Bonding to xo when not in use

[james Graham]

Recently our company installed a 480va 3- phase primary, to 208/120 volt secondary, dry type transformer.Backfeeding it with 208va and connected a conductor to the xo routed in metal race way terminated on neutral bar.This neutral had no load (not that we would have had 277va)But it was installed by habit.the neutral and ground became so hot that they burnt.Can this be explained? This incident has made us leery of bonding the xo to ground when a neutral load does not or will not exist. Should or does the NEC have anything covering this issue?

 

[don_resqcapt19]

You don't use XO when you back feed a transformer.
Don

 

[Pierre C Belarge]

James
Did you use X/O from the 208v side and install it to the 480v side?
If so, how did you terminate the equipment ground and the grounded (neutral) conductor in that disconnect or panel?

 

[james Graham]

x/o and bonding

x/o and bonding

I bonded x/o to ground in transformer and from X/O to neutral bar that was not bonded. like previously stated there was nothing else on that bar.
engineer says it was electro magnetic field.I'm sceptical.

 

[roger]

James, the XO tap has no physical tie to the H windings and would not help even if a neutral were needed.

However, it would be a very dangerous situation if it were connected to a circuit on the 480 volt side of the transformer.

Has the manufacturer of the transformer blessed this installation?

Roger

 

[ronaldrc]

X0 needs to be bonded to your grounding electrode system rather you use the neutral or not for safety reasons.

Without it if line 1 ,2 or 3 goes to ground then you have 208 from ground to the other two lines that are not grounded.

 

[ronaldrc]

I missed the backfeed part as long as the feed has a bonded neutral or Xo if would probably be ok because the feed breaker would kick and clear the short.

 

[eric stromberg]

In this case (excuse the pun), the X0 should not be bonded. Bonding it would be in violation of 250.24 (if it is being fed from a service) or of 250.30 (if it is being fed from a separately derived system).

Bonding the X0 would create a neutral to ground connection on the load side of service/separately-derived-system and would create alternate paths for neutral current to get back to the source, in violation of 250.6.

Eric Stromberg, P.E.

 

[jtester]

I agree that the X0, on the 208 side, should not be bonded but I disagree with why. I have encountered this many times, and there are numerous posts on this site.

What actually happens is that the grounded wye-delta transformer bank, 120/208 to 480, will balance the loads on the 208 side, high side in this case, of the system. I've seen installations like this where the 480 side has no wires connected to it, and the 120/208 breaker will trip.

If you are familiar with symmetrical components and sequence connections, the zero sequence connection for a grounded wye-delta transformer installation will show why that happens.

Jim T

 

[ronaldrc]

I agree I missed the part about the back feeding of a transformer.

I once went on a service call to a medical Bldg. here where the electrical contractor had forgot to bond the x0 on a 208 service to the ground and it did thousands of dollars of damage to the Equipment when a hot leg of a sub panel went to ground.

Every since, then when I hear some one say they are not going to bond the xo this comes to mind.But this is a different situation.

In this case it should not be bonded because it would make the equipment ground a parallel neutral.

 

[iwire]

In this case it should not be bonded because it would make the equipment ground a parallel neutral.

I do not follow you, what would be the complete circuit?

 

[ronaldrc]

Bob

Yelp your right what neutral? But if we had a neutral

 

[hardworkingstiff]

Curious, why did you need to boost the voltage? What did you feed with the 480-volts?

Was the transformer a delta/wye or a wye/wye (I assume it was a delta/wye)? I hope this is not a dumb question, I've not done much transformer work.

 

[ronaldrc]

Bob
I never got a response if James did what he said in his second quote you would agree that would be paralleling the neutral and equipment ground wouldn't it? :roll:

 

[sandsnow]

I've investigated this with two manufacturer's. FPE(new) and Eaton. Also talked with a GE engineer.

It is OK to backfeed a XFMR. You can take a Delta-Wye and feed the Wye as a Primary. That would make the "X" taps the Primary connections

It is NOT "recommended" over 75kVA

The reason being High Inrush magnetizing Currents. You need to size the Primary OCPD at max permitted by Code. Apparently this is not enough when the XFMR is over 75kVA. After this the theory gets a little beyond me.

You do not connect a primary neutral to Xo. You do not bond Xo.

I have some scary yet humorous pictures of a real life scenario of a person backfeeding a Delta-Wye.

 

[ramsy]

I have some scary yet humorous pictures of a real life scenario of a person backfeeding a Delta-Wye.

Larry, could you use a digital camera to copy those photo's here?

 

[Pierre C Belarge]

Larry
I agree, that in this case, X/O does not get bonded, even though this is the wye side of the transformer. And you certainly do not Bond X/O and then install the conductor from there to the 480 side panel or disconnect.

 

[jtester]

There are numerous transformer connections where the high side neutral must be bonded. An open wye-open delta, for example won't work without a ground on the wye side. A wye-wye with the secondary grounded is generally undesirable unless the primary is grounded as well.

Grounding the high side isn't generally a code issue, it is an issue regarding how the three phase bank works in the circuit.

Jim T

 

[sandsnow]

Here's a photo of a backfed delta-wye where the intent was to derive 48oY/277
Note the white that leaves with the B-O-Y connected to Xo

 

[sandsnow]

Here's the nameplate from above XFMR