transformer bonding

[tkirk911]

I HAVE AQUESTION REGARDING DRY TRANSFORMER CONNECTIONS. I HAVE BEEN TAUGHT BY DIFFERENT PEOPLE TO DO IT DIFFERENT WAYS. THIS IS HOW I DO IT NOW.
1) FEED TRANSFORMER WITH (3) HOTS AND (1) GROUND. CONNECTING GROUND TO METAL FRAME OF TRANSFORMER.
2) CONNECT BUILDING STEEL TO METAL FRAME OF TRANSFORMER.
3) INSURE BONDING STRAP IS CONNECTED ON TRANSFORMER XO AND FRAME.
4) FEED NEW PANEL WITH (3) HOTS AND (1) NEUTRAL CONDUCTOR.
5) BONDING METAL FEEDER PIPE WITH BONDING LOCKNUT.
6) MAKE UP MY NEW PANEL, MAKING SURE TO BOND MY NEW PANEL WITH BONDING SCREW.

I WAS RECENTLY TOLD THAT I AM NOT ALLOWED TO BOND THE NEW PANEL BECAUSE BONDING WAS MADE IN TRANSFORMER. I WAS REQUIRED TO RUN A GROUND WIRE WITH FEEDER WIRES AND SEPARATE EVERYTHING IN PANEL.

WHO IS CORRECT?

AN URGENT REPLY IS NEEDED IF POSSIBLE BECAUSE I HAVE 6 TRANSFORMERS TO WIRE IN THE NEXT MONTH.

THANKS

tkirk911@hotmail.com

 

[fc]

Re: transformer bonding

If your bonded in the transformer you don't bond at the panel.

 

[don_resqcapt19]

Re: transformer bonding

First, in my opinion, the grounding electrode conductor must connect directly to the grounded conductor and not to the transformer frame. See 250.30(A)(2). If you have bonded the grounded and grounding conductors together in the panel, you cannot also make a bond at the panel. See 250.30(A)(1). An equipment grounding conductor is required between the transformer and the panel and an equipment grounding bar is required in the panel. Note that metallic conduit is permitted to serve as the equipment grounding conductor, but not if there is a section of flexible conduit in the run.
don

 

[iwire]

Re: transformer bonding

In short no, you can not bond it in two places.

With it bonded at the panel and the transformer a large part of the Neutral current is flowing through the metal raceway.

On the transformer XO you should have the neutral conductor, main bonding jumper, and the building steel connection.

Equipment grounds land on the transformer enclosure along with main bonding jumper and carry through from one panel to the next.

 

[fc]

Re: transformer bonding

If your bonded in the transformer you don't bond at the panel.

 

[bennie]

Re: transformer bonding

I promise not to take this topic into my favorite subject

I also lie. A transformer should always be bonded at the X-O. This is to prevent a winding to winding fault from appearing on the premises wiring. A high voltage fault should be contained inside the transformer not at the panel.

Imagine a 13.2 KV primary blowing through to the 208 side.

Exception No.1, to 250.30(A)(1) permits bonding in both places for the ones who call a transformer a separately derived system.

I call the premises wiring a separately derived system if it meets the definition.

When the premises wiring is a separately derived system, the transformer is the source. The bonding can be done at both the source and system.

 

[iwire]

Re: transformer bonding

Bennie are you trying to confuse this simple question?

tkirk911 Exception 1 would not fit the installation you describe.

250.30 Grounding Separately Derived Alternating-Current Systems.

(A) Grounded Systems.

(1) Bonding Jumper. A bonding jumper in compliance with 250.28(A) through (D) that is sized for the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. Except as permitted by 250.24(A)(3), this connection shall be made at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices. The point of connection shall be the same as the grounding electrode conductor as required in 250.30(A)(2).


Exception No. 1: A bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded circuit conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

[ May 21, 2003, 06:44 PM: Message edited by: iwire ]

 

[gwz2]

Re: transformer bonding

An inspector is obligated to follow the words of the adopted code.

If the NEC has been adopted, then the words of 250.30 must be followed for the grounding of the installation.

In Indiana, the installer must follow the adopted state code, regardless if inspected or not.

 

[don_resqcapt19]

Re: transformer bonding

Bennie,
Exception #1 will only permit rebonding if there are no metallic paths, other than the grounded conductor, between the first disconnecting means and the transformer.
Don

 

[bennie]

Re: transformer bonding

When a transformer is the source for a separately derived system, the source and system must be bonded.

Edited for subject conformity.

[ May 21, 2003, 10:33 PM: Message edited by: bennie ]

 

[gwz2]

Re: transformer bonding

The original post question stated " transformer ".

 

[bennie]

Re: transformer bonding

When the source is a transformer, to a separately derived system, the transformer secondary must be bonded(shorted)to the supply source. This is to contain a through winding fault. A ground electrode shall connect to the X-O.

A premises wiring system, when a separately derived system, shall be bonded and connected to a ground electrode.

This is the source and the system.

 

[brian john]

Re: transformer bonding

Somebody seems to be beating a dead horse. If the installing contractor installs the primary to a transformer properly per the NEC, then installs the secondary once again per the NEC the issue of bonding the primary to the secondary is addressed.