Transformer XO

[svaurez]

I have a question about the equipment bonding jumper running from the transformer to the panel.Where does this wire terminate in the transformer?In the Soares book it shows it terminating to the transformer case.In the NEC Handbook it shows it terminating on the XO lug.Which is correct?

 

[bennie]

Re: Transformer XO

The ground conductor, with the supply conductors from another system, should connect to the X-O of the transformer.

This connects all star points together for fault clearing purposes.

 

[dereckbc]

Re: Transformer XO

svaurez, it depends on which option you choose. Refer to 250.30(A)(1)

Basically it depends on where you land the GEC.

1. If you land the GEC to the transformer Xo; the bonding jumper connects from the Xo to the transformer case or grounding terminal.

2. If you land the GEC on the neutral bus in the first disconnect device; then you connect the bonding jumper from the neutral bus to the ground bus in the disconnect device. The ground bus in the disconnect device is then bonded to the disconnect chassis. You would then run a EGC along with phase and neutral conductors between the transformer to the disconnect.

 

[svaurez]

Re: Transformer XO

It is my understanding that there are two bonding jumpers.Assuming that the GEC will terminate in the transformer,I run a bonding jumper from XO to the case AND THEN I run an equipment bonding jumper to the panel.Where does this second jumper terminate in the transformer?

 

[roger]

Re: Transformer XO

If you install the GEC at the transformer, you will install a bonding jumper to the transformer case. From the transformer to the panel you will install the ungrounded conductors and a grounded conductor which is connected to XO, this is the end of the transformer connections.

In the panel you will will install a bonding jumper from the neutral/ground bar to the panel can.

You could run an EGC from the XO of the transformer to the panel, but you would seperate (isolate) the neutral and "this EGC" in this case. (treat it as a sub-panel)

Roger

 

[dereckbc]

Re: Transformer XO

If you have the 2002 Handbook look at exhibit 250.13.

You run one bonding jumper from the Xo to the transformer case, and another from the Xo or neutral buss in the transformer to the ground bus in the disconnect device.

 

[svaurez]

Re: Transformer XO

Dereck in the Soares book it shows the second bonding jumper starting at the transformer case and terminating on the ground bus in the disconnect.Which one is correct?

Roger,I was told that there is no EGC on the secondary side of a transformer.It is called an equipment bonding jumper and sized by 250.66.
Maybe im just confused.

 

[roger]

Re: Transformer XO

Svaurez, I don't have the Handbook or the Soares book, but either of the diagrams below would be fine.


Maybe I'm the one missing the translation.

Roger

 

[svaurez]

Re: Transformer XO

In the diagram on the right the EBJ lands on the XO and not to the transformer case.Is there a code section that requires this or I can I land it in either position?

 

[roger]

Re: Transformer XO

Svaurez, you can make the connection to the transformer enclosure or the XO terminal.

Making it to the XO terminal is more of a direct connection to the fault clearing point of the source.

Roger

 

[dana1028]

Re: Transformer XO

svaurez - are you referring Fig. 12-7 in Soares 8th edition?....of so, there is an error in that diagram...Soares has numerous errors in the 8th edition, you must get their 'eratta' sheet and correct lots of diagrams.

 

[svaurez]

Re: Transformer XO

I was referrring to figure 12-5.The line side equipment bonding jumper terminates to the transformer case.In the NEC Handbook Exhibit 250.13,it terminates to the XO.

 

[bennie]

Re: Transformer XO

Transformer enclosures are for enclosing the transformer. They are not intended to be a ground plate or bus.

All connections should be on the appropriate bus and lugs.

 

[donnie]

Re: Transformer XO

You need to also keep in mind ? Art 250.30(A)(1).
NEC 2002, The bonding Jumper connection shall be made at the same point as the grounding electrode conductor. ( Not 2 diffrent locations ).


Hope this helps.

 

[tepres]

Re: Transformer XO

With regard to the graphic above, I do not believe the one on the left is correct.
If you connected a jumper from xo to chassis here, you would have a parallel path (NEUT/GND).
Also, You would need to connect the GEC to the panel bus.
250.30(A)(2)(a)

 

[roger]

Re: Transformer XO

tepres,

With regard to the graphic above, I do not believe the one on the left is correct.
If you connected a jumper from xo to chassis here, you would have a parallel path (NEUT/GND).

exactly where is this parallel path to and from? Please explain where you see the current flowing. Do you remove the factory strap from every transformer you install, and if so why?

Also, You would need to connect the GEC to the panel bus.
250.30(A)(2)(a)

where is this stated?
Once again, please explain.

Roger

 

[mclain]

Re: Transformer XO

I would agree with tepres about the parallel current paths. If you bond xo to ground at the transformer and bound it to the ground bar as shown in the diagram on the left you will have part of the neutral current flowing through the grounding cable. Most likely you would also end up with current flow through any metal decking or conductive material that made connection with the transformer as well. Electrons take all paths not just the path of lease resistance.

 

[roger]

Re: Transformer XO

McClain, if there were a conductive path between the xfrmer and the panel in the diagrams (as in metalic conduit for example) there would have been no reason for the second drawing, and we would modify the jumpers shown on the left.

If you bond xo to ground at the transformer and bound it to the ground bar as shown in the diagram on the left you will have part of the neutral current flowing through the grounding cable.

Where is this "grounding cable" you are seeing in the diagram on the left?

There is no EGC shown, only a Neutral.

Roger

 

[mclain]

Re: Transformer XO

Roger
In the diagram you show the neutral/ground bar bonded at the disconnect. The disconnect is required to be grounded by code. If you bonded the neutral at the disconnect and the transformer, then bond both to ground you have a parallel current path. In the diagram you also show a neutral/ground bar. If you connect the GC and EGC both to this bar you are creating more parallel paths. The neutral current will flow through the EGC to ground, and back to the ground connection at the transformer. You mentioned removing the bonding cable on every transformer in one of your post. I have installed a lot of transformers but have never seen one come with the neutral bonded to the transformer frame. The conductive path I was referring to in my post is your ground connections. I hope they are conductive.

 

[roger]

Re: Transformer XO

McClain,

In the diagram you show the neutral/ground bar bonded at the disconnect.

correct this is per code 250.4(B)(2) for starters.

The disconnect is required to be grounded by code.

I think your confusing bonding and grounding.

The GEC can be at any location on the line side of the disconnect.

If you connect the GC and EGC both to this bar you are creating more parallel paths.

In the left hand diagram how many conductors beside the gray conductor do you see? Explain where you see a parallel path and we will will modify the diagram :roll:

You mentioned removing the bonding cable on every transformer in one of your post. I have installed a lot of transformers but have never seen one come with the neutral bonded to the transformer frame.

You've got to be putting us on


Actualy I asked if Tepres removed the factory bond on every transformer he installed, but since you obviously haven't ever seen one in a transformer you have answered the question as far as your posts are concerned.


Roger

[ January 09, 2004, 01:08 PM: Message edited by: roger ]