208V Wild Leg Question

[texie]

Many POCOs prohibit connection of high leg to neutral loads in their service standards. Also some prohibit single phase line to line loads using the high leg as well to help prevent overloading open delta banks.
I've always been surprised that the NEC has not expressly prohibited high leg to neutral connections. Seems like this question comes up a lot.

 

[SKalkbrenner]

Thank you everyone for your input. The primary reason I was considering this instead of using 2 of the legs is for savings, TBH. Because I have 2 pole breakers in my subpanel, I have several that give me 240V using a 120 and the 208. After investigating higher usage than expected, a portion is coming from the 208 wild leg.

When drawing 20.8A, if my 240V is coming from both 120V legs, I end up with 4992W (120x20.8)+(120x20.8)=Approx. 5kW/hr of run time
When the same draw is getting 240V from a 120 and the wild 208, end up with 6822.4W (120x20.8)+(208x20.8)=6.8kW/hr
If I was able to use the 208V alone, the draw would jump to 24A, but I would still be at the 5kW level (208x24).

If my figures are correct (please let me know if I have anything messed up), all of my 2 pole breakers using the wild leg are adding 36% more usage.

If the wild leg isn't a viable option, I think pulling the 3 phase subpanel is the next best option. I will only use the 2 120V legs to feed the standard subpanel. Any major issues with that plan?

 

[ptonsparky]

Thank you everyone for your input. The primary reason I was considering this instead of using 2 of the legs is for savings, TBH. Because I have 2 pole breakers in my subpanel, I have several that give me 240V using a 120 and the 208. After investigating higher usage than expected, a portion is coming from the 208 wild leg.

When drawing 20.8A, if my 240V is coming from both 120V legs, I end up with 4992W (120x20.8)+(120x20.8)=Approx. 5kW/hr of run time
When the same draw is getting 240V from a 120 and the wild 208, end up with 6822.4W (120x20.8)+(208x20.8)=6.8kW/hr
If I was able to use the 208V alone, the draw would jump to 24A, but I would still be at the 5kW level (208x24).

If my figures are correct (please let me know if I have anything messed up), all of my 2 pole breakers using the wild leg are adding 36% more usage.

If the wild leg isn't a viable option, I think pulling the 3 phase subpanel is the next best option. I will only use the 2 120V legs to feed the standard subpanel. Any major issues with that plan?

Your calculations are all messed up.

Your 240v loads, and your power company do not care what the L-N voltage is. You are not using the neutral.

You are correct in the thought that if you do not have three phase loads, install a single phase sub panel.

 

[ptonsparky]

Look back at post #2.

 

[oldsparky52]

There's nothing wrong with using the high leg for 240v loads that do not require the neutral.

If you do, you have to use a non "/" rated breaker?

 

[ptonsparky]

If you do, you have to use a non "/" rated breaker?

That is correct.
The typical plug in style loadcenters won’t fit the bill for two pole breakers. They would be “/“ rated.

 

[winnie]

When drawing 20.8A, if my 240V is coming from both 120V legs, I end up with 4992W (120x20.8)+(120x20.8)=Approx. 5kW/hr of run time
When the same draw is getting 240V from a 120 and the wild 208, end up with 6822.4W (120x20.8)+(208x20.8)=6.8kW/hr
If I was able to use the 208V alone, the draw would jump to 24A, but I would still be at the 5kW level (208x24).

If my figures are correct (please let me know if I have anything messed up), all of my 2 pole breakers using the wild leg are adding 36% more usage.

As far as your actual metered usage is concerned, your calculations are not correct. The watts consumed by your single phase line-line loads are the same no matter which pair of phases supply them.

Also the actual power consumed by the loads is the same.

Depending on the actual configuration of the transformer bank supplying your service, the heating of the transformer coils (eg the 'kVA loading' of the transformer) might be different. If you had a single 208V 24A resistive load connected wild leg to neutral of an open delta transformer, then the VA supplied by the transformer would be 240*24 + 120*24 = 8.6 kVA even though the power consumption would be 208*24 = 5 kW.

-Jon

 

[oldsparky52]

If you had a single 208V 24A resistive load connected wild leg to neutral of an open delta transformer, then the VA supplied by the transformer would be 240*24 + 120*24 = 8.6 kVA even though the power consumption would be 208*24 = 5 kW.

-Jon

Power company provides 8.6 kVA and the customer gets charged for 5kW. The PoCo probably doesn't like this.

 

[SKalkbrenner]

As far as your actual metered usage is concerned, your calculations are not correct. The watts consumed by your single phase line-line loads are the same no matter which pair of phases supply them.

Also the actual power consumed by the loads is the same.

Depending on the actual configuration of the transformer bank supplying your service, the heating of the transformer coils (eg the 'kVA loading' of the transformer) might be different. If you had a single 208V 24A resistive load connected wild leg to neutral of an open delta transformer, then the VA supplied by the transformer would be 240*24 + 120*24 = 8.6 kVA even though the power consumption would be 208*24 = 5 kW.

-Jon

Jon - So there is no impact/difference when comparing a single phase 240V supply (2 120V's) and when I get 240V via a 120V and 208V? I confess I am simply following the basic equation of P=V x I with no in-depth knowledge of what is happening before the lines come into my building. I just figured when I take it back to incoming line feeds, the line with higher voltage would calculate higher than regular 120V lines.

If that is the case, I would see no benefit in switching out to a standard panel.

 

[winnie]

Jon - So there is no impact/difference when comparing a single phase 240V supply (2 120V's) and when I get 240V via a 120V and 208V? I confess I am simply following the basic equation of P=V x I with no in-depth knowledge of what is happening before the lines come into my building. I just figured when I take it back to incoming line feeds, the line with higher voltage would calculate higher than regular 120V lines.

