120/240 3 Phase High Leg Delta - High Leg Is 248 volts

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Vehikl

Member
Location
Seattle
What is the acceptable variation in voltage range for the high leg of a 120/240 3 Phase High Leg Delta service panel?

The 120 volt legs both read at 120 volts, but the high leg measures at 248 rather than 240. So the voltage across the high leg to neutral is not 208 volts but measures at 217 volts.

I realize this is only 4.3% over 208, but I am installing a new circuit for a 208 volt conveyor toaster for a restaurant and am wondering if connecting the 208 volt equipment to 217 volts will shorten the lifetime of the heating elements of the conveyor toaster.

If this is an actual concern, then what would be a the recommended approach to correct & lower the 217 voltage down to 208 volts for this circuit? Or is 217 volts in the acceptable range of voltage variation for the 208 volt heating elements of the conveyor toaster? (The product documentation does not give a + or - voltage range for the unit.)

Any info or tips appreciated, Thanks
 

ActionDave

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Durango, CO, 10 h 20 min from the winged horses.
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Licensed Electrician
....…..If this is an actual concern, then what would be a the recommended approach to correct & lower the 217 voltage down to 208 volts for this circuit? Or is 217 volts in the acceptable range of voltage variation for the 208 volt heating elements of the conveyor toaster? (The product documentation does not give a + or - voltage range for the unit.)

Any info or tips appreciated, Thanks
The actual concern is that you should not be using the high leg to connect any line to neutral loads.

My recommended approach would be to check and make sure the machine is in fact 208V only and not 208/240V. If it is not then run a 240V circuit and install a buck/boost transformer.
 

augie47

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Staff member
Location
Tennessee
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State Electrical Inspector (Retired)
I agree. The guys who understand distribution systems will advise against obtaining 208 using the high leg. In addition I don't believe you can find a properly rated SP breaker (without going to a higher rated overall panel).
 

jeremysterling

Senior Member
Location
Austin, TX
I agree with ActionDave and augie47.

Here in Austin, it is a local ordinance that prohibits line to neutral loads on the high leg of a midpoint grounded delta (120V/240).

As augie47 pointed out, finding a single pole breaker rated for your intended use may be difficult.
 

Vehikl

Member
Location
Seattle
I agree. The guys who understand distribution systems will advise against obtaining 208 using the high leg. In addition I don't believe you can find a properly rated SP breaker (without going to a higher rated overall panel).
There is no SP Breaker. The disconnect is by the restaurant's individual meter on the floor level above the restaurant. We have no other options but to use the high leg for 208 as the two 120 v legs Breaker positions are all taken. (half of them are tandems already). Pretty much nearly a third of the panel positions are unusable otherwise. A service panel upgrade is not an option as the building owner plans to add another story to the building & simply will not allow any changes to the the restaurant's electrical service. Their 208 v AC unit appears to be operating just fine using the high leg & neutral for the 208 v circuit. I am interested in why using the high leg & neutral to obtain 208 is not recommended. I called our area electrical company & they have no objection to it.
 

kwired

Electron manager
Location
NE Nebraska
What is the acceptable variation in voltage range for the high leg of a 120/240 3 Phase High Leg Delta service panel?

The 120 volt legs both read at 120 volts, but the high leg measures at 248 rather than 240. So the voltage across the high leg to neutral is not 208 volts but measures at 217 volts.

I realize this is only 4.3% over 208, but I am installing a new circuit for a 208 volt conveyor toaster for a restaurant and am wondering if connecting the 208 volt equipment to 217 volts will shorten the lifetime of the heating elements of the conveyor toaster.

If this is an actual concern, then what would be a the recommended approach to correct & lower the 217 voltage down to 208 volts for this circuit? Or is 217 volts in the acceptable range of voltage variation for the 208 volt heating elements of the conveyor toaster? (The product documentation does not give a + or - voltage range for the unit.)

Any info or tips appreciated, Thanks
I am a little confused with your voltage readings. Are you saying it is 248 from A TO B and B to C? Because you made it sound like it is 248 B to N then followed up with saying it is 217 B to N?:?

