I learned I have 3 phase "high leg" at my new location, and need help using it

[nickmckinney]

I just moved into a new building and didn't even realize I have the only 3 phase supply in the warehouse group. I picked up a 3 phase 220/440 machine with a 5HP motor I need to run. A couple buddies looked at my panel and I have 120V on each outer leg and 240V on the center and there is orange tape. I was told its a high leg configuration and that went right over my head. Anyway we are trying to figure out the best way to use the 3 phase machine right up to it. Originally I was going to get a phase converter but now..........

 

[Tony Wells]

3 phase machines with low voltage controls require that you identify the high leg and make sure it is not feeding the control transformer. Connect either of the low legs to the transformer, and note the tap. Some are tapped for 208, 277, 120, etc.. The other two can be swapped to make your machine rotation is correct.

 

[rdhem2]

Gentlemen:
It always seems I am the purveyor of contrary news. Three phase with a wild leg indicates a 240/120v delta connected at the source. This is a typical, common light industrial supply from the power company. In other words not enough power demand to warrant a 480/277 wye service. With the 480v connection it requires the customer to purchase and install his own transformers to obtain power for 120v devices. The 240/120v delta service provides three phase power at a higher voltage, slightly less amperage demand. Phase A and phase C providing 120v power when read to neutral. The B phase is read all over from 190v to 240v to neutral. We were taught in school by actual calculation with everything perfect by spec and book that the calculated value should be 196v to ground or neutral. This is because of where the neutral is tapped in the transformer. You are actually getting power through 1 and 1/2 sets of windings, in relation to the B phase.

Now the contrary news. Even though the voltage is high on B phase in relation to phase A and C. It is still 240v phase to phase no matter how you read it. Yes the B phase is high. Yes the B phase will fry anything connected to B phase and neutral. Hence the reason for the orange tape that is is required at the service panel and is supposed to be used to identify the B phase throughout the system. But any 240v item does not know or care what phase combination it is supplied by. A-B, A-C, or B-C, it just does not matter because they all have 240v potential between them. Single phase or three phase. Try it out.

Your 3 phase service panel is basically one third useless when needed for 120v loads. 240v single and three phase may be originated from any place in the panel. 120v loads stay away from that darn B, center phase or you will let the smoke out.!

What I have done in the past is to set a single phase sub panel fed from phase A and C, next to the main 3 phase service panel then supply all 120v circuits out of the sub panel. Keeps confusion down, and it always seems like you run out of space for single pole breakers.

 

[John Hasler]

Gentlemen:
It always seems I am the purveyor of contrary news. Three phase with a wild leg indicates a 240/120v delta connected at the source. This is a typical, common light industrial supply from the power company. In other words not enough power demand to warrant a 480/277 wye service. With the 480v connection it requires the customer to purchase and install his own transformers to obtain power for 120v devices. The 240/120v delta service provides three phase power at a higher voltage, slightly less amperage demand. Phase A and phase C providing 120v power when read to neutral. The B phase is read all over from 190v to 240v to neutral. We were taught in school by actual calculation with everything perfect by spec and book that the calculated value should be 196v to ground or neutral. This is because of where the neutral is tapped in the transformer. You are actually getting power through 1 and 1/2 sets of windings, in relation to the B phase.

Now the contrary news. Even though the voltage is high on B phase in relation to phase A and C. It is still 240v phase to phase no matter how you read it. Yes the B phase is high. Yes the B phase will fry anything connected to B phase and neutral. Hence the reason for the orange tape that is is required at the service panel and is supposed to be used to identify the B phase throughout the system. But any 240v item does not know or care what phase combination it is supplied by. A-B, A-C, or B-C, it just does not matter because they all have 240v potential between them. Single phase or three phase. Try it out.

Note that the "neutral" referred to above is not the three-phase neutral but the grounded conductor, which is a center tap on the transformer winding between A and C.

Your 3 phase service panel is basically one third useless when needed for 120v loads. 240v single and three phase may be originated from any place in the panel. 120v loads stay away from that darn B, center phase or you will let the smoke out.!

What I have done in the past is to set a single phase sub panel fed from phase A and C, next to the main 3 phase service panel then supply all 120v circuits out of the sub panel. Keeps confusion down, and it always seems like you run out of space for single pole breakers.

