3 phase 240VAC to 120 VAC single phase?

Jack, Maybe, it's me and I've only had coffee #2 this morning ... but your question reads as if you want to somehow meter 3-phase service and single-phase service for both areas from your location. It would seem the logical solution would be to have the leased area billing put in your tenant's name. Otherwise, combining single-phase and 3-phase service in the same panel would involve the use of transformer(s). This type of question would be much better put to your electric utility. Bob Swinney

What you have is corner grounded delta. What you're calling "neutral" isn't actually neutral. It is one of the 3 hot legs, which happens to be tied to ground. (Doesn't matter to the 3 ph, voltages to ground for a 3 ph delta are abritrary anyway, zero is as good a value as any.) This is a common way to wire industrial power. You can get 3 ph 240, or single phase 240 from this system. But you can't get 120 without another transformer.

The only way you're going to get 120 out of this is with a stepdown isolation transformer with its primary across one pair of legs, and its secondary feeding a separate 120 volt panel. Pick one of the secondary leads, call it neutral, and ground it in the 120 volt panel. Call the other lead hot, and feed it to your 120 volt breakers.

You probably should check with your power company to see if they'll change your service over to 4 wire delta with one phase centertapped. That way you can get 120 between neutral (actual neutral now) and either one of two of the three hot legs. The third leg, called the high or wild leg in this configuration, will read more than 240 to ground. Again this doesn't matter to the 3 ph, but you don't want to use it with the 1 ph wiring. This type of service is commonly used for small office buildings where the loads are primarily 1 ph, but with some 3 ph required for air conditioning, elevators, etc.

They'll probably want to charge you to make this change since it'll require setting a new transformer and changing your entrance drop (you'll also have to change your entrance panel). But you're going to have to buy a transformer anyway, and a separate 120 volt panel, so at least you'll see which would be cheaper. (I'm betting the latter in most cases.)

Another option would be to order a separate 240 single phase drop. That would be the ordinary residential type setup using

3 wires, hot, neutral, and hot. You could then install a panel for this and wire it like a house, giving single phase 240 between the hots, or single phase 120 from either hot to neutral. The big advantage of this over the other setup is you minimize the surges you'll get on the 120 volt circuits due to large 3 ph loads being switched on or off.

Note you probably don't want to change to 4 wire wye service. That would give you 120 volts to neutral from any leg, but the leg to leg 3 ph voltage would drop to 208. That would require rewiring all your 3 ph motors for 208 instead of 240. This was found a lot in older installations, but most power companies don't want to provide this any more.

In new work, the power company is going to want to supply you with 480 wye. That gives you 480 3 ph, and 277 1 ph (for lighting) from any leg to neutral. Now you need to supply stepdown transformers to get 240 3 ph, or 240 1 ph, or 120 1 ph. The advantage of this in a large building is that you can use 480 wiring to each subpanel, and site stepdown transformers at each subpanel to give you the 240 or 120 volts your applications actually need. This can save a bundle on distribution wiring in a large building, easily offsetting the cost of the extra transformers. But it is a pain in the ass for a smaller building.

Obviously, whichever way you choose (and whichever way your power company will let you choose), you're going to need the services of an industrial electrician to wire it all up. He, and the building inspector, are going to be the final authorities on what you can do in your jurisdiction.

Gary

Gary Coffman wrote: The third leg, called the high

Not so. The high leg or wild leg, also called the power leg, will be 208 volts to ground. I have some pictures of transformer connections on a web page. Scroll to the bottom of the page to see them.

Not so, again. This is still widely in use. At the power company I work for we install a lot of these, many in strip malls, supermarkets and small office buildings. Anyplace that has a large 120 load, and need some 3-phase for refrigeration and air conditioning.

Don

Not so.

Semantics.

The "third leg", often called the "high leg" or "red leg" really should be called the "orange leg" as all new work is required to indicate this leg with orange wire (small gauges) or orange tape (large gauges).

No single-phase loads are permitted on such a leg.

You're right, sqrt(240^2 - 120^2) = 207.846 volts.

Nope, you took that one out of a different context. I'm talking about 208 delta there. Your power company may still support it for new work, but none of the ones I know will (though they continue to support it in old work). They want to supply 480 wye instead. It saves them money via smaller cheaper transformers. In a stretched out facility it saves you money too, in reduced copper costs.

Gary

I'm talking about 208 delta there. Your power company may still support it for new work, but none of the ones I know will (though they continue to support it in old work). They want to supply 480 wye instead.

120/208Y is still the bulk of the new Wye installations, far outpacing 277/480Y.

Installations usually follow this sequence (as a function of intended usage and total premises volume):

1) 120/240 center-tap grounded Delta (mixed use, small to medium volume),

2) 120/208 Wye (mostly power, small to medium volume), and

3) 277/480 Wye (industrial, large volume).

