240V single phase condensers on delta 3 phase

[Kilivolt]

I recently had another electrical contractor contact me about a problem he was having with an HVAC contractor. The HVAC contractor had all but called him an idiot, and he called me to get a second opinion before he argued with him further. The job involved hooking up condensing units that were only 240V single phase units, but the service they were being connected to was a 3 phase Delta service with the 208V wild leg. The units are 240V only, with no 120V connection at all. According to the HVAC contractor the units would not run if one of the 2 lines of the condensing unit was tied to the wild leg (even though the voltage at the unit would still just be 240V), and advised the building owner that he would not warranty the units if problems would arise if they were hooked up that way. The only problem we could come up with was that if something was to go to ground in the compressor it would put the higher voltage to ground if that leg was the wild leg, but at that point it wouldn't really matter since if something had gone to ground inside the compressor it would probably be shot at that point anyway.

An engineer from the power company also met with him and the building owner and could not see any reason why the compressors would not run correctly when one leg was tied to the wild leg.

We tried to find any code references prohibiting the use of the wild leg on condensing units, but were unable to. To us it made more sense and helped balance the load by using all 3 connection possibilities for all of the units.

 

[augie47]

As long as they are 240v I see no reason you cant use the "high-Leg" as one of the phases.
(side note: You have to use "straight rated" and not "slash rated" breakers when connecting to the high-letg per 240.85)

 

[ActionDave]

I agree with augie.

Line to line voltage on a 240V Delta system is 240V. The only time you need to stay away from the high leg is when there are line to neutral loads.

 

[Ingenieur]

Typically the installer does not warrant the equipment
the mfg does
have him show you (and the owner) where it is prohibited in the install manual

should not make a difference
can't say for sure without a schematic
perhaps affects the start/run caps?

 

[Galt]

Where would he get such an idea? I have done this many times.

 

[Jraef]

It's a common misconception. That 208V line to neutral measurement freaks some people out. It's completely irrelevant for 240V loads. COMPLETELY.

That H-HAK guy has a lot of gall telling an electrician he doesn't know what he's doing. Some of the biggest electrical blunders I've ever witnessed were perpetrated by those guys biting off more than they can chew.

 

[iwire]

Typically the installer does not warrant the equipment
the mfg does
have him show you (and the owner) where it is prohibited in the install manual

True, but at the same time if I don't install it correctly and the manufacturer won't warranty it my boss is going to be paying.

 

[Ingenieur]

True, but at the same time if I don't install it correctly and the manufacturer won't warranty it my boss is going to be paying.

hence the comment, show me in writing
the install instructions usually have detailed electrical requirements (and prohibitions)

 

[Coppersmith]

I need a little education. Although I have wired transformers when working for other contractors, I've never had to purchase one myself. This thread got me curious about how to purchase one and what they cost. The supply house listings have a bunch of 480 delta - 240 delta/120 transformers but never mention a wild leg. Here's an example:

https://www.platt.com/platt-electri...20/Eaton/V48M22T15EE/product.aspx?zpid=684123

Am I correct in assuming that since it says it has a 120 volt lighting tap, that I can get 208 volts on one of the legs (B phase)?

 

[kwired]

I need a little education. Although I have wired transformers when working for other contractors, I've never had to purchase one myself. This thread got me curious about how to purchase one and what they cost. The supply house listings have a bunch of 480 delta - 240 delta/120 transformers but never mention a wild leg. Here's an example:

https://www.platt.com/platt-electri...20/Eaton/V48M22T15EE/product.aspx?zpid=684123

Am I correct in assuming that since it says it has a 120 volt lighting tap, that I can get 208 volts on one of the legs (B phase)?

Physically impossible from a basic delta secondary where the midpoint of one side of the delta is utilized as a "neutral" to not have a "high leg" on the opposite corner of the delta. Don't matter what the secondary voltage is either, it will be 1.73 times the line to neutral voltage of the other two corners.

If you want same voltage to neutral on all three "phases" straight from basic windings, you must use a wye connected system.

