# Connecting Motors to High Leg on a 3 phase 120/240V



## emtnut (Mar 1, 2015)

If it's a split phase delta, it should be reading 208V on the high leg.

If it's a corner grounded, you should have 240V to ground on all phases.
**edit** on 2 phases :icon_redface:


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## Dash Dingo (Mar 3, 2012)

The motors will like it. That's typically why that type of service was installed in the first place was for buildings with big motor loads.
I did not however read the PDF you linked for the motor.
I do it with RTU's quite frequently. Any motor rated with voltage between 208-230 volt it will take the 240 no problem.
Like others have said if it's a mid winding tap one of the phases should be around 208v to ground.
You will still read 240 volts between your phases no matter what.



Sent from my iPhone using Tapatalk


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## micromind (Aug 11, 2007)

there are millions (yes, actually millions) of motors connected to 240∆ systems and they are perfectly content with it. 

It doesn't matter at all which motor lead is the high leg, the motor is looking for phase to phase voltage and does not care about phase to ground. Same with the starter. 

One place you can get in trouble with a 240∆ is if you use 120 controls with the neutral and the control circuit is connected to the high leg. Stuff will blow up...... If there's a control transformer then this is not an issue.


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## varmit (Apr 19, 2009)

The original purpose of a "high leg" service was to supply motor loads and lighting from a single service. As posted above, motors are dumb. If the phase to phase voltage is in the motor's tolerance, they will run just fine. DO NOT CONNECT ANY 120 VOLT, TO NEUTRAL, LOADS TO THE HIGH LEG.


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## varmit (Apr 19, 2009)

Here is a previous post of mine. I hope it is useful.






There seems to be more confusion about corner grounded systems and high leg systems than any other subject on this forum. Neither are complicated, but high leg systems require a little more thought when connecting loads. Especially 120 volt loads on a 240/120 high leg service. 

A corner grounded delta system is very common in older industrial facilities. These were usually equipment power only, with either another service, or on site transformer(s) for general 120 volt lighting and outlets. 

WHEN WORKING AT A LOCATION, ALWAYS CHECK/TEST TO SEE WHAT TYPE SYSTEM YOU ARE DEALING WITH. Wear your PPE when doing this.

Note: Names will vary regionally. Voltages listed are nominal rating. This is a basic description of what you should find.

Delta, open or ungrounded 230 (or 460) volt
A to B 230 (460)
B to C 230 (460)
A to C 230 (460) 
Any phase to ground will read random values. Usually different on each phase.

Corner grounded Delta (One phase conductor is connected to the grounding electrode system. Normally the grounded conductor is A or B phase- but you never know until you test.
A to B 230 (460)
B to C 230 (460)
A to C 230 (460) 
A to ground 230 (460)
B to ground 0 (0) grounded phase conductor in this example
C to ground 230 (460)

High leg system ( Usually 240/120, but I have seen other voltages on rare occasions.) The B phase position is normally the high leg- but always verify. This system has a "neutral" (grounded conductor) for use on 120 volt loads connected to THE NON HIGH LEG PHASES.
A to B 240 
B to C 240 
A to C 240 
A to ground 120
B to ground 208 (high leg phase conductor in this example)
C to ground 120 

Resistance grounded ( A large resistor is connected to the supply transformer center tap and connected to the grounding electrode system. This helps to limit fault current to an acceptable level on larger services.Usually 460 volt or higher.)
A to B 460
B to C 460
A to C 460
All phases to ground will read a random value that will be about the same on all three phases.


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## Southeast Power (Jan 18, 2009)

vinnygalbo said:


> I've been called in to a very old warehouse building that has a 120/240V 3-phase 4-wire with a high leg reading 240V to ground and neutral wire on the C phase coming from the service disconnect from the power company. I know the power company used to have the high leg on the C phase but changed it to the B phase. I would like to know if 3-phase 240V motors can be connected to the the 3-phase power with high leg and not damage the motor. The motors in question are from SEW Eurodrive.
> 
> http://www.clrwtr.com/PDF/SEW-Eurodrive/SEW-Eurodrive-DR-Motors.pdf


The motor will never know.


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## lightman (Oct 14, 2015)

The motor will run fine. Its going to see the 240 phase to phase. The voltage you are seeing on the high leg should be closer to 208 (120 X 1.73) than 240. The industry standard for metering 4 wire services call for the high leg to go on the right, or "C" phase while the code calls for the high leg to go on "B" phase on customers equipment. Thats always been confusing.


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## Doohicky (Jan 6, 2022)

Southeast Power said:


> The motor will never know.


