Electric-D
Member
what exactly is a high leg?
high leg
- Thread starter Electric-D
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Alright everybody. Let me warn you all in advance that I will not allow any crude jokes. Or did my warning in and of itself at least imply the existence of crude jokes. 
Never mind. Just answer the question. :grin:
Never mind. Just answer the question. :grin:
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brian john
Senior Member
- Location
- Leesburg, VA
The high leg delta service was used in older industrial applications (In my expierence) where they had 3 phase loads and very little 120 loads. This connection gives you 120 VAC for lighting and utilization outlets.
Normally one would avoid the 208 to neutral connections as 120 VAC loads seem to fry at this voltage.
Normally one would avoid the 208 to neutral connections as 120 VAC loads seem to fry at this voltage.
catchtwentytwo
Senior Member
brian john said:
In my area, it is lovingly called the "B*stard leg". You can spot an installation using because one of the three pole mounted XMFRs is usually physically smaller. Often used where larger straight 240 volt 3-phase motors are in use. It helps with motor starting and maximized the motor horsepower You did have to put in a separate single-phase panel for the 120 volt loads.
Many years ago, I was sent to take over a small industrial installation that had been roughed by the shop "star " (AKA - runner & electrical installer) and the owner was his personal friend. Pulling into the parking lot, I saw the "smaller" can on the pole and knew it would be interesting.
Whoops, he hadn't read the prints which said 240/120 volt 3-phase 4 wire and had installed 3-phase 4-wire panel boards (omitting the single phase panel) and roughed all the 120 volt branch circuits using 4-wire cable to save money. The sheet rocking was done. The large motor load was an already installed 240 volt 100 hp compressor. He tried to argue by claiming that you could "get away with it" by merely blanking off any unused high-leg pole positions and marking them "do not use".
The small town had one "every trade" building inspector who never would have spotted or understood the situation. Good thing the service hadn't been energized. The owner ate the cost of the POCO replacing the XMFRs, got a 208/120 volt system and used the 240 volt motor at 208. The business failed before the motor did. The "star" blamed our boss for not telling him about the "B*stard leg" and went on to "run" more large jobs for the shop.
I won't go into the "star" roughing the spray booth feeds (PVC in the slab) into 4" square boxes in sheet rock walls...that's another story.
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- Location
- Massachusetts
catchtwentytwo said:
I believe that would be NEC compliant
As far as I know you simply do not use the high leg spaces and you are not required to mark them.
catchtwentytwo
Senior Member
Bob,iwire said:
I should have clarified that by stating our POCO (and most local AHJs where the "high-leg" is used ) wanted the single-phase panel. I'm told too many cases of someone eventually using the "available" single-phase space without realizing the consequences drove this. Not sure if that is still the case. Even with proper labeling, some electricians and "electrical installers" use the terms 208/120 and 240/120 interchangeably. I suppose some get use to that doing mostly residential work.
In this instance, using the 3-phase 4-wire cable for the 120 volt receptacle and lighting made it difficult to recircuit and skip the high-leg spaces. To say nothing of the already installed sheet rock. The branch circuits were already terminated on their breakers and the guy used the "high leg" spaces for 120 volt circuits.
Regards,
John
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- Location
- Massachusetts
catchtwentytwo said:
John, to me this makes little sense.
The person working in an electrical panel is supposed to be qualified.
An unqualified person could just as easily supply a 120 circuit with 208 or 240 by making the wrong connections.
IMO the AHJ or inspector would need a local amendment to require separate panels.
brian john
Senior Member
- Location
- Leesburg, VA
Single phase 208 VAC motors would operate off this voltage, but I believe this would be a NEC violation? Just curious or truthfully to lazy to go the the bathroom to retrieve Code book.
mdshunk
Senior Member
- Location
- Right here.
I think the issue is that there seems to be no single pole breakers available for the types of panels that are typically used on a high leg system that are rated for 208. If there were, it would be compliant, in my opinion. You'll still see this from time to time anyhow. People say that the high leg voltage can vary wildly, but that has not been my experience at all.brian john said:
brian john
Senior Member
- Location
- Leesburg, VA
Most of what I have seen is as noted by others at 208 + or - typical variances for a distribution system, couple of volts here or there.
big john
Senior Member
- Location
- Portland, ME
Would this be the transformer from which "B" phase is derived? I'd guess it's smaller because unlike the other two phases, it doesn't usually supply any single phase loads?catchtwentytwo said:
-John
It is used also when the primary loads are single phase 240/120 and there are jsut a few 3 phase loads, and in our area the local utility will many times set up a Open delta system, depending on the size of three phase loads.
An off topic question,
On a open delta system, is there a sine wave produced on the open side of the system, where the missing transformer is?
An off topic question,
On a open delta system, is there a sine wave produced on the open side of the system, where the missing transformer is?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Builder said:
In an 'ideal' system, then yes, the voltage on the 'open leg' (line to line voltage as measured by the terminals that do not share a common winding) would be sinusoidal.
In a _real_ system, there will always be finite impedance and a certain amount of non-linearity, so _none_ of the voltages is going to be a perfect sine wave. Because an open delta system is not symmetric, you can expect _different_ distortion on the various phases. But they will all be pretty close to sinusoidal.
-Jon
e57
Senior Member
- Location
- San Francisco, CA
K2500 said:
You could use it - it is possisible, but practical... NO! Thats a yes BUT no question? It's current path and impedances will vary with the load (AB/BC) and and make voltages unstable all around. (ABC. BCA, CAB) It is only measurable at 208 in a bananced system, and adding an unitentional load (and unintentionally reconnecting the transformer for a seperate voltage) through the load(S), (Portions of BC, and AB, and various portions of AN, and CN) changes the voltage and the true VA (loads) of all the phases.
If you drew a circle of the current path of a high leg to neutral path - it would look like a figure 8......
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K2500 said:
It could be used as far as it being present, so possible yes, but it is not intended to be.
It could be likened to using an EGC to complete a circuit, it would work but is not intended to be used this way, hence my use of the word unusable.
Roger
- Status