So assuming you have 120 A TO N, 120 C to N and 217 B to N, then the voltage is out of balance from where it should be but could be for a few different reasons. First you maybe have a lot of single phase load between A, N and C but very little load on the high leg - so it is pulling the voltage down on the 120/240 winding but the other(s) are not loaded much and are not being pulled down. Larger transformer and/or larger supply conductors may offset that if it is a problem.

POCO could have unbalance primary voltage, bad regulation/tap changer and it happens to be on the line that derives the high leg - especially with an open delta primary configuration.

217 volts on high leg would be normal if the other two phases were reading 125ish.

If you can shut things down see what voltages look like with no load.
 

kwired

Electron manager
Location
NE Nebraska
I realize this is only 4.3% over 208, but I am installing a new circuit for a 208 volt conveyor toaster for a restaurant and am wondering if connecting the 208 volt equipment to 217 volts will shorten the lifetime of the heating elements of the conveyor toaster.

I missed what was said here first time through. You don't have a 208 volt system if you have a high leg of 208 (in which 217 is about 5% above nominal 208 and is generally considered acceptable). What you have is a delta system with 240 volts phase to phase. Your conveyor toaster likely has all heating elements connected phase to phase and they would be running at a nominal 240 volts and not a nominal 208. If it has a neutral would only be used for some controls or maybe the conveyor or a blower motor.

It will put out more heat then it will at 208, if that is not acceptable you need to either change input voltage taps - (not often an option on this kind of equipment though), run through a transformer to get correct voltage - buck boost is likely the most economical way to go instead of full isolation transformer, find equipment rated proper voltage, or be prepared to possibly need to change heating elements more often then you may if they were connected to proper voltage - maybe purchase ones for proper voltage if it comes to needing to change them.

edit to add:

Or do you have a single phase unit and are thinking about connecting it from high leg to neutral? Most will tell you this is not a good idea, outside of the fact you will not find a properly rated single pole breaker to do this, I really don't know how bad of an idea it is.
 

Vehikl

Member
Location
Seattle
I am a little confused with your voltage readings. Are you saying it is 248 from A TO B and B to C? Because you made it sound like it is 248 B to N then followed up with saying it is 217 B to N?:?
Yes it is 248 from A TO B and B to C, & 217 B to N

So assuming you have 120 A TO N, 120 C to N and 217 B to N, then the voltage is out of balance from where it should be but could be for a few different reasons. First you maybe have a lot of single phase load between A, N and C but very little load on the high leg - so it is pulling the voltage down on the 120/240 winding but the other(s) are not loaded much and are not being pulled down.
That is exactly the situation as you describe.
Larger transformer and/or larger supply conductors may offset that if it is a problem.
 

Vehikl

Member
Location
Seattle
The actual concern is that you should not be using the high leg to connect any line to neutral loads. .
Upon checking Wikipedia's article on the subject I see that you are correct - Some manufacturers recommend that high leg to neutral not exceed 5% of the rated capacity of the transformer.
My recommended approach would be to check and make sure the machine is in fact 208V only and not 208/240V. If it is not then run a 240V circuit and install a buck/boost transformer.
. Thanks for the tip, the Buck Boost transformer appears the way to resolve the issue without altering the Service itself.
 

Vehikl

Member
Location
Seattle
You say there are only single spaces available in the panel.
I meant that there are only High Leg spaces available in the panel
You are wanting to supply a load from B to N. What am I missing?
That was what I was wanting to do. Evidently not a good idea since I have now learned it could damage the transformer if exceeding 5% of the transformer's capacity.
 

Vehikl

Member
Location
Seattle
I missed what was said here first time through. You don't have a 208 volt system if you have a high leg of 208 (in which 217 is about 5% above nominal 208 and is generally considered acceptable). What you have is a delta system with 240 volts phase to phase.
Yes, I said that in the title of the O.P. that it is 120/240 3 Phase High Leg Delta.
Your conveyor toaster likely has all heating elements connected phase to phase and they would be running at a nominal 240 volts and not a nominal 208.
I don't believe you are correct on this. This model of conveyor toaster ships in 3 different versions from the manufacturer ~ 208, 220 & 240 all three voltage versions are single phase.

If it has a neutral would only be used for some controls or maybe the conveyor or a blower motor.

