High Leg Delta

[Seven-Delta-FortyOne]

As far as the voltage goes, it will be the same between any two hot legs, regardless if one is the high leg.

High leg only comes into play with H-N circuits.

Generally. YMMV. I have not covered all possible scenarios, etc.

 

[LarryFine]

Yes, capacity issues considered, L-L loads may be served by any two lines.

 

[jim dungar]

From what I understand it would be better if it's a closed Delta vs Open.

It all depends on the sizing of the transformers and the amount of loading on A and C.

Also be aware that many electronic controls want to see no more than 120V to ground even when they do not use neutrals.

 

[electrofelon]

It all depends on the sizing of the transformers and the amount of loading on A and C.

Also be aware that many electronic controls want to see no more than 120V to ground even when they do not use neutrals.

I have always been paranoid about this, but never found it to be the case. My main client has Labs with high leg Delta and I've connected all sorts of stuff line to line with one being the high leg, and never had a problem.

 

[winnie]

Guys, this is an old thread that was resurrected with a new (related) question.

Hello guys
While everyone's on the subject of hi leg delta ...and not to change the subject but....
Any reason why we can use a single phase circuit (2 hot, 1 eq ground) to power our ductless splits at a restaurant renovation with a dated distribution system that were doing
One leg would be the 210 volt hi leg and the other would be A or C

There are 3 issues to address
1) Does the service have the necessary capacity on the high leg? Some high leg services are designed for a small 3 phase load on an otherwise single phase service, and don't really have the capacity for high leg single phase loads. Other services might have lots of unused capacity on the high leg
2) Does the service come from an open delta transformer? If so, then you shouldn't place a single phase load across the open jaw.
3) Does the load itself have any voltage to ground limits. The example of a L-L load with L-G voltage limits is a VFD. The surge suppression in the VFD is connected L-G and will fry if don't use a balanced supply. Your mini splits are likely rated for 240V L-G but you must confirm this.

Jonathan

 

[jim dungar]

I have always been paranoid about this, but never found it to be the case. My main client has Labs with high leg Delta and I've connected all sorts of stuff line to line with one being the high leg, and never had a problem.

Line to Ground connected filters and surge devices are the usual issues.

 

[Kyle1500]

Just to elaborate a bit, I am nearly certain no one makes a singe pole 240 rated plug on breaker. 2 poles are available (i.e for siemens its a R suffix, Q230R, for QO its an H suffix QO230H). All 3 pole breakers are straight rated. So to do this one would need one of the special 2 poles or a 3 pole, and the 2 poles are usually more expensive than the 3's. IF its a siemens true/bolt on panelboard, the BQD breakers are 277/480 slash rated so you could indeed get a single pole and they are the same form factor as the 240V bolt ons so will fit unlike some brands where the 240 vs 480 bolt ons are physically different.

Actually, GE makes (or used to) 240V straight rated single pole breakers, THQL1215 & THQL1220 for the 15 and 20 amp respectively. The slash rated version is THQL 1115 & 1120 respectively. I'm fairly certain GE was the only manufacture to have straight rated 240V single pole breakers in their lineup and from my understanding they are no longer manufactured, a few websites have them listed as available for around the $100 ballpark as of November 2025. These GE 240V single pole breakers are extremely rare, much moreso than a straight rated 240V two pole breaker of which is available from all the major manufacturers i.e. Eaton, Square D and Siemens although it's likely they will have to be ordered directly from the manufacturer as few electrical supply stores keep them in their stock offerings.

The GE THQL 1215 & 1220 was used in super rare scenarios typically for HID high bay fixtures with quad tap ballasts which can be configured for 120/208/240/277, with a few ballasts having a 5th tap for 480V. In this application the use of the high leg to neutral eliminated the wasted space in the panel which would otherwise leave every third slot empty or occupied by a three pole breaker, but usually in this situation you'd opt for a straight rated 2 pole and configure the ballasts for 240V because the fixtures will pull slightly less current thereby reducing voltage drop and heating of the conductors. Alternatively one could use a straight rated 2 pole and run a neutral with the circuit and have two lighting circuits, one at 120 the other 208V.

