Service With The High Leg

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solo618

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Orange County, N.Y.
I Have A Situation Where I Was Called By A Friend Who Had A New Service Installed In His Auto Garage And The B Phase Has A Higher Voltage Then A And C,now I Was Taught About This Some 13 Years Ago While In School And Never Have Come Across It In The Field So I'm Not Too Sure About This Kind Of Service. His Question To Me Was Would It Be Okay To Hook Up His Lifts To This Which Require Single Phase 208v At 20 Amps. And Also He Has A Three Phase Compressor At 31 Amps.
 
solo618 said:
. . . Would It Be Okay To Hook Up His Lifts To This Which Require Single Phase 208v At 20 Amps. And Also He Has A Three Phase Compressor At 31 Amps.
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High-leg systems are 240v line-to-line, not 208v. Therefore, the 208v 1-ph load must be rated to also run on 240v 1-ph. Same goes for the 3-ph load: 240v, not 208v.

As long as the loads have no line-to-neutral load, any two phases may be used. However, any breaker connected to the high leg must be rated for 240v, and not 120/240v.
 
Most 208v Wye loads will work just fine connected to his 240v Delta, except for maybe a slight decrease found on resistive heat loads. You should notice that the B phase has 2 and/or 3 pole breakers connected in his pannel, but no single pole breakers.
 
Minuteman said:
Most 208v Wye loads will work just fine connected to his 240v Delta, except for maybe a slight decrease found on resistive heat loads.
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Umm... I believe you got that backwards. There would be a slight increase in current putting a 208V resistive heating load on a 240V circuit.
 
Although as Minuteman said the 208 volt load may work well on a 240 volt system I would check with the manufacturer first just to be safe.

Also, Solo welcome to the forum. Maybe it's just me and my old eyes but I found your post kind of hard to read with every word capitalized.:wink:
 
Clarify please:

Clarify please:

solo618 said:
I Have A Situation Where I Was Called By A Friend Who Had A New Service Installed In His Auto Garage And The B Phase Has A Higher Voltage Then A And C,now I Was Taught About This Some 13 Years Ago While In School And Never Have Come Across It In The Field So I'm Not Too Sure About This Kind Of Service. His Question To Me Was Would It Be Okay To Hook Up His Lifts To This Which Require Single Phase 208v At 20 Amps. And Also He Has A Three Phase Compressor At 31 Amps.
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First, are you measuring line to neutral? If so, you should read 120, 120, and 208V.

Second, are you asking if it is acceptable to connect a 208V load between line B and neutral?

Yeah, I know there is no neutral in a delta system, but there is in the 240V single phase subsystem.
 
IF the single phase motor is only rated for a 208 Volt supply, then you can connect it from the B phase to neutral for that Voltage. Be sure to use a breaker rated for 240 Volts (not one rated at 120/240) on that circuit.

As for that compressor, check to ensure you have the correct size overload heaters installed for a 240 Volt source.
 
high leg service

high leg service

thanks guys for your responses. sorry for the limited info but it was given to me over the phone from an auto mechanic. ok so i went over to his place today and took some readings from the panel and this is what i got.

A phase to N 124V
B phase to N 249v
C phase to N 124V

A TO B 249V
B TO C 249V


Now his lift motor is a 2hp motor rated for 208-230v single phase,which he had temped out and was working from a two pole breaker on the c and a phase. Since i wasn't 100% sure about this i just left it until i could get more info. Now let me ask two more things about this. Could i bring just the b phase which reads 249v with a neutral for this . Second could i use the b phase with either the a or c phase which reads the same 249v for this motor.
 
solo618 said:
thanks guys for your responses. sorry for the limited info but it was given to me over the phone from an auto mechanic. ok so i went over to his place today and took some readings from the panel and this is what i got.

A phase to N 124V
B phase to N 249v
C phase to N 124V

A TO B 249V
B TO C 249V


Now his lift motor is a 2hp motor rated for 208-230v single phase,which he had temped out and was working from a two pole breaker on the c and a phase. Since i wasn't 100% sure about this i just left it until i could get more info. Now let me ask two more things about this. Could i bring just the b phase which reads 249v with a neutral for this . Second could i use the b phase with either the a or c phase which reads the same 249v for this motor.
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Your B-N voltage cannot be the same as your A-B and B-C voltages. The correct ratio is 208/240, so your B-N must have been 215V.

The motor can be connected A-B, B-C, or C-A. However if it is connected to the B phase at all, you need to use an expensive 240V 2-pole breaker NOT a standard 120/240V one.
 
solo618 said:
Now his lift motor is a 2hp motor rated for 208-230v single phase,which he had temped out and was working from a two pole breaker on the c and a phase. Since i wasn't 100% sure about this i just left it until i could get more info. Now let me ask two more things about this. Could i bring just the b phase which reads 249v with a neutral for this . Second could i use the b phase with either the a or c phase which reads the same 249v for this motor.
Click to expand...

No, you cannot connect just the b phase and the neutral. However, you can use ANY two phases and a 2 pole breaker. A-B, B-C, or A-C.
 
solo618 said:
Now his lift motor is a 2hp motor rated for 208-230v single phase,which he had temped out and was working from a two pole breaker on the c and a phase.
Click to expand...
Best advice: leave it this way.
 
solo618 said:
Jim i hear what your saying and i don't know how to explain it but when i tested b to n i got 249v, that i'm sure of.
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You are the one that took the readings, but this goes against the laws of physics.

In any case, what you have now is working using A-C phases so why would you want to change it?
 
Something is wrong:

Something is wrong:

solo618 said:
Jim i hear what your saying and i don't know how to explain it but when i tested b to n i got 249v, that i'm sure of.
Click to expand...

If you measured between Vb and a miswired "neutral" on a 120V receptacle, then you should get 240V.

I am with Jim on this one. What you report is impossible. Literally, the voltages do not add up.
 
Hrm, well, I have the same mysterious voltages going on with my "3 phase" service..

from the "supposed" neutral I have the following:
120/249/120

However between any 2 phases I get 240v. So, it "looks" like useable 3 phase with a questionable neutral.
 
To refer to the voltage on the high leg as "208" is really very confusing as that is the voltage on a 120/208 wye system, correct?

Around here I generally find the high leg on a delta system to be around 190-195 volts.


Generally, treat it like any other 3 phase system but don't put 120 circuits/circuitry on the high leg unless you want smoke.
 
Strangely enough, rechecked everything last night and got:

120/208/120...

I am going to put a scope on the 208 line and set it to trigger at 215 and leave it for 24 hours and see what happens.

If it doesn't end up triggered, I will go ahead and treat it like standard delta 3 phase with a center tap neutral.

In all honesty, I am hoping I was just really tired when I first checked it. ;) This is why checking twice and wiring once is important.
 
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