Bonding XO when not using a neutral conductor

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Mike_J_H

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Hello

Had a debate about the correct way to wire a transformer, hoping for some help. We have a 480V 3ph 4W panel feeding a 480V to 240V delta wye transformer. From the transformer, we are feeding a 3-phase enclosed circuit breaker, which is feeding a CNC machine. The panel is fed with a neutral, but there is no neutral being used anywhere in this particular feed. We have ground wires coming from the ground bus in the panel to the transformer, going out from the transformer to the ground lugs at the ECB, and from the ECB to the machine. There is also a ground wire ran from the transformer to building steel. All grounds are connected to ground bushings at each end of every conduit, and are connected to a factory installed ground bus inside of the transformer itself.

The question is whether or not we also need to connect the transformer ground bus to the XO.

Thank you very much for any help you can offer
Mike.
 
Yes, the transformer's secondary neutral must be bonded to the EGC system, so a fault to ground generates enough current to open OCP devices.
 
Yes, it's required as Larry stated because what you currently have is an ungrounded secondary and a WYE secondary is required to have the X0 connected with a system bonding jumper (SBJ) to the case/EGC terminal. Some helpful terminology, which is important to know when sizing the various "grounds":

EGC: Equipment Grounding Conductor- the conductor between the panel and the transformer primary and the enclosed circuit breaker and the machine
SBJ: system bonding jumper- connects the X0 to the case/EGC terminal on the transformer secondary
SSBJ: Supply Side Bonding Jumper- conductor run with the secondary conductors to the enclosed circuit breaker
GEC: Grounding Electrode Conductor- conductor run from X0 to the grounding electrode, in this case building steel
 
I think we need some clarification as I note that the OP says that the secondary is 240 volt. If this is really 240 volt 3 phase I think it is likely he has a center tapped 240/120 delta secondary.
 
Mike_J_H said:
Hello

Had a debate about the correct way to wire a transformer, hoping for some help. We have a 480V 3ph 4W panel feeding a 480V to 240V delta wye transformer. From the transformer, we are feeding a 3-phase enclosed circuit breaker, which is feeding a CNC machine. The panel is fed with a neutral, but there is no neutral being used anywhere in this particular feed. We have ground wires coming from the ground bus in the panel to the transformer, going out from the transformer to the ground lugs at the ECB, and from the ECB to the machine. There is also a ground wire ran from the transformer to building steel. All grounds are connected to ground bushings at each end of every conduit, and are connected to a factory installed ground bus inside of the transformer itself.

The question is whether or not we also need to connect the transformer ground bus to the XO.

Thank you very much for any help you can offer
Mike.
Click to expand...
Mistake or do you actually have a 240 volt wye secondary instead of 208?

IMO you don't even need a "neutral" to the enclosed breaker.

If this is intended to be a grounded system though then the neutral is the conductor to be grounded. It can be bonded at the transformer or at the first disconnecting means.

Between the transformer and first disconnecting means it is called the supply side bonding jumper - important to know that for determining what size of conductor it needs to be.
 
yes, as noted, required
in addition to fault clearing if a ph faults to gnd your grounding system/frame goes to ph-n voltage

if the X0 is floating, ie, 0 seq reactance, you will flow no 0 seq current, ie, gf current
upon a ph-frame fault the frame goes to phase voltage...and stay that way
not a shock hazard since 0 seq Z is infinite

your post is kind of hard to follow, the X0 is isolated/floating? no intentional ground bond?
or is it internally bonded to the xfmr gnd lug?
a sketch is worth a 1000 words
how do you get a neut from a 240 delta?
 
Ingenieur said:
yes, as noted, required
in addition to fault clearing if a ph faults to gnd your grounding system/frame goes to ph-n voltage

if the X0 is floating, ie, 0 seq reactance, you will flow no 0 seq current, ie, gf current
upon a ph-frame fault the frame goes to phase voltage...and stay that way
not a shock hazard since 0 seq Z is infinite

your post is hard to follow, the X0 is isolated/floating? no intentional ground bond?
or is it internally bonded to the xfmr gnd lug?
a sketch is worth a 1000 words
how do you get a neut from a 240 delta?
Click to expand...
He said " 480V to 240V delta wye transformer"

My guess is either 208 and mistakenly written 240 or he has somewhat rare system - 240 volt wye would have 138 volts each line to neutral.

Sort of doesn't matter to certain extent - he isn't utilizing any line to neutral loads.
 
kwired said:
He said " 480V to 240V delta wye transformer"

My guess is either 208 and mistakenly written 240 or he has somewhat rare system - 240 volt wye would have 138 volts each line to neutral.

