# Getting 120v Single Phase from 3 Phase

• posted

Can you get 120v single phase by picking off one line of 240v 3 phase as long as you have a neutral to carry the current back to the box? Thanks, Steve

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Not quite. The voltage won't be right for pure three phase. I think that with Wye connections, you will have something closer to 104V.

And many are delta which *has* no neutral. All may be floating.

But a frequent variation has one of the three sides center tapped (the way the standard residential feed is supplied, 240V center tapped with the center tap grounded and neutral connected to that.)

The breaker boxes for this have three buses, but only two of every three positions can be used for 120V single-phase breakers. The third phase is *way* too high.

However -- from *any* reasonable 240V 3 phase, you can tie two of the three phases to the primary of a 240-120V step-down transformer, and power your equipment from that.

Enjoy, DoN.

• posted

Yes, but be careful if you have 3 phase delta.. The high leg to ground (B phase if I'm not mistaken) yields 208 volts, not 120.

Harold

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I'm at a loss to understand that, DoN. Care to elaborate? I have wired three places with delta service, two of which used either the A and C phase and the neutral for 120V. All of it was done to code. The third place has a single phase panel along with the 3 phase, both of which are fed from the same taps from the transformers.

In this case, he's already suggested that there would be a neutral, so it would be a 5 wire system.

As stated above, I got around that problem in my current shop by having two panels, one strictly 3 phase, so none of the positions are lost.

Harold

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Typo? Should be about 140 volts from line to neutral with 240 wye (calculates to 138.6 or so)

208 wye gives 120 line to neutral...

center

transformer,

938-4564

--- Black Holes are where God is dividing by zero ---

• posted

Only in a special circumstance, which is a 4-wire, center-tap grounded delta service. This has one 240 V L-L circuit with a center tap, just like everybody's home service. Then, it has another transformer that develops the 3rd phase. In this system, there are only TWO of the three hot wires that will give 120 V to neutral. (The third will give ~207 V, so you will definitely be able to tell which is which with a meter.)

This service is pretty rare, at least around where I've lived. Corner-grounded delta is more common, but you can't get 120 V directly from that. You need a 240 - 120 step-down transformer. Corner-grounded delta is most obvious because 2-pole circuit breakers and disconnects are used. The 3-phase wires are hot, hot and neutral, and you can wire a 3-phase motor up to those 3 wires. (These are also labeled hot (A), neutral (B), and hot (C) phases, and therefore sometimes called grounded B phase.)

If you have 208 V WYE service (sometimes called star) you have three

120 V circuits, from any line to neutral. But, in this system, you can't get 240 V, without a transformer. Like, the building I work in, has 208 V Wye for the office section, and we have little autotransformers to step 208 up to 240 for the window air conditioners.

If you try this on a true 240 V Wye system, which has a neutral, you will get a rather high voltage of about 138 V. But, 240 V Wye is pretty rare. If you try this on a real delta system, you might kill all the lights in the building, as it may trip the ground fault protection. But, then, a true delta system doesn't have a NEUTRAL, although sometimes telling the difference between a neutral and a safety ground can be difficult. Delta transformers have a balancing transformer in them that makes it look like they are referenced to a neutral, but if you draw any current from line to neutral that unbalances it, the transformer should shut down. This normally wouldn't apply to open delta and corner or center-tap grounded deltas, as they are expected to feed unbalanced loads.

Is that more than you wanted to know?

Jon

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Yes, but depending on your load requirement, get an office building electrician to wire it. The entire USA gets 120 volt single phase from 3 phase sources.

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If it had a neutral it wasn't a delta service.

The 104V mentioned was a typo, it's really 138V and change. Square root of three thing for three phase power. 240V Wye service will give you

138V phase to neutral and 208V Wye service will give you 120V phase to neutral.

Known as Wye.

I believe this is often referred to as the "wild" leg.

There are / were a lot of strange variations on three phase power, but most anything new is going to be 208V Wye service. Larger industrial stuff will get 480V.

Pete C.

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BW sez:

"...The entire USA gets 120 volt single phase from 3 phase sources."

Cute - but not quite right. See John's answer above.

Bob Swinney

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Wrong!! One can have a three, four or five wire delta system.

I have a 5 wire system, and it *is* a delta system. It is not a wye, which does not have the wild leg. Mine does have. Ground is established by tapping the center of one coil, which results in the longer path to ground from the other two coils. 208 volts from phase to ground. It's not conjecture, it's measured.

Again, wrong. It *is* a delta system. He's talking about 240 volts, not

208. As far as I know, single phase service to the typical house is just one leg of a three phase delta system. Isn't that how it comes from the power plants, the primary service? How it's delivered to the customer depends on the transformers that feed them.

Agreed.

