Converting 240 delta ungrounded to corner grounded

[line_noise]

Job has a couple of 480 delta-240 delta transformers we are replacing, each serving a box with 3 fuse holders for transformer secondary OCPD, and taps to fused switches to serve 3 phase motor loads. Pretty sure it was all ungrounded, as I didn't find any evidence of a grounding electrode or bonding jumper, and the color codes are Black/Red/Blue. Loads are three phase motors. Lighting, receptacles etc. in the plant are fed from different transformers.

Owner has preference for solid grounding, so we are planning on changing to corner grounding and installing new panelboards to replace the old switches and gutters. We made sure the breakers have correct voltage ratings. So we are good there.

Only thing I can't get around, is Article 200 requiring that the grounded conductor be white. We can tape the bigger ones, but tracing and replacing all of the smaller ones will be a pain. Lots of stuff is 60 year old cloth wiring, and we'd have to repull all the circuits, which we weren't planning on, and is going to require more downtime.

Can anyone comment on the safety issue on a 3-wire delta system of having white grounded conductor in 200.6? We'd like to just ground the B phase and leave it red, but there must be some reason code says what it does. I guess if the inspector won't allow it, we can pull the bonding jumper and leave it ungrounded.

Any insight would be appreciated, I don't run across delta secondary systems often.

 

[jumper]

I can tell that I hate the rules for identifying grounded and grounding conductors smaller than #4.

There is no sound reason in my mind why this rule still exists.

A red #4 conductor with white identification tape is safe, but somehow a #6 isn't. Balderdash and phooey I say!!!!

 

[petersonra]

It is just a rule in the code.

IMO, the old cloth covered conductors are probably somewhat of a hazard after all these years, and I would be inclined to rip them out and start over.

 

[don_resqcapt19]

Assuming that you are talking about a 3 phase corner grounded system, even though the code requires the grounded conductor to be white, I have never seen that in the field.

The code requires it because it is a grounded conductor and it should be identified the people that work on it have a better understanding, but given that many have no idea what a corner grounded system is, it probably causes more confusion to have a white wire as one of the 3 wires that feed s 3 phase load. (not saying you shouldn't follow the code rules)

 

[highlegdelta]

If you are replacing the transformers anyway, why not get one with a center tap in one winding. Ground the XO and leave the three phases ungrounded.


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[line_noise]

If you are replacing the transformers anyway, why not get one with a center tap in one winding. Ground the XO and leave the three phases ungrounded.


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We thought about that, but are worried about putting 277v to ground on one phase of 230V motors.

 

[dkidd]

It would be 208.

 

[highlegdelta]

We thought about that, but are worried about putting 277v to ground on one phase of 230V motors.

It is 240v phase to phase and 208v to ground on one phase.

 

[line_noise]

It is 240v phase to phase and 208v to ground on one phase.

Thank you! It has been too many years since I've looked at this. Duh on me, I should have remembered that it was high leg because of the danger of using 1-pole breakers on b-phase and putting 208V on to loads that were expecting 120V.

For some reason when it came up this time, I thought that mid-tap ground was always called high leg because the voltage would be higher, so I divided line-line by 0.866 instead of multiplying. Looking at a triangle, there is no way for any voltage to ground to be greater than 240, regardless of where you put the ground.

 

[dkidd]

I thought that mid-tap ground was always called high leg because the voltage would be higher

The line-to-ground voltage is higher than that of the other two phases.

 

[winnie]

Depending on what sort of lead time or non-standardness you are comfortable with, you could use 480: 240/138V wye transformers and ground the neutral.

This gives you 240V to feed the 240V loads, and minimizes the voltage to ground. Of course you would still need 240V rated breakers; I don't think you can get slash rated breakers for this application.

-Jon

 

[kwired]

Depending on what sort of lead time or non-standardness you are comfortable with, you could use 480: 240/138V wye transformers and ground the neutral.

This gives you 240V to feed the 240V loads, and minimizes the voltage to ground. Of course you would still need 240V rated breakers; I don't think you can get slash rated breakers for this application.

-Jon

He had fuses in the OP anyway. If he goes corner ground he will need to eliminate all the fuses in the grounded conductor though, where a breaker would be permitted on a grounded conductor if it disconnects the other two lines simultaneously.

 

[don_resqcapt19]

He had fuses in the OP anyway. If he goes corner ground he will need to eliminate all the fuses in the grounded conductor though, where a breaker would be permitted on a grounded conductor if it disconnects the other two lines simultaneously.

Easy to do with dummy fuses.

 

[kwired]

Easy to do with dummy fuses.

And hope it is only serviced by qualified personnel.

When corner grounded systems were common around here on irrigation equipment you would occasionally get a farmer that moved the dummy to a fuse slot that kept blowing for whatever reason.

Of course that dummy wasn't always something specifically intended for the purpose either in those days, standard copper tubing was common, 3/4 copper water pipe was about right for a 30 amp 600 volt fuseholder.