Bonding a step up transformer

[Sthome25]

The situation - I am installing a pair of single phase transformers (240V to 480V then back to 480V) for a step up then step down configuration in an all exterior run to correct a voltage drop issue as determined by the design engineer. I am the electrician going between my company's engineer, the design engineer, and the customer's engineer. I have asked every seasoned electrician and engineer I know without being able to get an accurate answer. So this will be a learning moment for me, so I humbly ask for some guidance. Please look at the attached pictures.

My question - Do I bond the neutral (X2 and X3 from the picture) taps to ground on the STEP UP side of the first transformer. And if possible could someone explain it the way mike does as to why?

Conclusion - I have the X2 and X3 bonded on the step down side with a ground rod and the Equipment Grounding Conductors thanks to my Mike Holt education, but I am confused on the step up side. I did not choose the equipment and cannot change it. I know I do not need a neutral from the service to the first transformer, just the 2 conductors and 1 equipment grounding conductor. I hope I have provided enough detail to explain my situiation and thank you for the help!!!

 

[LarryFine]

My question - Do I bond the neutral (X2 and X3 from the picture) taps to ground on the STEP UP side of the first transformer. And if possible could someone explain it the way mike does as to why?

No, the X2-X3 on what will be the primary should be left floating; treat it as a single 240v primary.

Be sure the transformer is approved for step-up use.

Use H1 and H4 on the first unit, and H1 and perhaps one of the taps on the second unit after checking the output voltage using H4.

 

[jim dungar]

Simple rule of thumb: unless you are a utility do not 'bond to ground' the source side of any transformer.

PV installation, and other bidirectional systems, follow different rules.

 

[electrofelon]

you are grounding the 480 system correct?

 

[tortuga]

For a single phase long distance feeder circuit I do 240V: 240/480V -----long feeder ---- 480V: 120/240V
I use a step up transformer with a 240V primary winding and a center tapped secondary 240/480V.
I bond the center tap (240V to ground), to limit the voltage to ground on my feeder.
On the step down side I would have a standard 480V : 120/240 transformer and bond the center tap of the 120/240 side as you normally do.

 

[jaggedben]

PV installation, and other bidirectional systems, follow different rules.

Do they, really? I didn't think so.

 

[electrofelon]

For a single phase long distance feeder circuit I do 240V: 240/480V -----long feeder ---- 480V: 120/240V
I use a step up transformer with a 240V primary winding and a center tapped secondary 240/480V.
I bond the center tap (240V to ground), to limit the voltage to ground on my feeder.
On the step down side I would have a standard 480V : 120/240 transformer and bond the center tap of the 120/240 side as you normally do.

I agree that is the ideal setup. However I think that transformer may be hard to find. Note there is a difference between a (V)/(2V) and a (V)x(2V) transformer, and all I could find are X on the 480 side. I dont really understand why, but the X is only for series or parallel operations and not for 3 wire service. I guess you could still ground it though? Whats the worst that could happen?

 

[tortuga]

I agree that is the ideal setup. However I think that transformer may be hard to find. Note there is a difference between a (V)/(2V) and a (V)x(2V) transformer, and all I could find are X on the 480 side. I dont really understand why, but the X is only for series or parallel operations and not for 3 wire service. I guess you could still ground it though? Whats the worst that could happen?

How about this one?

https://www.larsonelectronics.com/images/product/Specsheet/151074.PDF

Here is how I'd configure the feeder:

 

[LarryFine]

Isn't there an NEC provision that would allow the high-voltage portion to have one ungrounded and one grounded conductor, instead of two ungrounded and one grounded conductor?

 

[winnie]

Agreeing with the others: you do not bond the neutral of your step-up transformer on its 120/240V side. Your service is already grounded at the service entrance, grounding the neutral would reground the system and cause problems.

In your diagram I don't see grounding of the 480V derived system. You need to have something, such as the midpoint grounding @tortuga described or grounding one leg as @LarryFine described, or ground fault detection.

The transformer nameplate you provided specifies primary and secondary rather that HV and LV. Check with the manufacturer that the transformer is suitable for reverse use.

Jonathan

 

[Sthome25]

Thank you all for your time and responses. In my original post i mentioned I cannot change the equipment but I should have given more detail.

The Transformer - This equipment, from what I can tell, is intended as a step down (480V SP to 120/240V SP) that we are feeding backwards at the first location. I have three engineers that have said its fine and I need to use the equipment that has been submitted and approved. I agree that it is not the best suited equipment but it is what I have to work with, I just want it safe.

I am going to reach out to schneider electric to see if they have any input about the way we are installing this equipment. I will not ground the midpoint of the transformer (X2 and X3), just connect them as the nameplate directs.

