High leg

[laketime]

so i hooked up a new receptacle on and existing 220v/40amp feed. i got a reading of 120v on one leg and 200v on the other, is that a high leg thing? I don't have a lot of experience with high legs. Does that cause an issue with 220v equipment?

 

[Power Tech]

so i hooked up a new receptacle on and existing 220v/40amp feed. i got a reading of 120v on one leg and 200v on the other, is that a high leg thing? I don't have a lot of experience with high legs. Does that cause an issue with 220v equipment?

It is 208V to ground. No lighting or 120V receptacles. It's okay to use for 1 phase and 3 phase motors.

 

[jim dungar]

All breakers installed on a high leg must be fully rated for 240V. Most standard 2-pole breakers are slash rated as 120/240V.

 

[220/221]

Does that cause an issue with 220v equipment?

In a straight 240V load, it won't cause any operational or code issues.

If it's a 120/240V load, you will have obvious issues.

I avoid using the high leg in single phase 240V loads just to eliminate any future confusion if any less experienced people like yourself work on the system.

At least mark the high leg orange and make a note of it on the appliance or JB/disconnect.



3 phase Delta has two legs 120V to ground and one at around 200V to ground. They all read 240V between them. Don't ask me how it works. I'm just a mechanic.

 

[iwire]

3 phase Delta has two legs 120V to ground and one at around 200V to ground. They all read 240V between them. Don't ask me how it works. I'm just a mechanic.

That seems odd, I would expect 240, 240 and 0 to ground or if it's ungrounded delta who knows what you will read to ground.

 

[infinity]

3 phase Delta has two legs 120V to ground and one at around 200V to ground. They all read 240V between them. Don't ask me how it works. I'm just a mechanic.

That seems odd, I would expect 240, 240 and 0 to ground or if it's ungrounded delta who knows what you will read to ground.

Bob,
I think that he's referring to a 3 phase, 4 wire Delta (he left out the 4 wire part) where the high leg voltage to ground would be about 208 volts and the remaining two phases to ground would be 120 volts.

 

[220/221]

Yeah...what he said.^

3 phase, 4 wire is the only Delta system we have here.


All that other stuff is just crazy talk.

 

[LarryFine]

At least mark the high leg orange and make a note of it on the appliance or JB/disconnect.

Not a bad idea, although only required where the grounded conductor is present.

3 phase Delta has two legs 120V to ground and one at around 200V to ground. They all read 240V between them. Don't ask me how it works. I'm just a mechanic.

Allow me.

The 120/240v part is exactly like the single-phase service in your house, and you can use it freely like that. Any line-to-line or line-to-neutral load that uses the neutral must not connect to the high (208v) leg.

The 3-phase part is exactly like any delta service where the grounded conductor is not used. Any 240v single-phase (or three-phase) line-to-line-only load may be connected between any pair of wires (or all three.)

 

[laketime]

Why have a high leg? What was the original intention of the design?

 

[roger]

Why have a high leg? What was the original intention of the design?

It can't be helped in this configuration of windings, it's just an unusable byproduct of a center grounded Delta.

Roger

 

[220/221]

The 120/240v part is exactly like the single-phase service in your house, and you can use it freely like that. Any line-to-line or line-to-neutral load that uses the neutral must not connect to the high (208v) leg.

The 3-phase part is exactly like any delta service where the grounded conductor is not used. Any 240v single-phase (or three-phase) line-to-line-only load may be connected between any pair of wires (or all three.)

Well I know that, I just don't know HOW.....or why.

 

[hillbilly1]

It can't be helped in this configuration of windings, it's just an unusable byproduct of a center grounded Delta.

Roger

I wouldn't quite say "unusable", they are used quite a bit where there is very little single phase load, since the voltage is higher to begin with, voltage drop is not as big of an issue, also, if I remember correctly the current is less in the windings of the transformer for the same KVA load, so smaller transformers can be used. (maybe some our utility guys can verify this) With the open delta it saves the utility one extra transformer where the three phase load is not that great.

 

[roger]

I wouldn't quite say "unusable", they are used quite a bit where there is very little single phase load,

Please give an example.

Roger

 

[hillbilly1]

Please give an example.

Roger

Convience stores for one, they have a large compressor loads/HVAC loads compared to the lighting and receptacle load. Welding shops, especially ones with spot welders, Industrial plants not large enough for 480 volt services, I have wired several clothing manufacturing plants, all of the machinery was 240 volt of German manufacture (sewing machines were 240 three phase, as were the steam generators for the clothing presses.) Small sheet metal stamping plants, such as A/C duct manufacturing, also German equipment. (some were Canadian 575 volt also)

 

[220/221]

I look at this and my head tilts to the side like a puzzled dog.

 

[roger]

I have wired several clothing manufacturing plants, all of the machinery was 240 volt of German manufacture (sewing machines were 240 three phase, as were the steam generators for the clothing presses.)

I think we have a communication breakdown here, the high leg will have no affect on phase to phase voltage, the voltage will still be 240 volts phase to phase.

Roger

 

[hillbilly1]

I think we have a communication breakdown here, the high leg will have no affect on phase to phase voltage, the voltage will still be 240 volts phase to phase.

Roger

Correct, but if you use a wye connected transfomer your voltage would be 208 phase to phase, not 240, that is the advantage of the delta connected transformer, you would have to use buck/boost transformers to work with the rated voltage on the German equipment. A lot of convience store compressors are 240 volt single phase, running them on 208 shortens their life.

 

[LarryFine]

Why have a high leg? What was the original intention of the design?

Think evolution. First, there was the battery... J/K

Imagine you have your every-day, run-of-the-mill 120/240v service in your, let's say, mill. You have motors, lights, etc., the usual stuff. You're running at full capacity, but could handle more business if you had a super-mill machine.

One day, you're reading the latest issue of Mill Digest, and you see an ad for a new-fangled super-mill machine, but it requires a 3-phase supply. Gee, how to get the power into your mill?

The simplest thing to do is add one more 240v utility transformer (but with no (or a non-bonded) center tap) to the existing one on the pole, and power it from one of the other primary phases.

Just bring one more conductor with the existing service drop, run it through a new meter (or add a CT), and you now have a 3-phase service. You can replace the existing panel, or add a 3-ph disconnect.

This will give you an open delta (which is very common in many older parts of Richmond), which is commonly done when the 3-ph load is a relatively small portion of the total load.

A friend of mine grew up in a house that was fed with the usual 3-wire service drop, with a fourth conductor (#10, I believe) alongside the three, which supplied only the central-AC compressor.

Almost nobody would specify a high-leg service unless it was to accomodate existing equipment, or vice versa. I've done several new services which had, and kept, the high-leg service, and saved $'s.

 

[LarryFine]

It can't be helped in this configuration of windings, it's just an unusable byproduct of a center grounded Delta.

I guess it would be beneficial to think of my previous post as the answer to "Why have a center-grounded, 4-wire Delta.

 

[LarryFine]

I wouldn't quite say "unusable", they are used quite a bit where there is very little single phase load . . .

In my experience, it's the opposite, especially with an open Delta.