3 phase service

I have seen this done by using a Delta transformer with a center tap that is grounded. Each hot side of grounded winding is 120 VAC and

220 VAC between them. This can lead to all sorts of issues when hooking up 3 phase with internal 110 taps. the 3 phase is no longer ballanced around ground. Lets just say some circute breakers got a work out untill I figured it out.

Unless you have equipment with conflicting requirements, I'd plan for a 208 wye service in the shop in order to get get both three phase and

120 single phase. Regardless of what you do in the future, it sounds like you have enough wire buried to feed the house from the shop.

That would be "208 volts".

I would just have separate transformers if it was up to me.

i

And how is he going to have 220 in the house?

i

He'll have 208/120, like most apartment buildings.

Then you're paying for the xformer losses. With a 208/120 wye service the power company keeps the transformers warm on their dime.

Two legs and neutral of 120/208V Wye three phase service is generally considered to be equivalent to 120/240V single phase service for most uses. 120V only appliances will see no difference at all, 240V appliances will be minimally affected and many have dual 208/240V ratings anyway these days since apartments often have 120/208V service.

The key thing to watch out for with three phase service is the rates such as the common peak metered commercial rates where you end up paying for your highest use day, not your actual use. The four wires are correct since either way, one end is the service end with ground and neutral bonded and the other is a sub.

Thanks Pete and Ned. This is a big enough job we don't want to do it twice.

Karl

There are essentially 4 ways to wire a 3-phase 2xx Volt service. There is 240 V delta, and 208 V Wye service. 240 V delta is not what you want here. 208 Wye service will work, but if you hook the house to 2 of the 3-phase hots, you get 208 V on the appliances that expect

240. Check the air conditioners, etc. carefully for their ability to run off 208.

Then, there are two variants of the above systems, you may not be able to get these installed by your local electric company. One is corner-grounded delta, that gives you 2 240 V hots, good for running machine tool motors, the advantage is you can use normal 2-pole electrical panels and 2-pule breakers. No 120 V in that service, so not suitable for the house.

The last form is center-grounded delta. It gives you both a 3-phase service, as well as one phase that is grounded at the center-tap. So, those two hots and neutral look exactly like single-phase home service. There is a third leg to provide the 3rd phase.

IF!! you can get your electric utility to provide this service, it probably is the best choice, as you get real 240 V single phase service PLUS 3-phase with only 5 wires (3 phases, neutral and ground).

Jon

Suggest the "center tapped grounded delta" system...

SEE:

--just don't forget....use phase "B" ONLY for 3 phase loads because it has a

208 volt potential to ground /neutral.

I"d leave the house as it is and pull out the long runs for use in the shop once done.

I have single phase myself and added three phase rotary. I could have the power line deliver me 3 phase - as I have 1500 feet 42KV 2 phase and the house and shop tap off only one of the two legs. I'm a side branch from the state highway - delivering power to my site - once a sawmill. Mill is long gone, but the power allows all sort of possible future expansions or changes.

Mart> "The Kid" is putting in underground wiring to his outbuildings. For

That last one is also known as "wild leg" delta service since the third phase is not at 120V relative to the ground/neutral, and that third phase is normally color coded orange to identify it. It wouldn't be a problem for the house, since that wild leg wouldn't be fed to the house, but it has been known to cause confusion and blown up 120V stuff in many installations. Another issue with this type of service is that some three phase machines may want 120/208V Wye service with it's neutral to phase voltage at 120V for all phases.

Generally, if you ask the utility for three phase service, you will be getting 120/208V Wye service unless you specifically ask for something different. Expect a lot of questions if you ask for three phase service and a lot more if you ask for anything but the common commercial

120/208V Wye service.

As noted, most appliances these days are 208/240V rated since 120/208V service is very common in apartment buildings. Check everything to be sure, but you're not likely to find much that isn't ok with 208V.

The 240 volt systems were designed to save the power company from installing an extra pole transformer for 240 volt single phase service.

With 208 single phase two pole transformers were needed, and if three phase were required you needed to install three transformers to obtain the star or wye configuration. By using the center-tapped transformer neutral transformer it eliminated a second transformer. If three phase was required it was necessary to add only one more transformer rather than two, except when higher power rating were required and a third transformer is used. That was the difference between open leg delta and a full delta secondary. A lot of people are confused by seeing only two high voltage primary wires and getting fed with three phase into their building but that is very common.

John

Guys, thanks for the discussion on types of three phase. Bit more to this than I thought. he's talked with the power company and will get FAR lower install cost if he provides and maintains the transformer. We're, of course, looking for a used one. With all the different primary voltages out there and now all the flavors of three phase, this looks like a tuff task.

Karl

Karl,

Search for pole transformers. If its only the installation cost that is different I would go for the extra buck and let the power company own the transformer. If you happen to have one blow from lightning or some other problem it will be a lot faster if they come and replace it rather than you searching for a replacement. Also wait till you check on some of the prices of those pole pigs. Unless you are going to draw a lot of power you can run open delta with one 240 center tapped grounded and one

240 single phase unit. You only need two legs of the delta to get the three phase but you get a little less regulation and can only pull the wattage of the two transformers. Most smaller shops on three phase only get two pigs on the pole.

John

He's probably looking for a pad mount transformer, not a pole pig.

Pad mount are more involved than hanging one on the pole. Security and limiting the access involve a secure fencing system or vault or some other secure enclosure. Someone will try to steal the copper and run up the electric demand. :)

John

We're talking on down the road here, but I'm pretty sure its got to be pad mount. They said $5-7.5K just for the transformer, so buying his own looks like a real option. I told him to buy two, one on site spare. Bet they go for a song IF you can find the right one.

I'm giving the kid a Mazak M4 lathe and a Matsuura MC-1000V twin spindle bedmill when he's ready. These machines both need serious three phase.

P.S. I'm testing the D.C. power supply for the mill this afternoon. Been debugging this panel the last day or so.

Karl

Um no..

These things are everywhere with no additional security.