It will put out more heat then it will at 208, if that is not acceptable you need to either change input voltage taps - (not often an option on this kind of equipment though), run through a transformer to get correct voltage - buck boost is likely the most economical way to go instead of full isolation transformer, find equipment rated proper voltage, or be prepared to possibly need to change heating elements more often then you may if they were connected to proper voltage - maybe purchase ones for proper voltage if it comes to needing to change them.

edit to add:

Or do you have a single phase unit and are thinking about connecting it from high leg to neutral? Most will tell you this is not a good idea, outside of the fact you will not find a properly rated single pole breaker to do this, I really don't know how bad of an idea it is.
 

Vehikl

Member
Location
Seattle
Thanks for all the input everyone. I am going to return the 208v conveyor toaster for a 240 version and add yet another tandem to the A or C leg to use with B leg & I end up with 248 volts which is only 3.3% over voltage for the 240 unit which should be within its operating voltage limits.

Furthermore I am considering doing the same thing to their Air Conditioner's circuit which is a 208 to 230 volt single phase unit that is now using the high leg to neutral & is running on 217 volts. As 248 volts is 7.8% above the unit's upper limit of 230 volts I would install a Buck Boost transformer to bring the voltage somewhere down to about the middle of its 208 to 230 volt range.

Any further input on a perhaps different approach I should consider? (Other than altering the Service, which is not an option for couple of reasons.)

Going to have a lot of tandems in this panel.
 
Last edited:

kwired

Electron manager
Location
NE Nebraska
Thanks for all the input everyone. I am going to return the 208v conveyor toaster for a 240 version and add yet another tandem to the A or C leg to use with B leg & I end up with 248 volts which is only 3.3% over voltage for the 240 unit which should be within its operating voltage limits.

Furthermore I am considering doing the same thing to their Air Conditioner's circuit which is a 208 to 230 volt single phase unit that is now using the high leg to neutral & is running on 217 volts. As 248 volts is 7.8% above the unit's upper limit of 230 volts I would install a Buck Boost transformer to bring the voltage somewhere down to about the middle of its 208 to 230 volt range.

Any further input on a perhaps different approach I should consider? (Other than altering the Service, which is not an option for couple of reasons.)

Going to have a lot of tandems in this panel.

Your air conditioner rated 230 volts is likely intended to be able to be used on 240 volts nominal which anything up to about 252 would still be considered in the acceptable range for that system.

Using high leg presents one more problem - circuit breakers connected to the high leg need to be rated for straight 240 volts, not the typical 120/240 rating you find on two pole breakers. Most manufacturers make them, most suppliers don't stock them because there is little demand for them. They are only needed for this purpose or on corner ground systems for the most part, but any situation where either line is more then 120 volts (nominal) to ground.

Other thing is if your load uses the neutral you would need to be very careful and pay attention to what supply lead is connected to loads that utilize the neutral. If both supply leads would happen to supply line to neutral loads you are totally out of luck on using the high leg to supply this equipment.

Might be time to consider adding a sub panel so future additions don't have this same trouble. Chances are the panel isn't listed for tandem breakers either, Iv'e never seen a three phase panel that was.
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Upon checking Wikipedia's article on the subject I see that you are correct - Some manufacturers recommend that high leg to neutral not exceed 5% of the rated capacity of the transformer.

Actually many single core 3-phase delta transformers are limited to 5% neutral load total not just from the high leg. This is one reason a 240/120V 3-phase delta this typically created using individual single-phase units.
Restrictions against using the high-leg to neutral connection are usually based on the sizing of the individual transformers and if they are connected in an open or closed delta.
 

kwired

Electron manager
Location
NE Nebraska
Actually many single core 3-phase delta transformers are limited to 5% neutral load total not just from the high leg. This is one reason a 240/120V 3-phase delta this typically created using individual single-phase units.
Restrictions against using the high-leg to neutral connection are usually based on the sizing of the individual transformers and if they are connected in an open or closed delta.
POCO's build smaller capacity services from single phase units, and may even supply both single phase and three phase customers from one bank. Larger capacity services they may have padmounts with single core - but chances are the amount of three phase load is pretty high in comparison to single phase load on those installations - so total neutral load in comparison to total kVA rating of transformer is typically pretty low.
 
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