Transformer X0 bonding

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Hoodood

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We have a 3 phase 480v step down to 208v transformer. There is no neutral required for the secondary load side. Does the X0 need to be bonded to structural steel if there is no need for a neutral?
 
One might be able to not ground the neutral - but only if it doesn't leave the transformer enclosure. You still have to ground something though, which leaves you with one of the phase conductors, or set up as an ungrounded system and provide ground fault monitoring.

If the neutral does leave the transformer then it definitely must be the conductor that gets grounded.
 
kwired said:
One might be able to not ground the neutral - but only if it doesn't leave the transformer enclosure. You still have to ground something though, which leaves you with one of the phase conductors, or set up as an ungrounded system and provide ground fault monitoring.

If the neutral does leave the transformer then it definitely must be the conductor that gets grounded.
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I do not think the code section I cited allows an ungrounded 208Y/120V secondary.
 
jumper said:
I do not think the code section I cited allows an ungrounded 208Y/120V secondary.
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Most will read it that way, but I don't know it is worded quite the way it is intended. Do you see a problem other than possibly complying with this section with having a standby generator that is 480/277 to back up a corner grounded delta system and grounding a phase/leaving the neutral float on such system?

Another example that may be a little more common would be a transformer to supply foreign equipment designed for 240 volt two wire system. In that case a common 120/240 transformer would work but ground one end and leave center tap float. I have no problem with that but strict interpretation might say no. But if one brought the center tap out for any use at all then it should be the conductor that is grounded to comply with this section.

JMO.

Add: I have seen 12 lead generator that can be set up for many different voltages, single and three phase. One will have leads that get left floating for different system arrangements - another reason I believe this rule should only apply to the conductors you bring out of the source for possible utilization. POCO's also have floating terminals on transformer banks - they are not utilized as a system conductor.
 
jumper said:
kwired, my little brain cannot figure out all that.

Can we just keep it simple and bond XO to the steel?

It a wye secondary, life is much easier that way.

No muss or fuss.
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99% of the time yes, you must bond that neutral. Some special situations one may not be using that neutral for anything, or even desiring to ground some other point of the source for a specific reason. You can't ground two points, that lets smoke out of insulation.
 
Bonded and done

Bonded and done

Thanks guys! I have always bonded X0 but I'm on another job & noticed no bond so before I open my mind to these guys I thought it best to reach out & verify. Thanks again!
 
for what its worth hood, yes, bond it but i always use a dedicated terminal block for equipments down low attached directly to the chassis, leaving the xo block with just one green bond jumper and a neutral..it also keeps that bare rebar conductor away from those ungrounded lugs... you know, when some one backs a fork lift into the tranny? :) oh, it happens... also is easy on the inspectors eyes B)
 
Eyes wide shut

Eyes wide shut

rambojoe said:
for what its worth hood, yes, bond it but i always use a dedicated terminal block for equipments down low attached directly to the chassis, leaving the xo block with just one green bond jumper and a neutral..it also keeps that bare rebar conductor away from those ungrounded lugs... you know, when some one backs a fork lift into the tranny? :) oh, it happens... also is easy on the inspectors eyes B)
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Yes, I agree, the AHJ always looks at a transformer so when the X0 is bonded they smile! Thanks again guys!!!
 
wouldnt that mean a neutral has to be pulled with the primarys? or lines..
we still want the separate der. neuty bonded, right?
 
rambojoe said:
wouldnt that mean a neutral has to be pulled with the primarys? or lines..
we still want the separate der. neuty bonded, right?
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EGC with primary yes, neutral only if the primary winding is going to utilize it. Delta primary winding doesn't utilize a neutral conductor.
 
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