Can you convert a 3-wire service to a 4-wire service calling the Equipment Ground the NEUTRAL ?

[bdn2004]

Below is a sketch of a service from a Customer owned 12.47kV-480V transformer. There exists another service that comes into this same building that feeds the 277V lighting loads. That total load is no more than 200Amps, 480V, 3-phase. This is being re-worked so that there is only one service. And the lighting circuits will be fed out of this larger transformer also.

There is a #3/0 EG ran with each of the #500's in parallel in 3" EMT conduits. 3/0 has an ampacity of 200A. So we have an ampacity of 1600A of wire running between the service entrance and the transformer.

The question is ... if we jumpered between the neutral bus and the ground bus at the service - we would have our 277V needed for lights and we could just re-feed those lighting panels directly. We realize we could install 4W transformers at the panels, but this would be the simplest way, but is it a Code violation?

 

[jraef]

In a word, no. It would not be Code compliant.
(Assuming you are in the US by virtue of 480/277V service)

If there is room in the conduit, use the existing ground wire to pull a new neutral and ground wire though to the switchgear.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bdn2004]

Thanks... this is a case right here where it seems good engineering judgment should be allowed to override the conservative safety design criteria of the NEC.

 

[itsmoked]

I'd have to say it's more of a screw-up by the system builders who didn't bother to provide for the neutral now trying to band-aide the result. While you may have a point that type argument never goes over well with the AHJ.

Can the ground be removed from the transformer's immediate location being moved to the ground-bar where it gets bonded to earth? That would solve it but I don't know what the rule is on just how close to the transformer the bonding must occur.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bdn2004]

Itsmoked,

So you are saying remove the earth ground at the transformer and only ground it at the building service, then bond the EG to the Neutral at that point ? But we still have to bring over the center tap of the transformer - which are undersized for a neutral pretty sure.

I think we are screwed trying to find a cheap fix. Pulling in another 500 in each conduit looks about impossible. Transformers at the 480/277V panels seems to be the most practical solution.

 

[JezNZ]

I can't understand how you would necessarily make it work with good engineering judgement.

In the IEC world, this would be converting the PE to a PEN conductor, however it would normally involve the use of a fully rated neutral conductor. In this case, the grounding conductor appears to be minimally sized for loop impedance and short-circuit temperature rise, which would be based on carrying no current under normal conditions, ie starting from a 20deg (or whatever your nominal temp. is).

As soon as the grounding conductor is used to carry current under normal operating conditions, those loop impedance and short-circuit temperature rise calculations are invalid.

Are you confident that the grounding conductor size would still be suitable under the new operating conditions?