Is this really true, though? Motors are dumb, but, we still need to be be smarter than them. If we put a three phase motor on a high leg system, do we not lock it up into a death struggle of (unknown) harmonic frequency voltage in which the two weaker phases can never overcome the coil magnetization of the dominant/stronger phase and our rotor just wiggle back and forth a few time per second while we also mainly hear the motor buzz and groan? This is what I have seen. Btw, no starter was involved...just direct wiring.


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## gpop (May 14, 2018)

Doohicky said:


> Is this really true, though? Motors are dumb, but, we still need to be be smarter than them. If we put a three phase motor on a high leg system, do we not lock it up into a death struggle of (unknown) harmonic frequency voltage in which the two weaker phases can never overcome the coil magnetization of the dominant/stronger phase and our rotor just wiggle back and forth a few time per second while we also mainly hear the motor buzz and groan? This is what I have seen. Btw, no starter was involved...just direct wiring.



According to the hundreds of thousands of motors running on high leg delta they seem happy to run that way. 

P.s your problem was a dropped phase / open motor winding or miss wired motor (normally 6 and 9 swapped)


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## micromind (Aug 11, 2007)

Doohicky said:


> Is this really true, though? Motors are dumb, but, we still need to be be smarter than them. If we put a three phase motor on a high leg system, do we not lock it up into a death struggle of (unknown) harmonic frequency voltage in which the two weaker phases can never overcome the coil magnetization of the dominant/stronger phase and our rotor just wiggle back and forth a few time per second while we also mainly hear the motor buzz and groan? This is what I have seen. Btw, no starter was involved...just direct wiring.


This doesn't have anything to do with the high leg, the motor was single-phased.


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## Almost Retired (Sep 14, 2021)

this horse still isnt dead ?


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## kb1jb1 (Nov 11, 2017)

Doohicky said:


> Is this really true, though? Motors are dumb, but, we still need to be be smarter than them. If we put a three phase motor on a high leg system, do we not lock it up into a death struggle of (unknown) harmonic frequency voltage in which the two weaker phases can never overcome the coil magnetization of the dominant/stronger phase and our rotor just wiggle back and forth a few time per second while we also mainly hear the motor buzz and groan? This is what I have seen. Btw, no starter was involved...just direct wiring.


A lot of ornamental words shows that you might not be an electrical contractor or even an electrician.


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## Almost Retired (Sep 14, 2021)

edited


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## wiz1997 (Mar 30, 2021)

3 phase motors do not normally operate to ground, they operate phase to phase.

The "high leg" exists only to ground, not phase to phase.

In my experience.


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## JRaef (Mar 23, 2009)

"High leg" has absolutely nothing to do with a 3 phase load, there is nothing "high" about the "leg" with regard to the other two phases. it is only high in relationship to ground (208V compared to 120V) which is irrelevant to a 3 phase load. So if you have a 230V 3 phase motor, the phase-to-phase voltage is 240V as it should be. In fact if you have a 230V 1 phase motor, any phase-to-phase voltage is STILL 240V so it is fin. 

The ONLY time it matters is if you are wanting to connect single phase line-to-neutral loads, where two of the lines will read 120V to neutral, the 3rd will read 208V, so you just don't use that.


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## kb1jb1 (Nov 11, 2017)

Maybe the OP is thinking if the phases are unbalanced from a very heavy load on the 120 volt legs. This might leave a situation where A-B is 240 volts, A-C is 240 volts, and C-B is 230 volts. I am guessing at voltages just to illustrate the possibilities.


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## micromind (Aug 11, 2007)

kb1jb1 said:


> Maybe the OP is thinking if the phases are unbalanced from a very heavy load on the 120 volt legs. This might leave a situation where A-B is 240 volts, A-C is 240 volts, and C-B is 230 volts. I am guessing at voltages just to illustrate the possibilities.


This ia about the only problem I've seen with open ∆. 

Since almost all of them are overhead and the PUCO likes to run about 2,000 amps down a #6 quadplex even if the voltages are balanced at the pot bushings, they won't be at the panel if there are lots of 120 loads and not much load on the high leg.


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## gpop (May 14, 2018)

It complicated to explain to someone that there are 3 sine waves 120 degrees apart that go to the motor then explain that the motor doesn't see the sine waves it sees the pressure difference between the waves. E.g it doesn't see 2 waves at 277 it see's one wave at 480 on each winding. Then explain its a 9 lead and each pack is seeing 240.


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## 460 Delta (May 9, 2018)

I wasn’t going to respond to this post but I can’t resist. If open Deltas won’t work, I’m wondering how all the R-Mix plants, sewage booster stations, and old filling stations converted to convenience stores are running the 3 phase motors?


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