My 3 phase delta 240/120 volt service was installed just 4 years ago, at my request. I did not want a wye service (for obvious reasons), and am transforming to 480V for one machine.

Harold .

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Nope -- trying to remember which way the voltage was off, and getting it wrong. :-)

Thanks, DoN.

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Nuttin' personal, but I'm always amused by the perceptions (or just commonly used expressions) that power or voltage comes out one terminal and goes back to the other one.

I usually have nothing better to do than to ask.. then where does it go?

In the context of power generation and distribution, does it go back to the generation source? What do they do with all the power they receive back?

In a battery context, does the battery determine that the user has gotten their money's worth, and stop giving the user more voltage? Why did it go dead if there is power coming back into the other terminal? Does it become saturated wih negativity? Is the battery recycling program about using the unused power in dead batteries to make new batteries?

Can folks visualize that the power is expended at the load? Transformed into light, mechanical energy, heat, and simply dissipated at the electrical load. Maybe it's been transferred into food to make it hot?

My food is too hot, can I return some of this wasted energy to the power company (and get a credit?).

Maybe visualization is too complex, but generally, there is always heat present and that can be felt or measured.

Ground is always zero, right? Depends on where you're coming from, pal. So I guess you're saying ground is never zero? Definite maybe.

What's neutral? Switzerland, I think. Again, depends on where you're coming from.

Prototypes of the generator powered by free radon should be available in the first quarter of 2007.

Be sure to check to see if you have radon. (which is the only useful info in this post).

WB .................

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It is quite simple! :) Since it is coming in to the OTHER terminal, it is coming back a "UNPOWER". As the battery fills up with unpower, pretty soon it balances out the power that is left. Then pffft nothing. But un power is a little more volatile, and so it evaporates just a little faster. This is why if you let the battery sit for a little while, you can get just a little more oomps out of it for a short time. :)

jk

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Huh? You always (unless you've got a 100yr old system) have a ground for your building system, so once you reach your building distribution you have a minimum of four wires for a strictly delta system, A, B & C phases and ground. A Wye system will have five, A, B & C phases, neutral and ground. The "Wild leg" delta system would also have five wires.

Your "wild leg" delta system is not very popular these days since most three phase services are provided by three phase pad mount transformers so the potential cost savings from using two smaller and one larger single phase transformers to service the load doesn't exist. The "wild leg" is also a potential safety issue to people and equipment which is why the NEC requires the orange color coding and placement in the middle position of the panelboard.

Ground is not established by the center tap on one transformer, that is a neutral of sorts. Ground is always established by the ground rod(s) for your service. The ground and the neutral are always bonded together at the service entrance panel and never at any sub panels.

Ok, the "wild leg" configuration is technically a delta configuration. It is however more often referred to with various derogatory terms due to it's disadvantages.

While older distribution was often fed in a delta configuration, that is being phased out for safety reasons.

When the distribution transformers are fed in a delta configuration, in the event that there is a circuit loss on one of the phases feeding the transformer due to a cable break of a fuse blow, the line remains hot due to power feeding from the other phase through the delta wired transformer(s) and back down the "disconnected" phase.

A Wye connected transformer does not present this risk since only one leg of the transformer winding is ties to a hot line with the other at neutral / ground potential.

Huh? What "obvious reasons"? 120/208 Wye service has no disadvantages that I know of. You can get 120v from any of the phases, allowing you to balance your single phase loads and single pole, two pole or three pole breakers breakers can occupy any panel position since all phases are equal to each other and to the neutral.

I consider there to be considerable disadvantages to a system with a "wild" leg. You have no way to even come close to balancing your single phase loads on the three phase feed and you have the "wild" leg which can cause safety issues and/or equipment damage if people are not paying attention. From what I recall this configuration was primarily used to cut transformer costs when serviced from three single phase transformers and not for any technical advantage.

Pete C.

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I always assumed that it was done for convenience. I have 3 hot wires (A, B & C(wild)), I can run all my 120 stuff off A to neutral or B to neutral. I can run all of my 240 single phase stuff off A-B and I can run all of my 240 3-phase stuff off A-B-C.

If I had Wye or corner grounded delta service then I would have to give up one of those conditions or purchase another transformer.

By the way: center tapped delta service is very common here in the Chicago area. Wye service is strictly used in office buildings and warehouses.

As a side note: I have a suspicion that Commonwealth Edison balances the overall service in an industrial park by locally grounding the center of alternate coils. IOW: that wire that comes into my building and that I call the wild leg, is not the same as my neighbor's wild leg.

George.

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Anything that uses 220 only, ( 220 with two conductors and a ground) can be run off of any two legs. Only if the thing uses 120 and 240 volts and has a neutral wire going to it do you need to use the centertapped phases.

Nope

It is good for the electrical company when there is a mixed use of single and light three phase users. They save a transformer when three phase is required.

