Gounding electrode conductor for Transformer

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Sierrasparky

Sierrasparky

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Location
USA
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Electrician ,contractor
I just got a call to install a transformer for a 12 ton roof top package unit. It appears that the original units voltage was 480v 3phase. I was wondering if we installed a transformer using the existing 480v conductors and no neutral connection was used can the Equipment ground be the entire ground connection. or will we have to go with a GEC several hundred feet away?
any thoughts?
 
What is the secondary Voltage of this transformer? kVA rating?

Are you feeding your RTU directly, or mounting some other piece of equipment first?

Why would your GEC have to run several hundred feet? No building steel connection available up there?

More info please.
 
Primary voltage is 480v 277v Y, Sec 208 3Phase no nuetral.
The roof was just done. Roof was some kind of rubber membrane. No one in a hundred miles to repair. If I go across the roof and down the side to pick up a ground it could be several hundred feet.

I was hopeing to find a trans that would not require a neutral connection or service ground

We calculated at 30 kva

the replacement unit requires 92 amps max fuse, min cir ampacity 82
Existing feeder #8 THHN @ 480v
I was hpeing to give a price in the am .. Guess thats not gong to happen.
I've never not used a cold water ground or building steel

Your suggestions are greatly apreciated.
 
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The primary side of the transformer will not need a grounded conductor, but it is a SDS and requries a grounding electrode conductor. You may be able to pull a GEC in the raceway with the primary conductors.
Don
 
unfortunaly the primaryis existing. and too small.
Looks like the HVAC contractor really made a blunder.
 
Crazy idea time:

Use an autotransformer rather than an isolating transformer?

I don't have the time to work through all of the ins and outs of article 250, but if you use an autotransformer then you are not creating a separately derived system.

I still can't figure out how to size the EGC for an autotransformer system, however.

-Jon
 
Also, is there any chance that you could pull #6 conductors in place of the #8 conductors? That would get you your MCA, withough any transformer hoops to jump.

-Jon
 
Sierrasparky said:
... and no neutral connection was used can the Equipment ground be the entire ground connection. or will we have to go with a GEC several hundred feet away?
any thoughts?
Click to expand...

Do I understand you would like to install an AC unit with no protective conductor for the sheet metal, panels, or jboxes?
 
pfalcon said:
Do I understand you would like to install an AC unit with no protective conductor for the sheet metal, panels, or jboxes?
Click to expand...

The exisitng conduit is grounded. there is no neutral just trying to avoid a grounding electrode conductor. because of the distance. and equipment necessary to install it. ( may need a man lift to run the GEC)

This mistake by the HVAC contractor could cost ten thousand dollars..
 
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(responding to pfalcon)
No. The OP has a 480V circuit with an appropriately sized EGC (possibly just the conduit, but a legal EGC none the less).

The load to be served is a 208V load, thus requiring a transformer.

A normal isolation transformer, installed as a 'separately derived system' requires a _grounding electrode conductor_. The OP is trying to come up with a reasonable and legal way to simplify the GEC requirement, use the already present EGC, or avoid the GEC in some other way.

-Jon
 
winnie said:
(responding to pfalcon)
No. The OP has a 480V circuit with an appropriately sized EGC (possibly just the conduit, but a legal EGC none the less).

The load to be served is a 208V load, thus requiring a transformer.

A normal isolation transformer, installed as a 'separately derived system' requires a _grounding electrode conductor_. The OP is trying to come up with a reasonable and legal way to simplify the GEC requirement, use the already present EGC, or avoid the GEC in some other way.

-Jon
Click to expand...

What's an OP
so do ypu think the Auto trans is the answer?
I don't have my code book on hand. I'll look this eve.That online version is very slow.
 
Sierrasparky said:
What's an OP
so do ypu think the Auto trans is the answer?
I don't have my code book on hand. I'll look this eve.That online version is very slow.
Click to expand...

OP means "Original Poster", the person who started the thread.

I don't know if an autotransformer is the way to go; it is too far out there to be an acceptable solution without significant engineering analysis. I was just throwing it out as a possibility; I believe that it would eliminate the need for a GEC, but would introduce other problems.

IMHO I think that you would be better off if you could pull a slightly larger circuit and not try to put a transformer on the roof. I'd bet you can get 3 #6 conductors in place of the 3 #8 conductors that you currently have, but it would be close. I'd bet that pulling a new 6/3 MC cable would be no more difficult that pulling a new GEC, and this again gets the transformer off the roof.

IMHO the way to go is to get the HVAC company to get a unit with the correct voltages, or to get motors swapped in the unit to get to the correct voltage.

Anything else will be more expensive to operate over the long haul. At 208V you will have greater voltage drop in the circuit, along with the losses in the transformer, versus supplying 480V.

-Jon
 
The HVAC Contractor really boxed himself in a corner. The unit was special ordered and for what ever the reason the ordered the wrong voltage.

Also The min cir ampacity is 82 amps .Max fuse 90 .
It looks like a rough pull through 1 " emt but I guess it could be done

There is no 208 at the roof othe than what is servicing other devices.
The feed that supplies the 480 is a poorly done tap from a 100amp switch that feeds a tennant space not supplied "conditioned" by this replacement A/C unit. I did not want to touch the tap at all. There are no load centers in the electrical room. The only thing I could possibly do make a new tap from the secodary side of the tranfomer in the electric room that is fed from the Primary 100 amp 480v fused switch.

Then the problem is pulling the larger wires in the existing 1" conduit.
 
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I think that you edited the 82A value :) Yikes. So you have a choice of #4 conductors (which would fit in 1" EMT, but you might just be hitting the 'jam ratio' of wire diameter to conduit diameter; that will take closer checking), or putting the transformer on the roof.

Okay, here is another wild idea (and remember, this is _not_ my well thought out code opinion, but just a wild ass idea): if the transformer is part of the equipment being served, rather than a transformer supplying a 'system', then does it need the GEC? Has anyone contacted the HVAC supplier, either to determine if the motors can be changed, or to find out if a transformer can be added to the unit and considered part of the unit itself?

One comment: if any of these 'wild' ideas are actually viable, be sure to balance the liability that you would be assuming versus saving the HVAC contractor money. They screwed up, and are looking for a way out. Helping at a fair price is the right thing to do, and trying to figure out how to minimize the unnecessary expenses is the right thing to do. But (for example), a 480V to 208V autotransformer has a potential failure that would put 480V across the load. This risk has to be balanced against the money saved.

I do believe that I've exhausted any sort of valuable input I can provide you. Good luck.

-Jon
 
Yes I did edit I had seen I had made a mistake. opps sorry
I'l ask about calling the trans [art of the equip...



I doubt if changing the motors is an option there are 3 5 ton compressors- 2 blower motors and control equip...

Thanks for all the help
 
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