High Leg on a 3 phase 120/240 4 wire delta system - Electrician Talk - Professional Electrical Contractors Forum

az101 · 02-13-2010, 06:03 PM

I`m a journeyman electrician in Arizona and I recently encountered a very old warehouse building that has a 120/240 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. I know I can use a rotary phase converter to create the 3 phase needed, but if not cool.

Bob Badger · 02-13-2010, 06:08 PM

Quote:

Originally Posted by az101

I`m a journeyman electrician in Arizona and I recently encountered a very old warehouse building that has a 120/240 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. I know I can use a rotary phase converter to create the 3 phase needed, but if not cool.

Yes you can connect a 3 phase motor to this supply, the motor is to stupid to care about the voltage to ground.

**MDShunk** · 02-13-2010, 06:10 PM

Yes, you're fine with 3-phase motors.

The only two things you have to worry about on a high leg system are 120V single phase loads on the high leg, or 240volt multiwire branch circuits that use the neutral inside the equipment for some 120V stuff (like clothes dryers and electric ranges). Treat your 3-phase motor loads as you would any other 3-phase load. Any three phase equipment that also has a neutral run to it should be investigated to determine what they're using the neutral for, so that you don't smoke something inside the machine that might use the high leg and neutral for something 120V. Some printing presses are this way, as one notable example. The cure there is to feed a transformer, instead of the equpment directly, and do delta primary and wye secondary. Omit the neutral with the branch circuit run from the high leg panel, and make a neutral with your wye secondary.

az101 · 02-13-2010, 06:23 PM

I also talked to a motor tech and he had said that the high leg would burn the motor out in a few weeks or months. The tech said each leg on the 3 phase should read 120V and anything higher on any leg meaning the high leg can burn the motor. I think he believes the motor is very smart.

**MDShunk** · 02-13-2010, 06:25 PM

Quote:

Originally Posted by az101

I also talked to a motor tech and he had said that the high leg would burn the motor out in a few weeks or months. The tech said each leg on the 3 phase should read 120V and anything higher on any leg meaning the high leg can burn the motor. I think he believes the motor is very smart.

He wasn't much of a motor guy if he told you that.

ohmontherange · 02-13-2010, 07:26 PM

Just don't connect any 120V single phase loads to the high leg.

A-B, B-C, & A-C IS 240V phase to phase.

All the high legs I've seen are typically 210-216ish phase to ground

Magnettica · 02-13-2010, 08:06 PM

Quote:

Originally Posted by az101

I also talked to a motor tech and he had said that the high leg would burn the motor out in a few weeks or months. The tech said each leg on the 3 phase should read 120V and anything higher on any leg meaning the high leg can burn the motor. I think he believes the motor is very smart.

Think of the motor as a painter, or a plumber.

brian john · 02-13-2010, 08:13 PM

Quote:

Originally Posted by az101

I`m a journeyman electrician in Arizona and I recently encountered a very old warehouse building that has a 120/240 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. I know I can use a rotary phase converter to create the 3 phase needed, but if not cool.

You are reading 208 VAC on C phase to ground NOT 240 VAC.

micromind · 02-13-2010, 11:04 PM

That 'motor tech' is way off. He needs a bit more education.

I can't even think of how many 3 phase motors I've connected to delta systems, and yet to have a problem. Some of these are 20 years ago.

I've seen plenty of motors that were installed in the '60s and before that ran just fine on delta systems.

One of the major reasons for the existence of the 240 delta system was to add a 3rd phase to a single phase system so that 3 phase motors could be installed.

Anyone who says that a delta system will damage a 3 phase motor doesn't know much if anything about motors or power systems.

Rob

chenley · 02-13-2010, 11:29 PM

I don't think I would take any advice from that "motor tech" from now on.

All you really need to watch for in motor applications is the rotation of it so it doesn't damage the equipment it is attached to.

