corner grounded delta service

domnic

Senior Member
Occupation
Electrical Contractor
cain a use a 3 phase panel for a 3 phase corner grounded service ( just use 2 of the lugs on the 3 phase breaker )
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
It seems that some electricians see corner-grounded systems as scary or an anomaly. It's no different than a grounded neutral.

Any one system (SDS) conductor can be grounded; all it does is "fix" the relative voltage(s) to ground of the other conductors.

As alluded to by Jim, a 1ph panel would handle a corner-grounded source just like any other with one grounded and two hots.

You can use a 3ph panel if you want OCP on the grounded line, too, as long as you use common-trip breakers, and not fuses.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
cain a use a 3 phase panel for a 3 phase corner grounded service ( just use 2 of the lugs on the 3 phase breaker )
You can...the biggest issue is the breakers...you must use a straight rated breaker for the installation. You cannot use a slash rated breaker where the lower voltage in the rating is less than the line to line voltage. ...

A better choice would be a single phase panel that is marked for use with corner grounded systems. Again, though no slash rated breakers could be used.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
It certainly will work.

But you have to check with the manufacturer.
Square D QO 1PH panels are rated for 3PH corner grounded.
In the few times I have done corner grounded work I always used 1 PH panels rated for corner grounding. I see no reason to use 3 phase panels for this. Am I missing something here?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
In the few times I have done corner grounded work I always used 1 PH panels rated for corner grounding. I see no reason to use 3 phase panels for this. Am I missing something here?
Not at all. A grounded conductor is a grounded conductor.

The relative phase angles don't matter as long as you don't exceed voltage ratings.
 

ElectricMatt

Senior Member
Location
Waco, tx
I deal with several corner grounded systems. Most of them are small water systems that require 3 phase for motor loads. We always use 3 phase panel/breaker arrangements.

I see using a single phase panel, I this arrangement, as incorrect. Am I right in this? All of our breakers are straight rated, no slash ratings.

There is no potential to ground on any grounded conductor, so using a 1ph panel and utilizing the grounded conductor would add no risk but seems wrong to me. Am I right on my thought process on this?


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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Am I right on my thought process on this?
No. As Jim said, some 1ph panels are even listed for the purpose.

Think about it; what's so different about it compared to 1ph use?

You have two ungrounded conductors with voltage between them and between each and the grounded conductor.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I deal with several corner grounded systems. Most of them are small water systems that require 3 phase for motor loads. We always use 3 phase panel/breaker arrangements.

I see using a single phase panel, I this arrangement, as incorrect. Am I right in this? All of our breakers are straight rated, no slash ratings.

There is no potential to ground on any grounded conductor, so using a 1ph panel and utilizing the grounded conductor would add no risk but seems wrong to me. Am I right on my thought process on this?


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Single phase panels and 2 pole breakers are fine for corner grounded systems as long as the panel is rated for it and the breakers are straight rated.
You mention "small water systems" and using corner grounded systems. Are you certain of this? More typical would be 120/240 3 phase delta with a center tap neutral. In that case of course you need 3 phase panels and breakers.
 

ElectricMatt

Senior Member
Location
Waco, tx
Single phase panels and 2 pole breakers are fine for corner grounded systems as long as the panel is rated for it and the breakers are straight rated.
You mention "small water systems" and using corner grounded systems. Are you certain of this? More typical would be 120/240 3 phase delta with a center tap neutral. In that case of course you need 3 phase panels.
I am certain they are 3ph 480v corner ground systems. Most of them are 30+ years old. I have 7 480v 3ph corner ground systems I take care of regularly.

If you measure from one leg to ground, at the service, you have 0 v.


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I just looked at the Siemens catalog and in their part number system for their P series panelboardes, for "voltage and system" they show "V" being for "240V 3Ø3W Grounded B Phase - P2, P3, P4, P5" I did not see one for 480V grounded B phase however.

I am guessing your typical loadcenter is not approved for a corner grounded system....But wait, it looks like QO is with limitations:

Yes, convertible Main Lug Only (MLO) load centers are suitable for this application, but the short circuit rating is limited to 5,000 AIR. Only two-pole QO-H branch breakers, rated 240Vac may be used. The MLO cannot be converted to a Main Breaker panel and maintain the rating. Main Breaker load centers are not suitable for the application. Please refer to the Technical Information pages of Catalog Section 1100CT9901 for further information.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Single phase panels and 2 pole breakers are fine for corner grounded systems as long as the panel is rated for it and the breakers are straight rated.
Well, technically speaking, it's not that they can't be slash-rated, but that the voltage to ground can't exceed the lower rating.
 

ElectricMatt

Senior Member
Location
Waco, tx
Let me clarify, we are not installing new 480v corner grounded systems. However, we do have to replace components on occasion. I am just making sure we are doing it correctly.


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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I am certain they are 3ph 480v corner ground systems. Most of them are 30+ years old. I have 7 480v 3ph corner ground systems I take care of regularly.

If you measure from one leg to ground, at the service, you have 0 v.


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We used to have lots of 480 corner grounded services in our region fo center pivot irrigation. Most of the POCOs are getting rid of them as time goes on.
 
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