3 Phase service question, need some expert advice

[Joe Jensen]

I am building a new shop and the local utility has two options for 3 phase service.


1) The utility can put 2 transformers in the Southeast corner of the property and then feed back to the meter. The meter has to be in the front part of the property where the utility can walk up to it. Probably on the fence wall. It would then feed back to the new workshop the feed from the meter to the shop would be about 75 feet. The voltage would be 120/240 volts- 3 phase 4- wire Delta. We can get 400 or 600 amps. The owner needs to make sure his equipment can run on 240 volts ( not the same as 208 volts). SRP has a limit of 43 kw of 3 phase power equipment that can be used at one time. The 30hp belt sander and the 7.5 HP dust collector put us at that limit.

2) The other option is to feed from the Northwest corner of the property, however this would require an 12’ wide easement all the wall along the East property line from the front to the back. We could get 120/208 volt 3 phase – 4 wire with this option. The metering location requirements would be the same as above.

I will be buying some new larger 3 phase machines, like a 10 HP planer, and a wide belt sander. Does it matter if I have 208 or 240? Which is more common?

[Jim Becker]

Joe, I find it strange that they need physical access to the meter these days...most utilities read them electronically from receivers on the poles that transmit the data back to the billing department. Our utility (PECO/Exelon) replaced pretty much every meter "everywhere" and there's been nobody doing a physical read for a couple years now.

That said, it sounds like your choice between the options is going to be dictated by what your new machinery requires/can handle. That's where I'd be starting if you've identified what you're going to buy.

[Joe Jensen]

Some more info. Ideally we run only one service to the shop. It's my understanding that if I get a true 208V three phase service I would not be able to run 230V single phase tools. Is this true?

[Joe Jensen]

the challenge is this will be my retirement shop and I intend to fill it with tools I buy on auction, and if my investments do better maybe some new Martin machines.

[Paul A. Hopkins]

I would go with the 208volt system the 240volt delta is a high leg system meaning that you will have 120volts to ground from 2 phases and 208volts to ground from the third phase which Ive seen that voltage fluctuate so you have to be careful. According to the code the High Leg is to be on B phase, colored orange or tagged. It somewhat limits where you can install 120volt circuits in your panel and if you have a lot of them it can make the electrical install a real pain. I don't believe you'll have any problems running 230volt single phase tools off the 208, they do make buck/boost transformers if you have a machine that your concerned about.

[Darcy Warner]

Can you get 480/277y service instead? It makes everything so much easier. I have 480 service, transformer to get 240v 3phase, transformer from 480 to my 240/120 single phase. I hate boosting voltage if I don't have to.

[Jim Becker]

I don't believe you'll have any problems running 230volt single phase tools off the 208, they do make buck/boost transformers if you have a machine that your concerned about.

I had to put one of these transformers on my MM S315WS a few years ago as the 240v utility power (single phase) was fluctuating enough to shut the machine down. So I know they work and are very easy to setup if you need to customize the voltage for a particular machine.

[Bill Space]

I would go with the 208volt system the 240volt delta is a high leg system meaning that you will have 120volts to ground from 2 phases and 208volts to ground from the third phase which Ive seen that voltage fluctuate so you have to be careful. According to the code the High Leg is to be on B phase, colored orange or tagged. It somewhat limits where you can install 120volt circuits in your panel and if you have a lot of them it can make the electrical install a real pain. I don't believe you'll have any problems running 230volt single phase tools off the 208, they do make buck/boost transformers if you have a machine that your concerned about.

Paul,

Please explain this further, as a three phase motor supplied by a 240 volt ac, three phase, delta source does not care what a given phase voltage to ground may be.

There is of course a virtual ground that floats somewhere between the three phases, that is a function of whatever the leakage resistances may be between the phases and ground. But the motor does not see this.

I do not think what you stated is correct. I know it is NOT correct in an industrial environment.

Please add some specifics.

[Paul A. Hopkins]

It doesn't effect a motor at all either 3 phase or single phase 240volt I didn't mean to make it sound like it did, what it does effect is 120volt circuits as you can not use the high leg phase for a 120volt circuit, if you have a lot of 120volt circuits its easy to run out of space in the panel and end up with empty spaces that you can't use. The system works fine as long as no one accidentally connects a 120volt circuit to the high leg phase and that is correct with this system even in an industrial environment.

[Joe Jensen]

I was assuming I'd have a 3 phase panel and a single phase panel. Is that not how it's done?

[Darcy Warner]

I was assuming I'd have a 3 phase panel and a single phase panel. Is that not how it's done?

You can if you install a sub panel off the main just for single phase. That's what I would do, no confusion then.

[John Lanciani]

I was assuming I'd have a 3 phase panel and a single phase panel. Is that not how it's done?

No, you’ll have a 3 phase panel with 1,2,or 3 pole breakers as needed. As for the service, my preference would be 208/120 volt four wire Y. The only reason the utility wants to give you 240/120 Delta is that they can get away with only using 2 pole mounted transformers in a broken delta configuration. It’s less than an ideal setup, most forward thinking utilities won’t even install that configuration any more.

[Frank Walker]

No, you’ll have a 3 phase panel with 1,2,or 3 pole breakers as needed. As for the service, my preference would be 208/120 volt four wire Y. The only reason the utility wants to give you 240/120 Delta is that they can get away with only using 2 pole mounted transformers in a broken delta configuration. It’s less than an ideal setup, most forward thinking utilities won’t even install that configuration any more.

This is essentially correct although "open delta systems" are quite common where there is typically a single load exceeding 5 HP (think small commercial set up with 10 ton AC). When I retired probably 25% of the systems I was designing were open delta. The center tap of one transformer is grounded so there is nothing floating but the stinger leg will be 208 volts to neutral/ ground. Both of the other phases will typically be 120 volts to ground and the line to line voltages are 240 volts vice 208 volts on a 120/208 3 phase Y system. From a pure electrical perspective the Y system will provide the more stable/ suitable service and would be my choice starting from scratch. As to motors operating on 208 or 240 volts it depends on the motor/ machine. Some will accept both operating conditions some will not.

[John Lanciani]

This is essentially correct although "open delta systems" are quite common where there is typically a single load exceeding 5 HP (think small commercial set up with 10 ton AC). When I retired probably 25% of the systems I was designing were open delta. The center tap of one transformer is grounded so there is nothing floating but the stinger leg will be 208 volts to neutral/ ground. Both of the other phases will typically be 120 volts to ground and the line to line voltages are 240 volts vice 208 volts on a 120/208 3 phase Y system. From a pure electrical perspective the Y system will provide the more stable/ suitable service and would be my choice starting from scratch. As to motors operating on 208 or 240 volts it depends on the motor/ machine. Some will accept both operating conditions some will not.

The utility I work for (27 years) hasn’t installed open delta services in at least that long. We even took straight delta services out of our new service options about 15 years ago. Protective devices respond much more predictably on grounded wye installations.

[Joe Jensen]

No, you’ll have a 3 phase panel with 1,2,or 3 pole breakers as needed. As for the service, my preference would be 208/120 volt four wire Y. The only reason the utility wants to give you 240/120 Delta is that they can get away with only using 2 pole mounted transformers in a broken delta configuration. It’s less than an ideal setup, most forward thinking utilities won’t even install that configuration any more.

I have an option to get either. The 208 will be more expensive as they have to run the wire another 500 feet. All the WW machines I looked at on Exfactory, as well as the Martin machines, have 240V motors. If I have a 208 service will I need to add a transformer for 240V motors?