220/221 · 02-14-2010, 12:28 AM

Quote:

I also talked to a motor tech and he had said that the high leg would burn the motor out in a few weeks or months

Quote:

I know the power company used to have the high leg on the C phase but changed it to the B phase.

Actually, POCO still connects to C but past their terminations, NEC requires it on B.

Quote:

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.

Sure. That's what you have. 120/240 3 phase as opposed to 120/208 3 phase where all legs are 120V

Although, IMO, it's not a great idea, you can connect any 240 load using the high/wild/bastard/stinger leg, even single phase, and it will work properly because you have 240V between any of the two phases.

Just don't ask me how it works.

Quote:

You are reading 208 VAC on C phase to ground NOT 240 VAC

Even though the math says it should read 208, for some reason they seem to generally read in the high 190's.

brian john · 02-14-2010, 04:50 AM

Quote:

Originally Posted by 220/221

Actually, POCO still connects to C but past their terminations, NEC requires it on B.

Sure. That's what you have. 120/240 3 phase as opposed to 120/208 3 phase where all legs are 120V

Although, IMO, it's not a great idea, you can connect any 240 load using the high/wild/bastard/stinger leg, even single phase, and it will work properly because you have 240V between any of the two phases.

Just don't ask me how it works.

Even though the math says it should read 208, for some reason they seem to generally read in the high 190's.

Because the phase to phase is 218-220 and single phase to neutral is 109-110.

HIgh Leg services were designed for INDUSTRIAL facilities, for areas with high number of motors.

jwjrw · 02-14-2010, 09:36 AM

Quote:

Originally Posted by Magnettica

Think of the motor as a painter, or a plumber.

Why not an electrical inspector or a multi inspector???????

:thu mbup:

electron_theory · 02-15-2010, 12:26 AM

Strange that a motor guy would say that, considering that Delta systems are designed for and used almost exclusively for pure power applications.

LawnGuyLandSparky · 02-15-2010, 12:38 AM

Quote:

Originally Posted by electron_theory

Strange that a motor guy would say that, considering that Delta systems are designed for and used almost exclusively for pure power applications.

...And the situation is going to get worse before it gets better. "Motor guys" know motors, not power systems.

Marshall175 · 02-15-2010, 04:10 PM

Quote:

Originally Posted by 220/221

Actually, POCO still connects to C but past their terminations, NEC requires it on B.

Sure. That's what you have. 120/240 3 phase as opposed to 120/208 3 phase where all legs are 120V

Although, IMO, it's not a great idea, you can connect any 240 load using the high/wild/bastard/stinger leg, even single phase, and it will work properly because you have 240V between any of the two phases.

Just don't ask me how it works.

Even though the math says it should read 208, for some reason they seem to generally read in the high 190's.

where does the NEC require the high leg to be B phase? Correct me if I am wrong, but I don't believe that is code, only common practice...

electron_theory · 02-15-2010, 05:42 PM

Quote:

Originally Posted by Marshall175

where does the NEC require the high leg to be B phase? Correct me if I am wrong, but I don't believe that is code, only common practice...

Here is an interesting read....
http://www.mikeholt.com/mojonewsarch...7~20041220.php

Marshall175 · 02-16-2010, 05:44 AM

yeah, I guess I posted too fast...I found the code section a little after I posted that...

TOO_SL:IM · 05-12-2010, 10:41 PM

The utility makes the call on where you land the wild leg.Usually they come in on L3 on the line side then on the load side we swap it to L2 "B" phase>I run into this problem a lot as one of our utility is a Rural coop and we do all the wiring from the Transformer on in to the Building.Was told that is how the meter reads it correctly, my 2cents

jrannis · 05-13-2010, 07:31 AM

Quote:

Originally Posted by az101

I`m a journeyman electrician in Arizona and I recently encountered a very old warehouse building that has a 120/240 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. I know I can use a rotary phase converter to create the 3 phase needed, but if not cool.

Yes you can, that's exactly what it was made to do.