Good idea. Also simplies changes down the road. I'd supply all single-phase loads from that panel if possible, not just 120 ones.

Let's hope nobody here ever has to deal with corner-grounded delta...

 

[DMS]

Many motors can be wired to run off of either a "Wye" or "Delta" 3 phase configuration. If you look on the side of your motor there are often wiring instructions (sometimes they are inside the wiring box). Basically a "high leg" is a "Delta" service (which would normally have 3 legs), but they center tapped one of the transformers to give you 120V for "normal" usage. This center tap is normally marked as the neutral, but you can't use it as a neutral in a 3 phase system. If your machines can be configured to run from a delta service, then just wire them up like the neutral is not there.

For what it's worth, I have only ever seen or heard of a few people with this type of service, and it always ends up with somebody scratching their heads going "huh?", so don't feel bad

 

[itsme_Bernie]

3 phase machines with low voltage controls require that you identify the high leg and make sure it is not feeding the control transformer. Connect either of the low legs to the transformer, and note the tap. Some are tapped for 208, 277, 120, etc.. The other two can be swapped to make your machine rotation is correct.

Wow, never knew about a "high leg". I don't have to worry about it in y situation, but interesting to know that in some situations you have to think about which legs you swap.

Thanks Tony!


Bernie

 

[Dave Smith]

Russ and John--what you describe makes sense to me---so if I am understanding you correctly--if you have a three phase motor that can be wired 480 or 220--then the lower voltage connection would be using all three legs --without needing a transformer and without caring that the third leg is actually higher voltage?----and to change rotation --any legs can be switched even though one is higher? --I understand that some 3 phase motors are high voltage only and need 277v on all three legs.---thanks both for you clearing this up ----Dave

 

[John Hasler]

Many motors can be wired to run off of either a "Wye" or "Delta" 3 phase configuration. If you look on the side of your motor there are often wiring instructions (sometimes they are inside the wiring box). Basically a "high leg" is a "Delta" service (which would normally have 3 legs), but they center tapped one of the transformers to give you 120V for "normal" usage. This center tap is normally marked as the neutral, but you can't use it as a neutral in a 3 phase system. If your machines can be configured to run from a delta service, then just wire them up like the neutral is not there.

As long as the motor neutral is not brought out and intended to be connected to the supply neutral (I've never seen such a motor) you can connect a wye-connected motor to a delta supply and vice-versa if the line to line voltage is correct.

Rewiring a motor from wye to delta (or vice versa) changes the line to line voltage it requires but the motor does not care how the source is wired: just what voltage it supplies.

 

[jim18655]

Several things here need clarification. If the machine has a 240 volt control transformer then it will make no difference where the high leg is connected. If the machine has a 4 wire cord - 3 phases, neutral, and ground then it will make a difference in the connections because they are getting 120v control power, or lights from one of the phases to neutral, assuming it needs 120v. It could have 240v controls. You must make sure the control circuit isn't connected to the "high leg." As for delta or wye connected motors that depends on the internal connections in the motor. All you do when you change voltage is series or parallel the coils. A 12 lead motor would give you the choice of wye or delta.

 

[rdhem2]

Mr. hasler, shall we try again? You guys are so close yet so far away. Forget the neutral when wiring motors. Motor does not use it, does not care. Motor name plate says 208/240v it can be wired either 208v WYE or 240v DELTA connected transformer. Why don't we care? Because it comes from the power company and you can do nothing about it anyway. 480/277v again forget the 277 as this is for a neutral connected item. Motor is not connected to neutral so it is straight 480v connection. 480v grounded delta hook up has been illegal for years do to dangerous conditions in installations not properly supervised and maintained.

When swapping phase legs for motor rotation. Forget the wild leg. All phases are 240v between legs. Only when read to the neutral, electrical ground, or the ground wire, equipment ground does it manifest itself as 196v to ground. When wiring a 240v single phase device, your welder, dryer, big hot plate, whatever, it does not matter which two legs you tap as they are all 240v leg to leg. Get out you multimeter or even your wiggies and take some readings. Maybe that is the best way to get it to come to reality.

Wild legs only matter when connecting a device to that leg and a neutral, electrical ground. Equipment ground does not matter because it is only there for your safety anyway. To carry away objectionable, dangerous, fault currents of short duration.

Wild Leg only matters for 120v connections.