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3 phase transformer grounding & bonding #35912 03/23/04 07:52 PM
Joined: Dec 2003
Posts: 11
Macon Circuits Offline OP
I am installing a 45kva 3 phase delta-wye 480/208/120 transformer in a vehicle repair shop. The transformer primary will be connected to a breaker in the 480/277 building service panel. The transformer secondary will feed a 100 amp 208/120 panel for power and lighting.

I want to be certain this installation is grounded & bonded properly.

The 480/277 building service panel is fed from the POCO through a meter base and disconnect.

Can anyone post a diagram of the proper grounding & bonding between the building service panel and the transformer primary; and grounding & bonding within the transformer case; and finally from the secondary to the 100 amp 208/120 panel?

I want to clearly understand the relationship between the neutral conductor and the equipment grounding conductor and bonding.

I hope I have posted this question clearly, if not let me know and I will do my best to clarify.

Thanks guys!

Work Gear for Electricians and the Trades
Re: 3 phase transformer grounding & bonding #35913 03/24/04 02:57 PM
Joined: Jul 2002
Posts: 241
SJT Offline
Normally, an equipment ground would be pulled in from the main panel, and also out to the secondary panel. All the grounds can go together at the transformer, and the shell of the transformer needs to have a jumper(may be there already from the factory). I believe the xo terminal is bonded at this point also, along with a ground conductor to the Building steel, if it's there. Grounding is critical.

Re: 3 phase transformer grounding & bonding #35914 03/24/04 05:07 PM
Joined: Jan 2003
Posts: 4,391
iwire Offline
Macon Welcome.

There is more than one way to do it and meet code, I will tell you the way I do it.

You will need:

3 single conductor lugs sized for the primary side conductors

3 single conductor lugs sized for the secondary side conductors.

2 three conductor lugs sized as close as possible to the secondary conductor.

Put 1 of the three conductor lugs on the XO.

After scarping paint from the enclosure bolt the other 3 conductor lug to the enclosure. 250.12

Land the primary and secondary side equipment grounding conductor to the enclosure lug.

From the enclosure lug to the XO lug install the correct size bonding jumper. Table 250.66

From building steel, or water pipe run a transformer electrode grounding conductor to XO. 250.30(A)(4) sized to Table 250.66

Now you have one spot left on XO to run the neutral to the panel.

Take a read through all of 250.30, it is not that bad and has the info you need.

Are you all set on overcurrent protection?


Bob Badger
Construction & Maintenance Electrician
Re: 3 phase transformer grounding & bonding #35915 03/24/04 07:02 PM
Joined: Jan 2003
Posts: 4,391
iwire Offline
[Linked Image from]

Bob Badger
Construction & Maintenance Electrician
Re: 3 phase transformer grounding & bonding #35916 03/25/04 06:52 PM
Joined: Dec 2003
Posts: 11
Macon Circuits Offline OP
Thanks for the reply iwire. I’m sure my question seemed pretty dumb to a lot of folks here. I sat through a 4 hour seminar last night on transformers at the local tech school. Only the final 15-20 minutes covered connections. There is much confusion in our industry concerning grounding and bonding. My particular confusion comes in where parallel grounding paths might occur.

The particular transformer in question came with a factory installed bonding strap. If the XO terminal is bonded to the enclosure, and the secondary neutral and equipment ground are bonded at the XO terminal, then why the need for the additional chassis ground?

Another confusing issue at times is the requirement to bond the chassis to the building steel in certain situations. This particular building has a steel roof supported by steel joists, but no evidence that this steel is attached to anything other than the masonry building. The transformer is suspended from these joists by threaded rod and steel channel. If this building steel was not attached to earth and a high amperage fault occurred through the chassis, I wonder what would happen?

The transformer is installed per the code and is operating lighting & power as we speak. I went ahead and bonded it like the local inspector wanted it which is pretty much like your drawing.

Sorry to be so long winded, but I want to get to a point where I feel I have a competent understanding of grounding and bonding, and the reasoning behind the code requirements.

Re: 3 phase transformer grounding & bonding #35917 03/27/04 05:51 AM
Joined: Jan 2004
Posts: 91
rizer Offline
look at the "bonding the neutral " question i posed last week.
hope it helps.
i noticed you said you suspended the xfmr from the steel trusses. i hope you used sprinkler type beam clamps and attached them to the top part of the truss? putting a large load on the bottom of a truss is not allowed in the state i work in, yet, it is rarely caught by the "electrical inspector".
if you are bonding to the building steel it has to be approved by the engineer who designed that building.

[This message has been edited by rizer (edited 03-27-2004).]


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