All utility companies alternate on the hookup of the primary to the three phase high voltage lines, the ones on the extreme top of the pole with the big insulators.

you neighbor may be on a different pole transformer. IF he is then his phasing is different than yours.

John

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One thing that is common in warehouses here is 208 Wye service. That gives 3, 120 Volt legs. Obviously, I can run a 240 3-phase motor on that service but it will only run at 208/240)^2 = 75% power. Worse yet, many electronic items just simply will not run.

Alternatively, I can get 240 Wye service and run my 240 stuff at full power. As you mentioned, you can always run 240 single phase stuff across any two legs. (I can also use my 208V light bulbs from the previous example) But how do I get my 110V toaster oven to work in that arrangement?

Same question for a corner grounded delta arrangement?

Yea, I figured as much.

We had an electrical storm here several years ago. Half the light bulbs in the building were getting dim while the other half were getting unusually bright. Then some of the bright bulbs began to blow. I figured that we must have lost the center ground and that the voltage was being split somewhere in the middle of the center tapped leg. No doubt based on the relative load on either side.

I ran round the building frantically shutting everything off as light bulbs popped all around me. I then called the electric company and explained the situation. I must hand it to them, they arrived in truly record time!

But I noticed that my neighbors lights were also bright in one window and dim in the next. I was just sort of pondering the fun that it would be to ground out one side of the transformer and then ground out the other side.

Com Ed arrived before I worked up enough nerve to put theory to practice.

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No, it runs at full power. Motors however draw more current and run hotter. The typical approach (inexpensive) is to simply put a buck/boost transformer at each item that really requires 240 volts.

Jim

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Thats one of the nice things about using 240 volts. Overvoltage is usually less harmful than undervoltage

I've never run across a 240 volt Wye supplied from an electric company. IF you had a neutral supplied with the Wye connection you would have

120volts for your toaster. a delta connection would require a transformer if there were no center tapped transformer.

the only place I've ever seen a corner grounde system was in a DC-6 aircraft, and that was 115 volts leg to leg.

Everywhere I've been it was the same. 208 Wye was used where most of the loads were 110/220 single phase in apartment buildings and stores, otherwise it was 240 volts red leg delta service.

I had the same thing happen in florida caused by a lightning strike. It was like in the sci fi movies.. the fans were going up and down in speed and the lights were flashing, It happened a day after the storm. The day before they changed the fuse on the pole and i told them there were other problems. I could smell the burned transformer oil.

John

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snip-----

That's what I've been talking about, a wild leg system. That's what I have. The wild leg measures 208 volts to ground, or neutral.

Yep! That's what I have, the wild leg as the B phase, and it's orange. One difference-----I have three transformers on the pole in my yard, all the same size. They are not pad mounted.

Yeah, and thanks for correcting me. I fully meant neutral, but my fingers got away from me. i hae a basic understanding of the difference between a ground, and a neutral.

My panel (Square D) provides for a neutral, and I have it. I have a true 5 wire system, run to each individual box in the shop.

That's an interesting comment. When I discussed three phase service with PUD, our provider, I was advised that our area, which is relatively remote, was in bad need of an update, that they were going to provide the second leg of the three phase service to lighten the load on the single leg, which is, as I recall, something like 14,000 volts. I was told if I would pay for the third leg, it would be installed simultaneously, saving me considerable money, which it did. It dropped the cost from my original inquiry many years ago from \$30,000 to just over \$18,000. The point being that, while it's not necessarily a popular service, they had no problems providing it to my specs. I was required to provide a large CT can, naturally.

Which likely explains the crazy voltages I found when returning from vacation many years ago when we resided in Utah. Our entire house was wired with 3 phase, and one of the lines connected to a transformer worked loose, enough to lose a proper connection. We lost a few things from crazy voltages. Our refrigerator had been out of service for a long time, spoiling everything inside.

But yields only 208 volts. A good friend moved from one shop to another, the second serviced by a Y service. His CNC machines didn't like that one bit. Regardless of the fact that machine tools should have motors that can run on either voltage, I very much prefer to have the higher voltage, regardless of the inconvenience of losing the B phase for 120V service. I've managed to work around that very nicely in all situations.

Read above. Unless one has 208 volt motors, they tend to run hotter than necessary if 240 volt motors are run on 208. I don't consider that an advantage.

I can't argue with your thoughts, but I'm very comfortable with delta service, the only three phase I've used for more than 36 years, although my first service was an open delta. You know what they say about an old dog. PUD wasn't nearly as concerned about my load balance as you appear to be, given the fact that they are the ones that wired my two panels, from the pole to the CT can and meter base (one for three phase, the other for single phase, which serves both the shop and house, a 375 amp unit). They were more than aware that I was using the delta service in both capacities.

Harold

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