Rewire transformer from 1992.

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So I took a job as a maintenance electrician in a factory. I noticed a green ground screw bonding the neutral block, in the disconnect, on the secondary side of the transformer. 277/480 step down to 120/208. Separately derived system, not the service by a long shot. I know the bonding should take place at the source, not in the first disconnect.
So I pulled the cover of the transformer yesterday. They took the 2 neutrals (parallel) from the xo bar to a 4 barrel lug (mounted to the left side of transformer case) then off to the secondary disconnect. They took all the grounds, to a 3 barrel lug, mounted on the right side of the case. Building steel, and the grounds in the pipes. There was no actual wire connecting the neutral and ground lugs. I assume they were using the metal case of the transformer as a bonding jumper.

I reworked it, so the proper bonding is now done.

I guess my question is, was this legal back in the early 90's? I know it wouldn't fly today. Also they had the "service disconnect" sticker on the secondary side disconnect.
 

iwire

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Massachusetts
The NEC still allows the bonding of an SDS to be done at the first disconnecting means.

As far as the details of what they did I am not sure there were any violations.
 

ActionDave

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Licensed Electrician
Your pic is blurry. From what I can make of it you are saying there was a bond in the first disconnect of the SDS. That is acceptable under the code and is as safe electrically as a bond in the transformer. It's the way I was taught and the way I usually bond a transformer.
 

iwire

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Staff member
Location
Massachusetts
I think you may have misunderstood what I mean. See if this helps.
View attachment 14326



You had said

I know the bonding should take place at the source, not in the first disconnect.

I was pointing iut the NEC allows it.

250.30 Grounding Separately Derived AlternatingCurrent
Systems.

(A) Grounded Systems.

(1) System Bonding Jumper. An unspliced system bonding
jumper shall comply with 250.28(A) through (D). This
connection shall be made at any single 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, in accordance
with 250.30(A)( I )(a) or (b). The system bonding
jumper shall remain within the enclosure where it originates.
If the source is located outside the building or structure
supplied, a system bonding jumper shall be installed at
the grounding electrode connection in compliance with
250.30(C).



But from you description it seems they bonded it at both which is a problem.
 

kwired

Electron manager
Location
NE Nebraska
Bond can be at transformer or at first disconnect - Iwire quoted code that applies.

What is not clear yet is if you have a bonding jumper at both locations.

Sounds like you have bonding jumper in first disconnect for certain, and that a grounding conductor runs back to the transformer housing.

There should be no jumper from transformer XO to housing, or no bonding jumper in first disconnect - take your pick which one suits your needs better.


The "service disconnect" label is wrong because this is not service equipment.

Add: grounding electrode conductor needs to run to which ever point you place the main bonding jumper
 
Thanks for the code reference. I was always taught to bond in the transformer.

But if they are using the transformer case as a bonding jumper, then they shouldn't of bonded in the disconnect as well. Even if the bond in the transformer was just the case steel, and very sketchy. The paint was half assed scrapped off. Please excuse the phrase.
 

kwired

Electron manager
Location
NE Nebraska
Thanks for the code reference. I was always taught to bond in the transformer.

But if they are using the transformer case as a bonding jumper, then they shouldn't of bonded in the disconnect as well. Even if the bond in the transformer was just the case steel, and very sketchy. The paint was half assed scrapped off. Please excuse the phrase.

You still have to bond to the transformer case, but if your "system bonding jumper" is in the first disconnecting means then you will not directly connect the case to XO terminal of transformer - instead a "supply side bonding jumper" is run back from the first disconnecting means and bonded to the transformer enclosure.

You mentioned this was originally installed in 1992, some terminology used now didn't exist then, but installation requirements were still mostly the same.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
You mentioned this was originally installed in 1992, some terminology used now didn't exist then, but installation requirements were still mostly the same.
And IMO the alphabet soup of terms they have added since then, MBJ, SBJ, SSBJ, along with equipment bonding, and bonding conductor has made everything worse.

One bond at the service or SDS, one connection to earth there too, make it possible to clear a fault with equipment grounding, keep the current on the insulated wires. Done.
 
K, I got to work and pulled out my handbook, and looked at the pictures. I guess the best way I can describe the original bonding of the transformer, is that it was a hodge podge of crap. It looks like it is either done at the source or the first point of disconnect, not a litttle in each place. Like the original installer did.
But it is now correct, and I did learn something from you guys.
Thanks
Shawn
 
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