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4 wire vs. 3 wire service

4 wire vs. 3 wire service

4 wire vs. 3 wire service

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

You need to bring the neutral in to the service equipment and ground per NEC requirements.This ground can be solid or reaistance.
The neutral is your ground fault return path.

RE: 4 wire vs. 3 wire service

Thanks Reddog.  This was my plan - I was pretty positive that was my only option, but I kept hearing from folks that the neutral could be terminated at the transformer.  Just didn't sound right.




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

I agree with antigfk. What Reddog said (bringing N to service equipment) is only applicable to systems of less than 1000V.

NEC also requires "additional" ground connection of X0 at the "outdoor" transformers. (with or w/o impedance).



Rafiq Bulsara

RE: 4 wire vs. 3 wire service

We used bringing N to service equipment only to systems less then 1000V AND 4 WIRE.

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.  

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