a GEC connection which he made to the main ground rod which happens to be the main ground rod for the switch board.
He was hesitant to bond the neutral to the case and GEC as he claimed the transformer creates it own ground, which is false if no bonding is in place as its a SDS. He also ignored my request for the bonding jumper and the GEC conductor to be separately connected to the neutral bar and instead hooked the GEC to an external ground lug supplied by the transformer manufacturer making the GEC connection share the neutral to case bonding jumper. After everything was in place, he flipped the feeder breaker and pop, it immediatly tripped. He again tried flipping it on and again it popped. In a huff he said "It's that stupid wire from the neutral to the case, there are too many grounds!" He removes the neutral bond jumper goes back and successfully flips the transformer feeder on. "See I told you that wire was no good! I've been doing this for 40 years." He measures the line-line and line-neutral voltages and everything checks out.
I couldnt' let that go and decided to look over everything again. With the feeder breaker off I lifted the single secondary neutral connection he made and megged the transformer to ground which was infinite indicating no line faults. I double checked everything and see no reason for the feeder breaker to trip with that neutral bonding jumper in place. Since the 480V machine is located 60 feet from the transformer, I had him install a 480V fused disconnect next to the transformer. That disconnect was in the off position and had the fuses removed at the time he tried energizing the transformer. Since there is no need for a neutral, he ran a #6 EGC from the transformer secondary neutral to the fused disconnect and off again to the machine. In the disconnect, he bonded the EGC to the case which will cause a parallel path to the transformer if the neutral jumper was in place. My thought it he could bring the GEC to the disconnect and bond everything in there.
My only conclusion for the tripping is the breaker he sized is too small for the transformer inrush current and somehow on his third and final try, got lucky. The removal of the neutral jumper was just coincidence. The 200A feeder is too small and should be sized for 125% which would be ~260A which would be up-sized to the next breaker size of 300A. Problem is, the panel box won't take a breaker bigger than 225A (GE style Q line) and the Siemens 1200A switch board has a maximum branch breaker rating of 250A. The sub panel is already occupied by an 80 and 60 amp three phase breaker and overloaded. He did agree that he should move the transformer feed from the sub panel to the switch board where there is one remaining space for a branch breaker. That might solve our problem but we are still under the 260A 125% rule though not by much.
Any thoughts? I think he got lucky and the bonding jumper had nothing to do with it and it was all related to the undersized feeder breaker.
Basic diagram of our setup for reference:
He was hesitant to bond the neutral to the case and GEC as he claimed the transformer creates it own ground, which is false if no bonding is in place as its a SDS. He also ignored my request for the bonding jumper and the GEC conductor to be separately connected to the neutral bar and instead hooked the GEC to an external ground lug supplied by the transformer manufacturer making the GEC connection share the neutral to case bonding jumper. After everything was in place, he flipped the feeder breaker and pop, it immediatly tripped. He again tried flipping it on and again it popped. In a huff he said "It's that stupid wire from the neutral to the case, there are too many grounds!" He removes the neutral bond jumper goes back and successfully flips the transformer feeder on. "See I told you that wire was no good! I've been doing this for 40 years." He measures the line-line and line-neutral voltages and everything checks out.
I couldnt' let that go and decided to look over everything again. With the feeder breaker off I lifted the single secondary neutral connection he made and megged the transformer to ground which was infinite indicating no line faults. I double checked everything and see no reason for the feeder breaker to trip with that neutral bonding jumper in place. Since the 480V machine is located 60 feet from the transformer, I had him install a 480V fused disconnect next to the transformer. That disconnect was in the off position and had the fuses removed at the time he tried energizing the transformer. Since there is no need for a neutral, he ran a #6 EGC from the transformer secondary neutral to the fused disconnect and off again to the machine. In the disconnect, he bonded the EGC to the case which will cause a parallel path to the transformer if the neutral jumper was in place. My thought it he could bring the GEC to the disconnect and bond everything in there.
My only conclusion for the tripping is the breaker he sized is too small for the transformer inrush current and somehow on his third and final try, got lucky. The removal of the neutral jumper was just coincidence. The 200A feeder is too small and should be sized for 125% which would be ~260A which would be up-sized to the next breaker size of 300A. Problem is, the panel box won't take a breaker bigger than 225A (GE style Q line) and the Siemens 1200A switch board has a maximum branch breaker rating of 250A. The sub panel is already occupied by an 80 and 60 amp three phase breaker and overloaded. He did agree that he should move the transformer feed from the sub panel to the switch board where there is one remaining space for a branch breaker. That might solve our problem but we are still under the 260A 125% rule though not by much.
Any thoughts? I think he got lucky and the bonding jumper had nothing to do with it and it was all related to the undersized feeder breaker.
Basic diagram of our setup for reference:
