Getting 120v Single Phase from 3 Phase

That's how everything used to be and I believe both the wild leg and open delta configurations were primarily used as a way to save transformer costs.

Most panels these days are fairly modular and provide bolt in options of neutral and ground bars.

Much of the grid in this country is badly in need of an update unfortunately. Between cheap utilities not wanting to invest in plant upgrades and wing nut eco freaks the grid has been festering and decaying while the load just keeps getting larger.

Oddly enough the grid and service here in fairly rural North Texas is pretty good. They seem to prefer using individual transformers for each house or two instead of a secondary bus on the poles with a dozen homes off of one transformer. My neighbor and I share a 50kva unit on the pole between us.

Similarly bad things happen if you loose the neutral on a typical

120/240 residential service.

Nope, a Wye connected transformer yields whatever you spec it to yield,

208/120 is just the most popular for light commercial use, if you need more power you get 480/277 Wye. Of course if you get the 480/277 then you also need a transformer to give you 208/120 as well.

As for the CNC machines, back when I worked on them I seem to recall every one I worked on could be wired for a pretty wide range of input voltages. They also have the small buck/boost autotransformers you can use on the few machines that truly can't be restrapped for 208.

It would certainly be my preference to use a few autotransformers on one or two problem machines rather than go with a wild leg system.

Most machines I've seen can accommodate a wide input voltage range, for the few machines that can't you use the small inexpensive buck/boost autotransformers.

The utilities aren't that concerned with balance since they can just switch a few residential streets between phases to balance things. I just like my power system to be balanced, since that seems to be the only place in my life where I have any hope of achieving balance. Of course, not running any large business, I'm still stuck with my rotary phase converter.

Pete C.

Reply to
Pete C.
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You mention in thi reply below that you have seen a corner grounde system in a DC^ aircraft. That surprizes me. Most other aircraft use

120/208 3 phase Y configurations. Maybe I dont understand whata corner grounde is. I thought it would be a delta with one corner grounded.


Reply to
Jerry Martes

Well there are disadvantages from the 208 voltage, a lot of motors will not run at 208 with out overheating them. And my band was blade welder didn't really like it either, had problems welding the larger blades. The oven elements also don't seem to get as hot, cloths dryer, etc.....


Reply to

Buck/boost transformer.


Reply to
jim rozen

Exactly! I'm still at a loss to understand why the delta system is such a problem. Seems to me the Wye system, *for a machine shop*, would be far more disadvantageous. You'd have to be completely out of your mind to request it if delta was available.


Reply to
Harold and Susan Vordos


Regardless of reason, machine shops are typically provided with this service. The higher voltage is very desirable, and in some cases mandatory. I question your logic about saving transformer costs if individual machines would require buck/boost transformers. Between the area required to store them, and the increased cost of labor for installing them, seems to me it's a terrible waste of money when it can be dealt with by installing the delta system instead of the wye.

But you've overlooked the fact that it does *not* yield 240 volts, the optimum voltage for machine tools. I'm having more than a little trouble understanding why you feel delta is such a bad deal when it solves all problems aside from the wild leg issue. I can't think of one small machine shop that is wired wye------not one. Many of my friends are still in business in Utah, all of which have the delta service. Could be it's a regional thing. Dunno.

Or you could use 3 phase delta and ignore buying buck/boost transformers and their inherent problems. I thought that was an excellent idea.

You keep speaking of these problems, but I've had 3 phase delta systems since 1967, and aside from the one failure, which would have occurred be it delta or wye, I've never had any problems. From that I conclude that the problems, while possibly serious, are highly unlikely to plague the average person. I'm more than willing to gamble on these ethereal problems than request wye service and know for damned sure I'm going to face other problems, which I would. I get the idea you're geared to light commercial, where wye service is the norm. Machine shops do not use it----for obvious reasons. Places that have a lighting load that tends to be the largest power demand is where you find lots of wye service, at least in my experience.

We've already kicked that around. Why buy more transformers when you can get the proper voltage?

Yeah, the one that puts out 240 volts---------do you get my drift?

Funny, I get the distinct idea you think I made a mistake by installing the delta system. You couldn't be more wrong if you tried. It serves my purpose perfectly, very unlike a wye system.


Reply to
Harold and Susan Vordos

The transformer cost savings was for the large oil filled cans on the pole, by using only two for open delta, or two smaller and one larger for the unbalanced wild leg.

The buck/boost autotransformers are quite small and inexpensive. They are dry type autotransformers, not full isolating transformers with multiple windings so they are much smaller than you would expect for their capacity. The KVA size required for the autotransformer is not the full KVA of the load. The sizes you would need for an average machine are about shoe box size and around $150 new.

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The other point is that many machines have appropriate taps and connections on their motors and/or internal transformers and would not require the buck/boost.

How is 240v "optimum" for machine tools? A lot of machines larger than bench top size can be strapped for operation on 480v input as well. When I did CNC service, I don't recall seeing a single machine that didn't have a 480v input option.

I would consider "optimum" voltage for a machine to be any voltage that it can be strapped for. If it was designed with those taps then it should operate just as well on any of them.

The "optimum" voltage that you would want to use based on external factors would be one of the higher voltage options based on smaller required wire gauge, reduced voltage drop, etc. From that standpoint

208v and 240v are essentially equal.

If a machine has taps for both 208v and 240v then there is no advantage or disadvantage to either, only the convenience of what you have available. If there are a sufficient number of machines that can take

480v to fill a decent portion of a 480v panel then that is an even better choice.

I didn't say that delta was bad, I said that delta with a wild leg was bad, they are two different things. Take a (pure) delta service for the three phase and use a proper transformer to provide 208/120 Wye and I'd be happy. No wild leg anywhere, and you've still got your 240v delta that you like, as well as the familiar and flexible 208/120. Of course it would still be preferable to go with the 208/120 Wye to begin with and save the complexity.

I also didn't say that there were not plenty of shops with the wild leg service. If it's already in place in a shop and in good condition there is little reason to change it. I would not use it for a new installation.

What inherent problems? For the few machines that might require them in a decent sized shop cost should not be an issue. They are quite small so space should not be an issue. They are not difficult to wire so installation should not be an issue.

Many years ago I worked for a mid sized printing company. The building had 208/120 service, and there was only one machine in the entire company that required the buck/boost transformers, a press made in Sweden.

Failures due to poor maintenance can and will occur with any power system. Annual IR camera inspections really are cheap insurance since they can usually spot these problems developing before they can do any damage.

These days when you can get a handheld IR thermometer for $50 at Sears you can do the inspections yourself and save even more money. Granted it's a little slower than with an IR camera, but not that much slower. Do a monthly scan, input the data to a spreadsheet and pickup problem trends even faster.

Wye service is indeed good for buildings with a lot of lighting load,

480/277 is particularly good for that.

The main thing is that you are indicating that there is a big disadvantage to 208v vs. 240v which I just don't see. I think your making more of the difference between 208v and 240v than there is justification for.

The allowable voltage range for your 240/120 3ph delta wild leg service is from 220v-254v at the service entrance (from a chart referencing ANSI C84.1-1989). If the service is considered acceptable over a range of 34v I just don't see a 32V difference between services as significant.

Large motors are built to handle widely varying power and load conditions, and power supplies for controls either have plenty of tap adjustment range for older machines, or switching supplies for newer machines that are happy on anywhere from 98v - 250v. Your motors might run a few degrees warmer but still well within their specified operating range and your controls should be perfectly happy as well.

Again, because not many machines would actually require it, and the fact that your "proper" 240V service is allowed to vary over a 34v range anyway.

Basically I think the 14% or so difference is of little to no significance for 95% of the possible machines you might run, and for those few the fix is easy and inexpensive.

Er, no, a couple weeks ago I had 269V! A call to TXU had that fixed in less than one hour fortunately (good response).

And actually the 240V is some 9% high for the machine, as it's motor is rated for 220/440.

I think my main point is that I feel the Wye system would have served your needs equally well and would have simplified the installation by avoiding your use of separate panels for your single and three phase loads to avoid the wild leg issue.

Pete C.

Reply to
Pete C.

Peter, Do you think it would be OK to run the 240 motor on 197.6V ? Because that's what could happen when you hook it up to a 208 service. Most all power companies have a 5% tolerance on their service voltage. So a 240 service can go from 238 to 252. And a 208 service can go from 197.6 to

218.4. The purpose of a 208 service is to serve buildings with large lighting loads and small 3-phase loads. There is a lot of commercial 240 3-phase and a lot of houses that have their own 3-phase wells will also have a grounded center tap delta. I happen to have a 10HP 3-phase pump with a straight 3-phase service, 3hot wires and no neutral to the pump. The bank across the street from my house has a 25KVA and 2 10KVA transformers, in a center tap grounded delta. My house 120/240 single phase also comes off of this same bank.


Reply to
Don Murray

You're right, Harold. I think the 3 transformer delta service is the better of the two. One advantage is that when one transformer goes out a trouble shooter from the power company (me) can go out and rebuss it into an open delta and have you back up and running in 30 minutes. The two transformer open delta is only good for 86.6% of the KVA rating of the two transformers. So you might have to shed some load. :)


Reply to
Don Murray

I think something is wrong with your math there or you have a typo. At a

+/- 5% tolerance the nominal 240V service would range from 228V to 252V. According to that ANSI spec 220V to 254V is acceptable.

The different sized transformers is what I referenced about cost savings for the wild leg configuration, with the 10KVA transformers costing less than the 25KVA transformer.

A quick look on the Grainger site doesn't even find any 240V 3ph motors at all. They all seem to be spec'ed for 208-220/440 or 208-230/460. The Baldor site is also devoid of any 240V rated three phase motors, they list motors with 208-230 range and some at 220V or 230V.

The Baldor spec's indicate for a 2HP motor the FLA difference between

208V and 230V is .5A (12 vs. 11.5). If the entire current difference was dissipated as heat, which it is not, that's a whopping 104W extra to dissipate from the surface of a beefy 60# motor - hardly a big deal in just about any application.

So once again I think that there are very few machines that would actually have any adverse effects from running on a 208V service.

An existing building with wild leg delta service that was in good condition would be fine with me. For a new installation I would not spec. a wild leg delta service.

Pete C.

Reply to
Pete C.

You should also be able to replace the transformer in about the same time period. I watched a crew (two guys in buckets and one ground guy) change one in about 15 minutes. Even with the first guy (you) getting on the scene and then calling in for another truck to bring out the transformer while you remove the bad one it should not take very long.

Pete C.

Reply to
Pete C.

Chuckle! You, apparently, live in town. Not only could they not replace one for me in 15 minutes, it's highly unlikely they could do so in a couple hours. It would take no less than 45 minutes for someone to respond to our remote address, then the time it would take to get a transformer on location, assuming they had one in the yard.

Yeah, I know, small chance it would ever be an issue, but it's yet another one of the things that make me sure I did the right thing. I say go with the wild leg delta! In my experience, many of the older machines had motors wound for 240V, not 208V. That tells me they would be inclined to run hot. Do keep in mind I am not an electrician, so my exposure has been quite limited. Could be I'm all wet.


Reply to
Harold and Susan Vordos

Yeah, times 2 or three per machine, then the labor to install them. I think you shot yourself in the foot here, Pete. My point is that 3 phase delta is already the proper voltage----you need not find space for more electrical apparatus, nor pay for its installation. So far, I'm not convinced, and I'd refuse a wye service given a choice, which is exactly what I did when I requested my delta service.

Agreed. The vast majority of the large shops in which I worked had 480 volt service. I have no idea if they were delta or wye.

I would consider optimum voltage one that, given anything but a proper match, would be higher voltage instead of lower voltage, so motors didn't run hot. You may not think that's a problem, but I can tell you from the perspective of a guy that knows how to move metal when roughing, it's damned important. It's not unusual for a machine tool to be subjected to a 125% demand on horsepower. I've seen machines that shut down because they've been ridden hot and put away wet. No thanks, no 208 voltage for me.

But *not* equal.

My (very limited) experience has been that motors that are capable of running both voltages are wound 230 volts. I know my Bridgeport is so wound. That means, while you're still within tolerance, the motor is still running hotter than is necessary when operated on 208V. I may be wrong, but I don't think you have the luxury of changing internal wiring to reflect

208 or 240 volts. The only changes are to accommodate higher voltage, i.e. 460 volts, in my case.

Funny, I was thinking the same about your position. I can't imagine how the things you have spoken of are not a problem, buck/boost transformers for individual machines (cost and inconvenience of placement), motors that run hotter than is necessary.