If that is the case, I would see no benefit in switching out to a standard panel.

P=V*I when you are talking the voltage across and current through a given circuit element. The voltage to ground is not relevant. So you can't really say that the 'high leg' is somehow delivering more power than the 120V leg.

-Jon

 

[kwired]

Never seen one of those, all of our open deltas were using regular three phase panels or loadcenters. Looks like they were using an existing single phase panel as you said, and using regular wire for the third “buss”. Interesting!

Delta breaker was typical where all loads are single phase but you maybe had that one load (probably air conditioner) that was three phase.

NEC does not allow them anymore, hasn't for a long time. One issue is you can turn off the main breaker but that doesn't disconnect the third phase and said third phase can still back feed though the connected load and leave you with voltage on the other lines. Likely wasn't a problem with typical air conditioners as the controls would drop out if the main were opened and that would open contactor so you wouldn't have a backfeed condition anyway, but still have misleading situatiuon that the main removes all voltage in the panel

 

[hillbilly1]

Delta breaker was typical where all loads are single phase but you maybe had that one load (probably air conditioner) that was three phase.

NEC does not allow them anymore, hasn't for a long time. One issue is you can turn off the main breaker but that doesn't disconnect the third phase and said third phase can still back feed though the connected load and leave you with voltage on the other lines. Likely wasn't a problem with typical air conditioners as the controls would drop out if the main were opened and that would open contactor so you wouldn't have a backfeed condition anyway, but still have misleading situatiuon that the main removes all voltage in the panel

I remember seeing the prohibition in the code about them, but didn’t know that was what they were talking about.

 

[kwired]

Jon - So there is no impact/difference when comparing a single phase 240V supply (2 120V's) and when I get 240V via a 120V and 208V? I confess I am simply following the basic equation of P=V x I with no in-depth knowledge of what is happening before the lines come into my building. I just figured when I take it back to incoming line feeds, the line with higher voltage would calculate higher than regular 120V lines.

If that is the case, I would see no benefit in switching out to a standard panel.

For motor loads the total usage remains the same, the motor will draw more current because of the lower volts but other than pretty minor inefficiencies draws same energy. A resistance load will draw less energy at a lower voltage though. Going from 240 to 208 results in about 25% less power if the resistance remains the same.

 

[hillbilly1]

You have to keep in mind the progression of electrical installations. Think of it like stoplights: we now know what kind of traffic control is necessary for given traffic intersection types and sizes, but we didn't always have, or need stoplights for roads with 5 lanes in each direction. We learned by trial and error as traffic evolved.

Nobody today would ask for a high-leg open-delta service (nor would you get one if you asked), but they came into use as the ever-growing need and availability of larger machinery mesh as they grew. Many buildings with electrical installations already existed as new and improved utilization equipment became available.

Which came first: 3ph power or 3ph equipment? Hmmm?

Actually put one in a Church several years back, because Church’s do not use a whole lot of power, the poco didn’t want to spend money on three transformers with no payoff. They were originally putting in three phase a/c compressors, but found out they could get single phase residential units much cheaper, even the elevator was single phase, though it used a freq drive to convert back to three phase. But your right, they are getting to be very rare.

 

[kwired]

Can you find a single phase breaker rated for 208??.

For typical 120/240 miniature breaker styles I don't think you will find a single pole breaker rated more than 120. You can find double pole breakers with a straight 240 rating - they aren't cheap, and are likely a special order with most suppliers. Three pole breakers have straight 240 rating.

You could use a 277 volt rated breaker for this purpose though, but seldom will you run into a 277/480 panel on a 120/240 system. You might run into something like an I-line distrubution panel and would be able to get a single pole breaker rated at least 208 volts for that sort of panel (again likely going to be a 277 volt rated or even 600 volt rated).

 

[hornetd]

I'm having trouble following this thread because some are talking about a 2 transformer open delta supply and some are talking about a 3 transformer closed Delta supply. First let me say that I have never even seen an open Delta service nor transformer set. When I was doing a lot of rural work; which is several decades ago; I saw a fair number of Closed Delta services and transformer sets. It's easy to spot them because One of the transformers is larger than the other 2 and is the only one that has a center tap.

What I often saw in small to medium commercial premises was 2 separate service drops and Service Disconnects. Both had neutral conductors in the drop but the 3 phase one was only bonded to the service equipment equipment grounding busbar. There was no neutral busbar in those panels. Those panels were all listed for 240 volt 3 phase and nothing else. Only Delta Breakers would actually connect to the stinger phase. When I first saw one I was impressed at the power cooperative's approach to providing 3 phase power with maximum safety. The Coop separated the the two types of power at the Pole and they weren't allowed to be mixed in any way inside the building. Some 3 phase equipment had to have a transformer to provide 120 volt control power from the 120/240 phase of the 3 phase supply.

--
Tom Horne

 

[LarryFine]

First let me say that I have never even seen an open Delta service nor transformer set. When I was doing a lot of rural work; which is several decades ago; I saw a fair number of Closed Delta services and transformer sets. It's easy to spot them because One of the transformers is larger than the other 2 and is the only one that has a center tap.

Well, here's what an open delta transformer bank looks like. The single-phase section is identical to a 1ph service. In fact, the high-leg open delta began as a modification to existing 1ph services to accommodate a new single 3ph load, like A/C and refrigeration.

 

[hornetd]

Well, here's what an open delta transformer bank looks like. The single-phase section is identical to a 1ph service. In fact, the high-leg open delta began as a modification to existing 1ph services to accommodate a new single 3ph load, like A/C and refrigeration.

Thank you for taking the time to provide this!

--
Tom Horne