In (3), a customer-owned "dry type" transformer or transformers is (are) required to provide 120 single-phase, 120/240 single phase or 120/208 Wye and/or 240 Delta three-phase.

Gary,

I don't think I took anything out of context, I am going to cut and paste the paragraph from your original post so you can see why I thought you were talking about

208 wye.

"Note you probably don't want to change to 4 wire wye service. That would give you 120 volts to neutral from any leg, but the leg to leg 3 ph voltage would drop to 208. That would require rewiring all your 3 ph motors for 208 instead of 240. This was found a lot in older installations, but most power companies don't want to provide this any more."

I've never heard of a 208 delta, other than the high leg on a 240 bank.

The determining factor in whether you will get a wye or delta secondary is the voltage of the service. You can see the wye secondary transformer connections on my web page have the secondary coils paralleled. What determines whether the high side is delta or wye is the actual primary voltage of the line and the nameplate rating of the transformer. Where I work, we have a lot of 12KV primary and a 20.8KV primary that we use the same transformers on. The company stocks a lot of 12KV 120/240 transformers. So if you are going to hang a bank to serve a 208V 3-phase service in the 20.8KV you pop the lid off the transformers, parallel the secondary coils, and hang a wye-wye bank. If you wanted a 240V 3-phase bank in the 20.8KV, it would be a wye-delta. If you are hanging these 12KV transformers in the 12KV primary, it would be a delta high-side. And again the secondary configuration would be determined by the secondary voltage you want, 120-208V requires you to parallel the secondary coils and wye the secondary side. 120-240V would be delta secondary.

The 20.8KV system is a common neutral system. That is the neutral carried in the secondary position is shared by the primary and secondary, and is a metallic return to the substation.

Don

Geeze. Our entire plant is set up for 208 star. Because most of the loads in the labs are 120.

But then the building is supplied with two 13.8 kV feeders. I think we do our own on-site transformations from there on down.

Jim

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But then the building is supplied with two 13.8 kV feeders. I think we do our own on-site transformations from there on down.

Most likely.

While at my former employer we NEVER allowed one of our customers to connect to our transmission systems (115 kV ac through 500 kW ac, and 800/1,000 kV dc), we DID allow our large industrial customers/municipal partners to connect to our subtransmission systems (34.5 kV, most, and 69 kV, a few).

Note that MOST (but not all) system voltages are multiples of 115 ...

13.8 kV = 120 X

34.5 kV = 300 X

69 kV = 600 X

115 kV = 1,000 X

138 kV = 1,200 X

230 kV = 2,000 X

287.5 kV = 2,500 X (Hoover to L.A., Circuits 1 and 2, e.g., in L.A.'s system)

345 kV = 3,000 X (not used in L.A.'s system, but used within the "Western System", APS, e.g.)

But ...

500 kV / 115 = 4347.8261 (Hoover to L.A., Circuit 3, e.g. and the "Western System" ac Intertie, e.g.), and

765 kV / 115 = 6652.1739 ("Eastern System" ac Intertie, e.g.).

Oh, well.

I misspoke (miswrote), I meant 208 wye.

Gary

Well. I figured you meant star. It's the only thing that makes any sense there.

Jim

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I don't normally put photo captions on that page, as the pictures don't usually stay up very long. I just post them for a specific discussion, or to show a friend. I can't speak to the logic of it, they were designed before my time. I've only been in the business 31 years, some of that stuff is nearly a hundred years old.

That's all correct. Some other working rules on an arc circuit are you never work it hot, but you always work it like it's hot, as it can be wrapped in the 4KV and still be working. You never ground it, and you never open it. You put a shunt across a light when you are working on it.

That sounds interesting, I'd like to see pictures of it.

Don

been in the

Looks like they use the same type system for the series-circuit HPS streetlights in the Valley, with the big pole-mount RO transformers... I've seen the whole string cycle before, probably from one lamp that's at end-of-life and blows out, and knocks the whole string down...

All in all, I'll go for regular 120/240/277V feed Cobra-heads, thank you. Easy to troubleshoot and repair, or swap out. And you don't dump the whole circuit because of one bad lamp.

Orange Empire Railway Museum, Perris CA.

I'll have to see if I have pictures - someone should film it doing a startup/shutdown cycle for historical purposes. Open contactors and arc chutes, brush lifter motors...

The best part is the little worm-and-ball 'rotor wiggler' that moves the rotor in the sleeve bearings to keep the brushes from taking a set on the commutator, as it coasts down you can hear the ball coming off one end and going 'plink' as it goes back to the other end of the worm for another cycle...

And it should be good for another 100 years, as the GE Shops completely rewound and rebuilt the rotary converter with modern insulation a few years ago - there's a good story behind that one, it was supposed to go in for a dip-and-bake and minor stator coil work (with a fixed bid), and they kept digging...

It was the technician's last hurrah before he retired. :-P

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