480/277 Y Secondary 4-Wire Service Question
480/277 Y Secondary 4-Wire Service Question
(OP)
I have a question or two that I hope someone can aid in answering.
I have a 2500kVA transformer with the secondary (480/277 Y) feeding a bus with the following 4-wire configuration:
3-400 KCMILL/ phase, 3-4/O neutrals (X0), and 3-#2 equipment grounds.
Question 1:
what happens if one of the current carrying conductors faults to ground?
Question 2:
Does code say that a 4 wire services such as this must have a neutral conductor equally sized with the current carrying conductors? I can't seem to find it if it does.
Your help is appreciated.
I have a 2500kVA transformer with the secondary (480/277 Y) feeding a bus with the following 4-wire configuration:
3-400 KCMILL/ phase, 3-4/O neutrals (X0), and 3-#2 equipment grounds.
Question 1:
what happens if one of the current carrying conductors faults to ground?
Question 2:
Does code say that a 4 wire services such as this must have a neutral conductor equally sized with the current carrying conductors? I can't seem to find it if it does.
Your help is appreciated.






RE: 480/277 Y Secondary 4-Wire Service Question
Could any one help me?
My transf is (440 D/ 220 y) on IT System.
Regards
RE: 480/277 Y Secondary 4-Wire Service Question
1. You will have a ground fault. If there is no secondary protective device between the fault and the transformer, the result may not be very good. If lucky primary device may open but no guarantees. LG fault may soon turn in to a L-L or a 3 phase fault, etc. That is why unprotected secondary conductors have restriction on length and installation.
2. No, but there are some minimum requirements. See NEC 250.24, especially 250.24.C. And of course the neutral needs to be sized to carry anticipated unbalanced loads.
Rafiq Bulsara
http://www.srengineersct.com
RE: 480/277 Y Secondary 4-Wire Service Question
1. 400MCM is good for about 335A, you have a transformer that is 2500kVA which is good for about 3000A at 480V, so your phase cables are way undersized or your transformer is way over-sized, either-or.
2. Your neutral conductor should be the same size as your phase conductor. If you do not need 277V, then solidly ground the neutral and use a 3W switchboard. Otherwise it is just good practice to use the 100% neutral. Never cared to even look at NEC to find out since its standard practice in our industry.
3. If you solidly ground the neutral at transformer and provide a main binding jumper inside the switchboard and ground the service per NEC, you do not need the equipment grounding conductor from the transformer to the switchboard.
4. For this service you will need to have ground-fault protection on your main overcurrent device, at minimum.
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: 480/277 Y Secondary 4-Wire Service Question
Rafiq Bulsara
http://www.srengineersct.com
RE: 480/277 Y Secondary 4-Wire Service Question
Note, there are no 277V loads.
RE: 480/277 Y Secondary 4-Wire Service Question
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.