4 wire vs. 3 wire service
4 wire vs. 3 wire service
(OP)
I have a 3 wire vs. a 4 wire question for you folks out there. My primary transformer has a 12470 volt, 3 phase delta primary - 4160Y2402 secondary. However, my loads on the secondary of the transformer are all 3 phase, 3 wire loads (large motors). Can I ground the XO terminal on the secondary of the transformer at the transformer and not pull a fourth wire into my switchgear, or is that neutral wire required all the way through the system?






RE: 4 wire vs. 3 wire service
The neutral is your ground fault return path.
RE: 4 wire vs. 3 wire service
Jim
Thanks,
Jim
RE: 4 wire vs. 3 wire service
You kept hearing from folks that the neutral could be terminated at the transformer because it is common practice and done all the time. You take a ground conductor - terminate it on the X0 bushing and terminate the other end to your ground system... usually a ground rod. The transformer tank will also have a ground pad which should be connected to your ground grid via a ground cable.
RedDog is right that the connection between X0 bushing and ground can be solid or it can have an impedance (resistor or reactor).
The NEC requires you to run a ground conductor(s) with the phase conductors from the transformer to the switchgear. Conductor 1 will be run from the X1 transformer bushing to phase A bus at the switchgear. Conductor 2 will be run from the X2 transformer bushing to phase B bus at the switchgear. Conductor 3 will be run from the X3 transformer bushing to phase C bus at the switchgear. The ground conductor must be connected to the transformer ground bus and the other end must be connected to the switchgear internal ground bus. The switchgear termination box will usually have a ground-bus near the bottom of the enclosure that is bonded to the transformer tank - which should be bonded to the ground grid.
RE: 4 wire vs. 3 wire service
NEC also requires "additional" ground connection of X0 at the "outdoor" transformers. (with or w/o impedance).
Rafiq Bulsara
http://www.srengineersct.com
RE: 4 wire vs. 3 wire service
RE: 4 wire vs. 3 wire service
If your transformer will always be used to supply 3 wires connected loads, you could save the neutral cable between the transformer and switchgear. However, you may want to consider installing a CT inside transformer's LV junction box, to supply your earth fault protection scheme.