 

[winnie]

I'd still go back and ask the engineers about the grounding of the 480V output of the first transformer.

As drawn it is completely ungrounded, with no fault current path. I'd ask if that was the design intent.

Jonathan

 

[jim dungar]

Do they, really? I didn't think so.

I said generally to not bond on the source. Which side is the source in a bidirectional system?

 

[jim dungar]

I am going to reach out to schneider electric to see if they have any input about the way we are installing this equipment. I will not ground the midpoint of the transformer (X2 and X3), just connect them as the nameplate directs.

When I worked for Schneider our recommendation was to let the X0 (X2-X3) connection float on step-up applications.
First, the NEC says you cannot rebond the neutral-ground after the service.
Second, connecting this neutral point to the service neutral can affect the voltage regulation of the step-up by not allowing the primary voltages to shift based on the secondary loading.

 

[winnie]

Do they, really? I didn't think so.

A PV system isn't really bidirectional. The primary of the transformer is the utility grid side, because that is the side which gets energized first and which defines the voltage of the system. This is true even though power is flowing from secondary to primary.

Where you get _different_ rules is because of utility 'effective grounding' requirements. These are intended to maintain utility phase to earth voltages as stable during fault conditions. The utility intentionally has multiple connections between circuit conductors and earth, which would violate NEC codes.

Sometimes these utility requirements extend into PV installations. It is because of these requirements that you will sometimes see things like wye:wye transformers used in PV installations where delta:wye transformers would be expected in a pure NEC installation.

I bet there are sometimes fights about where the NEC ends and utility rules begin.

Jonathan

 

[jaggedben]

I said generally to not bond on the source. Which side is the source in a bidirectional system?

The utility. So it's not different. See 705.30(C).

(Unless perhaps you have some bidirectional system in mind that isn't an interconnected power source like a PV system, but I don't know what that would be.)

See Jon's post. (Which I knew all of that already, but you guys are the engineers.) As a residential and small commercial PV installer I've never had to deal with utility 'effective grounding' requirements. That doesn't seem to kick in for PV systems under a megawatt or so. Nothing like that will apply to the OP here, even if there's a PV system he's not telling us about. Just to be clear.

 

[electrofelon]

Isn't there an NEC provision that would allow the high-voltage portion to have one ungrounded and one grounded conductor, instead of two ungrounded and one grounded conductor?

Yeah 250.30(A)(1) exception #2. Allows for only two conductors since the grounded conductor also serves as your EGC. So actually, yeah, I have to take back what I agreed with @tortuga about grounding the midpoint being ideal, , that would make running a third conductor necessary

Granted I don't know all the details but I find this to be a horrible design. First they are NOT using the aforementioned exception so it's an extra conductor, there seems to be an extra disconnect that is not necessary, and I question whether all this is worth it over just using larger conductors. Probably yet another case of people ignoring voltage drop in the transformers and thinking they have perfect voltage regulation.

 

[electrofelon]

How about this one?

https://www.larsonelectronics.com/images/product/Specsheet/151074.PDF

Here is how I'd configure the feeder:
View attachment 2571372

Yeah nice find, appears to be intended as a step up too. But as I just mentioned I think I would prefer a skipping the third conductor. Sure having it grounded in the center lowers the system to ground voltage, but does that really matter? What does that gain you?

 

[jim dungar]

The utility. So it's not different. See 705.30(C).

(Unless perhaps you have some bidirectional system in mind that isn't an interconnected power source like a PV system, but I don't know what that would be.)

See Jon's post. (Which I knew all of that already, but you guys are the engineers.) As a residential and small commercial PV installer I've never had to deal with utility 'effective grounding' requirements. That doesn't seem to kick in for PV systems under a megawatt or so. Nothing like that will apply to the OP here, even if there's a PV system he's not telling us about. Just to be clear.

As mentioned PV is not truly bi-directional until it is up and running, but it could be if there there is a battery storage or maybe even a generator on the same side of the transformer.

I guess this is why I said it was a 'simple' rule of thumb.

 

[augie47]

Thank you all for your time and responses. In my original post i mentioned I cannot change the equipment but I should have given more detail.

The Transformer - This equipment, from what I can tell, is intended as a step down (480V SP to 120/240V SP) that we are feeding backwards at the first location. I have three engineers that have said its fine and I need to use the equipment that has been submitted and approved. I agree that it is not the best suited equipment but it is what I have to work with, I just want it safe.

I am going to reach out to schneider electric to see if they have any input about the way we are installing this equipment. I will not ground the midpoint of the transformer (X2 and X3), just connect them as the nameplate directs.

You might want to acquaint the engineers with NEC 450.11(B)