 

[herding_cats]

In the Delta configuration the wild leg will vary in voltage. There is no "single pole 208". You might have 208 for a few min on that wild leg, but it fluctuates based on grid load. 2-pole 240 is consistent voltage on delta configurations. There is no such thing as 240 wye or 208 Delta. There are transformers avail to convert 240 delta into 208 wye. I've done many installs for this.

 

[Joethemechanic]

In the Delta configuration the wild leg will vary in voltage. There is no "single pole 208". You might have 208 for a few min on that wild leg, but it fluctuates based on grid load. 2-pole 240 is consistent voltage on delta configurations. There is no such thing as 240 wye or 208 Delta. There are transformers avail to convert 240 delta into 208 wye. I've done many installs for this.

In Philly where we had 2 phase, lots of the old 2 phase equipment got hooked up to a high leg service when it got sold or moved. If you ran all 4 conductors out to the motor including the grounded neutral, you could run it with a 3 phase disco, and contactor. You had a phase from A-C and a 90 degree phase from B-N. The problem is the two different voltages A-C 240 and B-N 208. Then there was the impedance on the B-N path being kinda high.

It might not have been right, but it got done that way quite a bit, and it did work

 

[tortuga]

There is no such thing as 240 wye

I'd say 240Y is not super common but it does exist for special applications, I am not a VFD expert nor a distribution engineer but some say 240Y/139 is a good replacement option for corner grounded 240 delta with one or two motor loads that can't be run at 208Y120. Say a pump station.
The 139 is not used for anything, some 240V VFD's supposedly perform better off a wye system, so these 'drive isolation' transformers are out there just not super common.

 

[Birken Vogt]

I'd say 240Y is not super common but it does exist for special applications, I am not a VFD expert nor a distribution engineer but some say 240Y/139 is a good replacement option for corner grounded 240 delta with one or two motor loads that can't be run at 208Y120. Say a pump station.
The 139 is not used for anything, some 240V VFD's supposedly perform better off a wye system, so these 'drive isolation' transformers are out there just not super common.

It gets done on mobile generators here in Delta land quite often, you have a permanent delta gen that breaks down, roll up a mobile gen that is only wye connectable and you get to choose whether to run the installation at 208 which is low voltage for the motors, 240/139 which is high voltage for the controls, or re-wire the mobile generator to 240 delta which is seldom practical, or adjust it to the Mexican standard of 220/127, every situation requires a different approach.

 

[electrofelon]

Actually, GE makes (or used to) 240V straight rated single pole breakers, THQL1215 & THQL1220 for the 15 and 20 amp respectively. The slash rated version is THQL 1115 & 1120 respectively. I'm fairly certain GE was the only manufacture to have straight rated 240V single pole breakers in their lineup and from my understanding they are no longer manufactured, a few websites have them listed as available for around the $100 ballpark as of November 2025. These GE 240V single pole breakers are extremely rare, much moreso than a straight rated 240V two pole breaker of which is available from all the major manufacturers i.e. Eaton, Square D and Siemens although it's likely they will have to be ordered directly from the manufacturer as few electrical supply stores keep them in their stock offerings.

The GE THQL 1215 & 1220 was used in super rare scenarios typically for HID high bay fixtures with quad tap ballasts which can be configured for 120/208/240/277, with a few ballasts having a 5th tap for 480V. In this application the use of the high leg to neutral eliminated the wasted space in the panel which would otherwise leave every third slot empty or occupied by a three pole breaker, but usually in this situation you'd opt for a straight rated 2 pole and configure the ballasts for 240V because the fixtures will pull slightly less current thereby reducing voltage drop and heating of the conductors. Alternatively one could use a straight rated 2 pole and run a neutral with the circuit and have two lighting circuits, one at 120 the other 208V.