The voltage spread would, likewise be similar for 208, if not the same. It's greater than 10%, the normal tolerance for motor voltage. Am I wrong? Isn't that why newer motors are wound 230 volts? To keep them within tolerance? Wouldn't that mean that older, 240 volt motors, would be running out of tolerance if applied to 208 volt service?

Tell my friend in Utah that he only imagined the problems he had with his CNC grinders when he moved his shop to one wired wye. You make light of it, but it was a serious problem for him. His machines wouldn't run. I do not recall how they rectified the problem, this was about 12 years ago.

But it *is* outside the 10% tolerance zone. I don't agree. I also don't think of buying buck/boost transformers @ a few hundred dollars as not being expensive. Especially commercially, where they *must* be installed by a licensed electrician. I can see how such an installation could turn into several hundred dollars per small machine, and in the thousands for large ones.

Yeah, 9% high, and in tolerance, unlike 14% low, and out of tolerance.

Wrong! I have *no* wild leg issues. In fact, my split panels provide a good an valuable service beyond just addressing that problem. I'm on a demand meter with my 3 phase, but not the single phase panel. The moment I go above 50 KW (my induction furnace is a 50 KW unit), I pay more for my power. The way I've wired the shop, I avoid the demand load except where absolutely necessary, which is for the 3 phase equipment. My lighting load alone could reach about 75 amps, if I was to run all my fluorescent lights at one time. Each one is individually switched at the fixture so I can use only those that are necessary. In turn, they are wired in six banks, so each series (according to shop location) can be switched at the door.

My 42 position (Square D) single phase panel is all but full, without accounting for any 3 phase devices. In other words, I'd have had to install two panels, regardless. As it is, I have three, one being a 400 amp disconnect that feeds the induction furnace.

I am better off for the way I've installed my service, and I still have no regrets for having selected delta. If anything, you've further convinced me I did the right thing. For that, I thank you.


Reply to
Harold and Susan Vordos

That case was indeed in town and it was a planned replacement. However, if you give them sufficient information when you log the trouble call I see no reason they couldn't have the correct transformer with them when they come out. Given the stock yards I've seen I think it's pretty unlikely that they would not have a suitable replacement on hand. So 15 min to replace + travel time to get there.

Well, the motor (original) on my old old 1J Bridgeport is 220/440 rated. Looking at the Grainger and Baldor sites I was not able to find any 240V rated motors. I suspect you'll find the machines have 220V or 230V rated motors.

The difference in current draw between operation at 208V and 240V is quite small and only a portion of that difference would be dissipated as heat. For a 2 HP Baldor motor I looked at the total difference was 104W and even if all of that was dissipated as heat across the 60# motors frame it would be pretty insignificant.

Most all motors are going to run hot on continuous duty and that's why they have the different insulation classes. If the motor runs 4 or 5 degrees hotter on the lower voltage it's not very significant.

Pete C.

Reply to
Pete C.

For the perhaps 5% of machines that might actually require them? A smaller shop may well not have a single machine that legitimately required the boost transformers.

I think you're under the impression that a motor will run hotter if it's strapped for it's lower voltage rating. A motor should run at the same temp for the same load and duty cycle conditions when operated at any voltage it can be strapped for.

Operation at the higher voltage setting is for the benefit of smaller wire gauge to feed the motor, lower voltage loss on those wires and smaller switch gear to service it, not for the benefit of the motor.

If you are pushing your machines past their continuous ratings for significant amounts of time you should fully expect them to shut down on you. Brief excursions past the continuous rating are ok, but sustained use there is not.

Correct 208V /= 240V. However the ~ 14% difference is not significant from a practical standpoint. With either 208V or 240V vs. 120V you're getting the benefits of the smaller wire and switch gear requirements and lower voltage drop.

My Bridgeport motor is 220/440.

Again you're assuming that most machines will require the transformers, which is not the case. The hotter part is a small amount, perhaps significant for a big shop in the desert, but not significant in most cases.

I think that any older motors that are truly rated for 240V are also old enough to be overbuilt beasts from the days when the line regulation was nowhere near where it is today and consequently would likely not care about running at 208V which is likely what they saw anyway on old services with significant voltage drops within the building.

By changing the taps on the control transformers within the machines and adjusting the set points for the motor starters?

