480/277 Y Secondary 4-Wire Service Question

[SparkOmatix]

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.

 

[hugobellon]

I have a same question, but now I had a problem. Earth leakage on system monitored by Insulation resistence monitoring device (BENDER). I finded out phase loss at L2 with 3.2Vac another one (L1,L3) with 227Vac until 231Vac, that should be 127Vac until 130Vac. All loads connected on the system are lightings and some controls, and all secondary panels are unballanced. The temperature of transformer is a little less of limit and high noise like overload noise. My doubt is whether I can identify where(circuit or load) is with Watt clamp meter, checking phase, angle, whatever...

Could any one help me?

My transf is (440 D/ 220 y) on IT System.

Regards

 

[rbulsara]

SparkO:
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

 

[VTer]

When you say feeding a bus, I assume you are referring to your transformer feeding into the switchboard. If that is the case, there are multiple things here that I would point out:
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

 

[rbulsara]

Yes, the 3-400/phase is way undersized, unless it is terminating in a 1000A or smaller breaker.

Rafiq Bulsara

http://www.srengineersct.com

 

[SparkOmatix]

I mistakenly put "400" when it is indeed 500. There are parallel feeds coming off the xfmr. They feed 3 independent starter racks (think guttered bus). Each rack is fused with 1200A time delay fuses(LTL-1200).

Note, there are no 277V loads.

 

[alehman]

Phase conductors are probably way undersized. 3-500's in conduit are good for 1140 amps. If there are no neutral loads, either bond the transformer X0 at the transformer, or the neutral must be at least 12.5% of the combined phase conductor area.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.