Thats some cool rare trivia, thanks!

 

[electrofelon]

In the Delta configuration the wild leg will vary in voltage. There is no "single pole 208". You might have 208 for a few min on that wild leg, but it fluctuates based on grid load. 2-pole 240 is consistent voltage on delta configurations. There is no such thing as 240 wye or 208 Delta. There are transformers avail to convert 240 delta into 208 wye. I've done many installs for this.

Hmmm but why would the high lug fluctuate any more that other system voltages? I could see the impedance being a little higher because the load is passing thru more windings. but in practice Ive never seen any more variation that other system connections.

 

[electrofelon]

This is an older thread and I cant remember if I mentioned this, but earlier this year I hooked up a machine to the high leg and neutral, single phase 208V. I just used a three pole breaker. The reason was the machine needed single phase 220 nominal, and I looked at the various drives and stuff and a little higher than 208V was a fit for all the drives whereas a little higher than 240 was not. Plus there is slight risk of giving something two hots when is expecting one leg to be grounded. The machine has been working fine.

 

[herding_cats]

Hmmm but why would the high lug fluctuate any more that other system voltages? I could see the impedance being a little higher because the load is passing thru more windings. but in practice Ive never seen any more variation that other system connections.

It's just the way the transformers are wired. The wild leg is called "wild" because of the wild voltage variations. I've seen 135v, 165v, 189v and 200v in the same day (hot to neutral). POCOs told me it's normal based on the building load, and grid load both.

 

[winnie]

I suspect that the variation on the wild leg is likely much greater on an open delta system than on a closed delta system, in much the same way that the 'open jaw' of an open delta system isn't supposed to be used.

 

[hillbilly1]

It gets done on mobile generators here in Delta land quite often, you have a permanent delta gen that breaks down, roll up a mobile gen that is only wye connectable and you get to choose whether to run the installation at 208 which is low voltage for the motors, 240/139 which is high voltage for the controls, or re-wire the mobile generator to 240 delta which is seldom practical, or adjust it to the Mexican standard of 220/127, every situation requires a different approach.

Luckily here, most of the roll up generators < 100 kw have a rotary switch that you can select 277/480, 120/240 delta or 120/208, so you don’t have to mess with the taps.

 

[Joethemechanic]

I've got a open delta high leg feeding my place.



I rarely use the B-N for anything except one led wall pack light. The 3 phase runs a 15 HP compressor, welders, and some smaller machine tools, 5 to 10 HP and not really multiple machines running at the same time (At least not often). I never notice any wild voltage swings. I really haven't been looking at them, but when I bought the building I loaded stuff and looked for voltage drop. And yes I did find some high impedance in the single phase fuse boxes that I replaced with breakers anyway.

Now PP&L doesn't offer 240/120 4 wire delta. Only legacy customers. They only offer 208/120 WYE service from PPL owned equipment. That's all great and wonderful, but if you are a typical small businessman around here you aren't always buying brand new equipment, and most used equipment seems to be 240/480.

 

[Birken Vogt]

Luckily here, most of the roll up generators < 100 kw have a rotary switch that you can select 277/480, 120/240 delta or 120/208, so you don’t have to mess with the taps.

I've seen it, but only on a couple hundred kw units, under that they only seem to have wye. And it makes little sense to me because bigger installations are seldom delta, usually 480, delta is normally only 20 kw pump stations and below, and the like.

 

[WD40]

I've got a open delta high leg feeding my place.
..
Now PP&L doesn't offer 240/120 4 wire delta. Only legacy customers.

Whats funny is in PECO territory its actually easier to get a *new* two phase service as in 2Ø 5-Wire than a 3-phase 240 Open or Hi-leg Delta, as they severely limit the size of hi-leg Delta to something really low 15kVA per phase, two-phase you can go up to 100kVA still, not that anyone would want a two-phase service. (last I checked the 'blue book')