# 480v 3 phase Corner Grounded Delta



## deeplist (Aug 11, 2015)

I'm working on the system at an very old industrial building that was built in the early 1900s. Outside there's a 3 transformer array on the pole and 3 lines coming in to the main service box outside. (There's a secondary box next to it that houses the meter.) Here's what the inside of the box looks like on the outside of the building...










I'm trying to understand this, because if it's a 3 phase system, there should be a neutral in there too, unless it's a corner grounded delta system with no neutral. But then if that's the case, why are they only metering two of the legs? From the looks of what I saw in the panel outside, one leg goes directly into the building.

Here's a picture of the main disconnect located on the inside of the building.



















Of course nothing in here is labeled or colored correctly. The leg on the left hand side isn't fused, so I assume it's the grounded leg because it's bonded to an earth that runs back outside and into the ground. A volt meter shows 480v across A-B, B-C and A-C.

This main disconnect then feeds a few 3 phase disconnects for the elevator motor, a heating unit and something else that I can't remember. It also feeds a transformer (which appears to be mounted upside down) that steps down to run another panel that all the lighting and receptacles go through.

The smaller disconnects that run the 3 phase equipment come off each leg with no ground, but the lighting panel transformer comes off the center and right leg with the third tied in with the ground bond... which you can see in the close up of the main interior disconnect.

I'm trying to understand the fundamentals here because I've never saw anything like this before.


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## don_resqcapt19 (Jul 18, 2010)

You only need two current transformers to monitor the power in a 3 phase, 3 wire system.

I agree it appears to be a corner grounded system.

As far as marking, I have never seen a corner grounded system where the grounded conductor was identified as required by the rules in 200.6. They may exist, but I have never seen one.


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## varmit (Apr 19, 2009)

Yes, this would appear to be a corner grounded system. These are very common on older facilities. 

The disconnect, with the jumper in place of a fuse, is the correct (code compliant) way to do the fusing as the grounded phase is treated as if it were a neutral for fusing. ( Neutrals are not normally fused.) While I have never seen it anywhere, it was compliant to install a single phase panel using the two ungrounded phases on the breakers or fuses and the grounded phase on the "neutral" bar. this could really be confusing to some folks. The more common method is/was to use three pole breakers. This is code compliant because if the breaker is tripped or turned off it will open all lines at the same time. The grounded phase is also a phase conductor from the transformer secondary. 

The principle reason for a corner ground is to provide a fault path to trip the over current device in case of a second ground fault. There is an ongoing discussion on this forum concerning other positive/ negative benefits.

As for only two CTs: Since all of the loads directly connected to this service are phase to phase, with no true single phase to ground/ neutral loads possible, monitoring the current on any two phases will read the total current. This also saves the poco a little money on equipment.


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## retiredsparktech (Mar 8, 2011)

I honestly, never had any experience with 480 volts, corner grounded delta. Both plants had ground fault indicator lights. I didn't think, it even existed. I was well experienced with the 240 volt version.


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## deeplist (Aug 11, 2015)

Great info guys. I appreciate the insight.


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## Bad Electrician (May 20, 2014)

don_resqcapt19 said:


> You only need two current transformers to monitor the power in a 3 phase, 3 wire system.
> 
> I agree it appears to be a corner grounded system.
> 
> As far as marking, I have never seen a corner grounded system where the grounded conductor was identified as required by the rules in 200.6. They may exist, but I have never seen one.


Don they exist along with unicorns and triple rainbows. I have never seen one but I am told they exist.


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## deeplist (Aug 11, 2015)

Would mounting a transformer upside down like that have any ill effects? It's dry type, non ventilated. I'm not sure if there are any clearances not being met. Nothing is mentioned on the front plaque. I would have much rather had it bolted to the floor, but it's been up there on the wall like that for years... probably since the late 70s or early/mid 80s.


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## Bad Electrician (May 20, 2014)

deeplist said:


> Would mounting a transformer upside down like that have any ill effects? It's dry type, non ventilated. I'm not sure if there are any clearances not being met. Nothing is mentioned on the front plaque. I would have much rather had it bolted to the floor, but it's been up there on the wall like that for years... probably since the late 70s or early/mid 80s.


What makes you think it is upside down?

It looks to be encapsulated in which case the answer should be no.


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## deeplist (Aug 11, 2015)

Bad Electrician said:


> What makes you think it is upside down?


I worked in the GE factory in Fort Wayne Indiana (now defunct) where those transformers were produced and assembled. They were always the other way around. I can also tell the plaque on the front has also been removed, turned around and re-secured to the unit.


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## Bad Electrician (May 20, 2014)

deeplist said:


> I worked in the GE factory in Fort Wayne Indiana (now defunct) where those transformers were produced and assembled. They were always the other way around. I can also tell the plaque on the front has also been removed, turned around and re-secured to the unit.


All encapsulated transformers I have wired are bottom wired, the heat is excessive for top wire in all the ones I have worked on.


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