Sort of doesn't matter to certain extent - he isn't utilizing any line to neutral loads.
Click to expand...
I have installed a few 240Y/138 transformers used as drive isolation transformers. 250.20(B)(1) requires this transformer secondary to be connected as a grounded system.
 
Hopefully he's not using a 240Delta/480VWye in reverse.

He needs to show a photo of the tag so we can see what he's working with.

He may not need a neutral for any loads, but it's likely that the drives in his CNC want to have a wye output for the MOV protection.
 
Thanks for all the input guys, I really appreciate it. Sorry I wasn't able to reply sooner. I made a mistake in my original post, the transformer was indeed 480/208, not 480/240. There are a total of three- one 480/208 delta/wye and two 480/240 delta/delta --- I got them mixed up while typing.

XMFR T3.jpg

This is how we ended up wiring the transformer, with the line from the panel coming in on the right, and the load out the ECB on the left. We have a jumper from the X0 to the ground bus bar, and the fourth wire from there goes out to building steel.

(spec on this job is for wire to be Red Orange Black regardless of voltage)
 
Mike_J_H said:
...This is how we ended up wiring the transformer, with the line from the panel coming in on the right, and the load out the ECB on the left. We have a jumper from the X0 to the ground bus bar, and the fourth wire from there goes out to building steel.

(spec on this job is for wire to be Red Orange Black regardless of voltage)
Click to expand...

I have no personal argument with that spec, but how did you comply with 215.12(C)
 
Agreed on the 215.12C, feeders should have different colors or tagging to differentiate the different voltage systems and be posted on equipment, for personnel to be able to tell the difference of each nominal voltage system. Since you have three nominal voltage systems, you should have three different color or tagging arrangements.

On the delta to delta transformers, does the low voltage side have a center tap XO? Did you bond it the same way as the Y transformer secondary?

Is your bonding jumper from the XO terminal sized in compliance with 250.102 (C) (1) ?
 
Methinks confusion exists around XO as a bond or as a component of an SDS .

I just went through this with an install, not mine, where i could not get a reading from any hot to ground......

iCXPOOt.jpg


Realizing no reading equates to not grounded, led me to the manufacturer.

I had not seen an XO and was confused, but in fact one did exist under the coils, not out front with the rest of the terminations

simply bonding it allowed for any given secondary hot to read to ground.

you wouldn't believe the generation of sparks who had introduced all manner of extraneous and redundant EGC's downstream ,in a futile effort to ground the secondary circuitry, all of it nonfunctional as a path to OCPD performance.

two feet of CU cured it all


~RJ~
 
MTW said:
Agreed on the 215.12C, feeders should have different colors or tagging to differentiate the different voltage systems and be posted on equipment, for personnel to be able to tell the difference of each nominal voltage system. Since you have three nominal voltage systems, you should have three different color or tagging arrangements.

On the delta to delta transformers, does the low voltage side have a center tap XO? Did you bond it the same way as the Y transformer secondary?

Is your bonding jumper from the XO terminal sized in compliance with 250.102 (C) (1) ?
Click to expand...

I agree 100% on phase colours, but this facility has their way of doing things and we're forced to comply if we want to keep working there. Every piece of equipment we hook up has engraved labels telling voltage (input/output in the case of transformers), what it feeds, and where it is fed from.

One delta to delta transformer has an X4 and the other an X6, center tapped between A & B and between C & A (I believe). For both: voltage was 240V between each phase, 212V highleg to ground, and 120V other two legs to ground. For both, X4/X6 are bonded to building steel and bonded to a ground bus on the transformer case.

Bonding jumpers were oversized to 2AWG on all three transformers.
 
Kinexis said:
the paint under that ground bar doesn't look very conductive. was that in there from the factory?
Click to expand...

That's a factory bar I don't understand why factory EGC terminals do not have to remove the paint but field installed ones do.
 
infinity said:
That's a factory bar I don't understand why factory EGC terminals do not have to remove the paint but field installed ones do.
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it's tested by a recognized laboratory, either way i would remove the paint and use that grease you used on the transformer connections.
 
Kinexis said:
it's tested by a recognized laboratory, either way i would remove the paint and use that grease you used on the transformer connections.
Click to expand...
Even field installed bars sometimes get attaced to holes already tested and intended to accept the bar or lug. I never remove paint on those.

They aren't any better or worse then many loadcenter "bonding screws". Seen a few of those fail because they maybe weren't properly tightened when installed - they they fail when called upon to carry fault current.
 
Kinexis said:
it's tested by a recognized laboratory, either way i would remove the paint and use that grease you used on the transformer connections.
Click to expand...

I agree that is the reason, I would just use the bar as is from the manufacturer why bother with the extra work?
 
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