 

[kwired]

Only reason to not connect this AC to a high leg conductor would be if the appliance instructions say the voltage to ground must not exceed 120 volts nominal. Straight across the line controlled units are not very likely to have that instruction. Variable speed compressor units, may have that instruction not because the compressor motor can't handle it but maybe the controller can't handle it, and most likely because of integrated surge protection devices that are designed for 120 to ground nominal voltage. Put 208 to ground on one of them and it will burn in short time out because it is continuously trying to clamp what it sees as excess voltage to ground.

 

[Coppersmith]

Thanks. I assumed the 120 volt lighting tap meant a grounded midpoint on one phase, but wanted to confirm. Wish I already knew everything about this trade, but I don't.

 

[domnic]

delta 3 phase

delta 3 phase

Check voltage with load on it. it could bad connection at weather head or the pole.

 

[euboss]

Euboss

Euboss

I often see single phase 240 volt loads connected to an open delta connected high leg 120/240 volt system. It works fine. I have had a discussion with an engineer from Florida Power and light about this. He says FPL does not recommend putting any single phase loads on the high leg for it is intended for 3 phase loads only. Their second pot is sized smaller than the pot producing the 120/240 volt loads. The system is intended to save the power company money on the initial install because they can use 2 instead of three transformer pots. Most users for this system has only a nominal amount of three phase loads. Other wise they would elect to use a 120/208 volt wye system. I also have discovered that most modern single phase equipment that is rated for 240 volts will operate on 208 volt single phase.
Just a footnote. I knew a A/C Contractor that would connect his unit on the high leg to a single pole breaker and connect to the neutral to save a few dollars on a circuit breaker. Not very wise. I agree some of the A/C Techs should not be doing electrical work.

 

[Kilivolt]

Thanks for all of the input. I can't say I've ever seen anyone put a 208V load on a single pole breaker like Euboss mentioned, but I bet around here it's just because they haven't realized that will work. I hadn't thought of the surge suppressor idea of Kwired either, but from knowing the quality of equipment that this HVAC contractor uses I don't imagine there is anything like that on his units. I think he was just trying to find a way to get out of having to warranty his equipment if it fails for other reasons in the future.

 

[kwired]

I often see single phase 240 volt loads connected to an open delta connected high leg 120/240 volt system. It works fine. I have had a discussion with an engineer from Florida Power and light about this. He says FPL does not recommend putting any single phase loads on the high leg for it is intended for 3 phase loads only. Their second pot is sized smaller than the pot producing the 120/240 volt loads. The system is intended to save the power company money on the initial install because they can use 2 instead of three transformer pots. Most users for this system has only a nominal amount of three phase loads. Other wise they would elect to use a 120/208 volt wye system. I also have discovered that most modern single phase equipment that is rated for 240 volts will operate on 208 volt single phase.
Just a footnote. I knew a A/C Contractor that would connect his unit on the high leg to a single pole breaker and connect to the neutral to save a few dollars on a circuit breaker. Not very wise. I agree some of the A/C Techs should not be doing electrical work.

Not all high leg systems are open delta derived. If there is three transformers the same size there is usually intended to be more 240 volt load whether it be three phase or single phase. You should still try to somewhat balance the load across all three phases in that situation. I do frequently run into full delta on sites that have a lot of motor load or maybe even a fair amount of resistance heating load, and not so much 120 volt load.

If they would use the proper breaker when connecting a high leg to neutral or even high leg to one other leg - they won't save at all on breakers as you need one rated at least 208 volts and not a 120/240 volt rated breaker - there is little demand for such breakers in the typical "miniature breakers" like QO, CH, QP THQ... and they are expensive compared to the 120/240 rated breakers. Three pole breakers are rated straight 240 so you don't have to watch for that with those.

 

[kwired]

I hadn't thought of the surge suppressor idea of Kwired either, but from knowing the quality of equipment that this HVAC contractor uses I don't imagine there is anything like that on his units.

As I said, probably not an issue at all with most any unit that has a contactor and starts across the line, it would be any unit that has a variable speed compressor that may have such a surge protector built into the controller.