bonnie1

Greetings - I am new to this group, but need to ask the best minds out there since I have a question I think I know the answer to, which can be very dangerous. Question: I have a 1200 amp 3 wire 480 volt distribution panel. No ground on the transformer bank. The local utility is putting in a new system and will install a 277/480 transformer (padmount). I am being asked by a City to bid on a job to convert this existing panel from overhead to underground feed by installing a new termination box on the wall of a building, intercept each leg and run it into the existing panel. My contention is that you cannot do this without rewiring the full panel and heavy equipment inside. The City is telling me that they can still "peel" 3 ea. ungrounded 480 v legs from a 277/480 services. Any comments?

micromind

This certainly can be done, the 480V equipment doesn't care if it's fed from a wye or a delta. It only cares about phase to phase voltage. It doesn't matter if it's 480V single phase or 3 phase.

What does matter is where, or if, the original system is grounded. If it never was grounded (rare, but possible), it can be connected to a grounded wye without modification. Simply ground the neutral (center of the wye) that the POCO supplies.

If it was a corner grounded system, also known as a grounded B system, you'll need to locate the bonding jumper and remove it. Otherwise, you'll have two grounds, one on the center of the wye, and one on B phase. Once this jumper is removed, turn everything on in the entire building and megger to ground. If it's still shorted to ground, that must be cleared up before energizing.

Another issue is that code requires ground-fault protection of all 480V circuits above 1000 amps on a solidly grounded wye system. If this cannot be easily done, consider an impedance grounding system.

Rob

nap

MM, IF the system was installed correctly, the only ground to phase connection that should be removed would be at the service (with one exception explained later). After that point, an EGC would be used as a grounding conductor and all the phases should be isolated from ground.

The grounded phase is now a grounded conductor and should have been treated the same as any other neutral/grounded conductor with no "in feld" to ground connections.

exception explained: The only other place there should be a ground to phase connection would be in a transformer within the plant to alter voltages somwhere within the plant.


SO you are going to get a ground to phase reading with any such transformers online, including a control transformer within a machine, if it has a grounded leg. (and there is not neccessarily anything wrong with this)

I would expect, with everything turned on, there will be some ground to phase connections, which may or may not be intentional and/or correct.

micromind

What I meant to say in my previous post was if the original system was a grounded B, after the bonding jumper was removed (at the service), the entire B phase must be checked for ground faults. Obviously, A and C have no such faults, they're 480V above ground. B, however, could have been inadvertently grounded at some downstream of the service, and because it's already grounded at the service, you'd never know until you energized the service with a grounded wye.

As for control xfmrs, etc., they're to be grounded only on the secondary side, electrically isolated from the primary. Correctly installed, they would not constitute a ground fault on the 480V side.

Hopefully, this clears up any confusion.

Rob

bonnie1

Thank you both for your information. The tx bank on the pole feeding this service is an ungrounded delta 480 3 pot bank. The new tx will be a 277/480 padmount. There is a 3 wire service o/h going into the 1200 amp panel and there are the various metering and protection components located inside the building as part of the panel. From there, there are several 120/240 step down tx as well as several 480 ungrounded (as far as I can see....) motor services feeding cranes and a smelter. We will go ahead and drop a new 1200 amp switch on a pad, ground at the pad, remove the metering and protection from the inside, buss it through where the metering is removed then place the new metering on the outside. I will make sure we megger everything out and look for other grounds. Thanks again for the info and suggestions. You just don't run into 3 wire 480 much anymore.

micromind

Your plan sounds good. The new 1200 amp switch will most likely have ground-fault protection built into it, so you're up to code there. This is also the proper place to put the bonding jumper, and the old service becomes a 3PH 3 wire feeder. Provided there are no ground faults (I'll bet it comes out OK), it'll work.

Rob

JohnJ0906

Sounds like you will have to check phase rotation on all the motors after the changeover.

Besides that, I agree with the other replies.

__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.

khippy

The code states that if it is a wye system the fourth wire must be taken to the first overcurrent distribution point from the pad mount so you can not just take three wires to the panel board if it is the first distribution point.