wire a 120/240 high leg to 277/480

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cbjrcele

New member
Location
manassas VA
Good morning,


I'm looking for a wiring diagram for the connecting a 30kva Delta Wye transformer to convert a 120/240 High Leg circuit to 277/480....A little more information,,,We're going to be feeding a unit heater so we don't need a neutral for the 480 volt circuit.
Thanks
 
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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
All you need is 240 volt, 3?, 3 wire feeder to the transformer and wire the WYE secondary as you normally would. The X0 can be used for grounding purposes and need not be brought to your unit heater equipment.
 

kwired

Electron manager
Location
NE Nebraska
I don't mean to be condescending here but what you ask is really the simple part of such an install, and if you are having trouble with that you will have trouble with other aspects of the installation as well.

The transformer very likely has a wiring diagram right on it. If not should come with some papers that do show the wiring diagram.

Grounding and bonding of the secondary of that transformer and applying proper overcurrent protection can easily be more complex then the basic primary/secondary connections.

If you are having confusion over the 120 and/or high leg - you can disregard both of those facts as they mean little to the transformer - the transformer will only care that it sees 240 volts between each input terminal.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I don't mean to be condescending here but what you ask is really the simple part of such an install, and if you are having trouble with that you will have trouble with other aspects of the installation as well.

I'm not so sure that you succeeded. :roll:

Dealing with a high leg system is pretty rare for many electricians.
 

jumper

Senior Member
Dealing with a high leg system is pretty rare for many electricians.

For me, working with a high leg system was pretty common as we had a lot of them in southern VA, but the corner grounded system is a type I have not seen or worked on before.

I learned about them here and would have been completely baffled had I encountered one before I joined this forum.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Dealing with a high leg system is pretty rare for many electricians.
The first time you do you never forget it.

I remember when I was an apprentice standing in front of a panel where we were adding a circuit and me asking, "What do you mean we need to free up a space? There is lots of free spaces." and my journeyman saying, "Put your meter on one of them."
What the.....????

I think he left me standing there for at least twenty minutes poking around before he explained anything.
 

kwired

Electron manager
Location
NE Nebraska
I'm not so sure that you succeeded. :roll:

Dealing with a high leg system is pretty rare for many electricians.

I guess I didn't think about that, I see them all the time. I also see 240/480 three phase with a 415 volt high leg quite often, most all of those are open delta with limited load supplied where the 120/240 systems have quite a few open and quite a few full delta.

I still find it hard to believe they are not common at least in older commercial installations, especially old "downtown" stores/offices/etc. that had all 120/240 single phase loads except for a three phase air conditioner.
 

kwired

Electron manager
Location
NE Nebraska
For me, working with a high leg system was pretty common as we had a lot of them in southern VA, but the corner grounded system is a type I have not seen or worked on before.

I learned about them here and would have been completely baffled had I encountered one before I joined this forum.
There used to be a lot of corner grounded systems supplying irrigation services in this area, but they have converted all of them (in the 2 or 3 rural POCO areas that I do the most work in) to 3 phase 4 wire systems, most are 480/277, but there still is occasional full delta secondary (240/480 with 415 volt high leg) that you run into from time to time. I'm guessing those delta systems were once corner grounded - every one I have seen seems to be older equipment, may even still have a white wire in the center leg of the disconnect with a grounded conductor that appears to been added at a later time pulled to the disconnect.

Many POCO's used to provide conductors all the way to the irrigation equipment and you will find that when they converted from corner grounded system they buried an additional grounded conductor out to equipment - but it was a few feet away from the existing buried conductors which doesn't help with keeping impedance down when you have a ground fault problem, long distances alone are enough of a problem:(

When I was still a first couple years apprentice, we ran into a farmer that needed us to resolve an underground problem on a corner grounded system(480 volts to irrigation equipment). It was common in those installations for POCO to run three conductors on their meter pole to a three pole fused disconnect with the middle grounded phase "slugged" instead of installing a fuse. This guy told us he kept blowing one fuse and decided to move that "slug" to the position where the fuse kept blowing. He then later had to call POCO to tell them there was a fire at the pole top in the transformer bank:eek:hmy:, but switched the "slug" back to it's original position before they came out.

480 volts to ground is not a good combination with unqualified people having as much control over the installation as irrigation use tends to have, and is probably main reason these POCO have abandoned such systems. The users get into enough trouble as is with 277 volts to ground on these systems, but at least with those systems there generally is no neutral loads so they get out of the situation of using an EGC as a current carrying conductor - most of the time anyway.
 

kwired

Electron manager
Location
NE Nebraska
cbjrcele, I want to apologize for my initial comments, please give us more details of what it is that you may be having trouble understanding on this installation. Lack of details in the OP maybe set my mind in one direction and really needed to be asking more questions first.

This first came across to me as something a member of this site should not have that much trouble with. If the presence of a high leg system is your main concern, maybe I was a little harsh on you. It was pointed out to me that (and I did realize it but looked past that) that not everyone sees high leg systems much, and some never do and many of them are still very good electrical professionals.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
For me, working with a high leg system was pretty common as we had a lot of them in southern VA, but the corner grounded system is a type I have not seen or worked on before.

I learned about them here and would have been completely baffled had I encountered one before I joined this forum.
My first experience with it was on a shop we moved into where it had 120/240 open delta, so two transformers on the pole, one of them center tapped, and it was twice as big as the other. It looked just like this, schematically anyway.
DeltaOpenconfiguration.jpg

I can't attach this image, but here's what the pole pigs looked like.
http://powerlineman.com/lforum/attachment.php?attachmentid=4035&d=1322009045
 
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jumper

Senior Member
The first time you do you never forget it.

I remember when I was an apprentice standing in front of a panel where we were adding a circuit and me asking, "What do you mean we need to free up a space? There is lots of free spaces." and my journeyman saying, "Put your meter on one of them."
What the.....????

I think he left me standing there for at least twenty minutes poking around before he explained anything.

My first time did not work out so well. A few pieces of equipment died that day.:(
 
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