Actually if they are installed integrally to the machine then they would bypass any licensing requirement. Licensing requirements do not extend past the power input terminals on the machine. Bolt the boost transformers to the side of the control cabinet and run all the connections inside the cabinet and they become part of the machine tool.

And you're paying extra for the second service (meter) vs. a single meter if you used Wye service. The whole peak meter thing can get pretty hinky, particularly when it is based on rolling averages, not hard setpoints. If it's based on rolling averages then a consistent lighting load has little effect. You really have to analyze your loads to see what is best.

If you've got individual 50KW loads you may well be big enough to have benefited from 480V service.

Pete C.

Reply to
Pete C.

It's the power co that saves on the transformers, not you. You get to supply the buck/boost transformers if you want them! Also the buck/boost transformers are on your side it the meter so you get to pay to keep them warm ( this is not as bad as keeping the big ones hot though)


Reply to


I fully agree, but I have no motors that can be strapped for 208. As I said, my Bridgeport is wound for 230. I don't recall what the Graziano demands, and the motor is internal--not readily available. I just know that it runs well on 240, so why risk anything? The induction furnace is wired 220/440. It would be within range, but the huge amperage demand makes running at a slightly higher voltage an advantage over running lower. Not necessarily for wire size, but, again, for heat control.

I understand that, but the cooler operation is certainly a benefit.

No, what I'm assuming is that the majority of the machines would be running in the tolerance zone, unnecessarily. Given a choice, why would I want any of my machines to run under voltage? Even within tolerance? Over voltage is far superior to under voltage if one must deviate from the ideal.

Could be, but I'd not be willing to gamble on it. Good example is the induction furnace, although it's not one of the machines that falls out of the tolerance zone (220/440 in this case). The motor generator weighs almost three tons, and is water cooled, not fan cooled. That means it has the potential to run hotter than I would like because it relies on heat conduction with small coils of circulating water wrapped around the exterior of the assembly. Adding any unnecessary heat to its operation would not be in my best interest, especially if it had the potential for failure. I can't speak for you, but I couldn't afford to pay for a rewind on something of that magnitude.

Dunno. As I said, I don't recall, but he has since built his own place and has it wired according to his needs. Can't help but think it's delta, but I couldn't swear it is. For all I know, he may be wired 480. I'll try to remember to ask him the next time we talk on the phone.

All well and good if you choose to use a wye service, but I chose to eliminate that problem. I'm as happy as if I had good sense. Even *if* one can get around the licensed electrician, you still face the cost of the buck/boost transformers, which, at a minimum, would cost at least $300 per machine, assuming you could get away with two instead of three. You also have the cost of mounting them, and the inconvenience of them being in place, adding considerably to the bulk of the machine, with no benefits. No thanks.

Nope! Wrong again. I am paying for two services in spite of my choice of delta over wye. *ANY* 3 phase service is considered industrial where we live. Had I run everything through the CT can, avoiding the meter base, I'd be paying for my power at a higher rate because I'd exceed the maximum (demand meter). Our meter base is a 375 amp unit, so we feed both the shop and the house from it, at a residential rate. It's simple------if I wanted three phase service, and I did, I had to pay for two services, like it or not (and I don't). As much as I may not like the cost, I'm very happy to have the convenience. I explored the idea of phase converters and quickly dismissed the idea. Way too expensive, considering the load I had with the induction furnace. I had two choices had I chosen phase converters-----buy one huge unit and pay for the idling current endlessly, or buy a few small units and bring them online as needed, still paying for idling current. It was cheaper, and far more convenient, to pay for three phase service.

To the best of my knowledge, I chose the right service for my needs. Considering I have nothing that is wired for 208, nor can any of it be rewired to accommodate that voltage, I sure as hell didn't want it in the shop. If industry today is headed that direction, and machine tools are so wired, that's a whole different situation, and I'd have no quarrel with the choice. With the machines I have, that wasn't one of the options, and I sure as hell wasn't going to spring for transformers that weren't necessary, not when all I had to do is request the right service.

Again, a judgment call. I have one device with that load. Why would I pay for 480 service, add an additional panel, then transform for everything else? Personally, I think I made some excellent choices. Everything I have runs as it should, and I haven't filled the shop with unnecessary transformers. Idling current alone would have been expensive in the long haul had I chosen that path. I have only one machine that *requires* 480 volts, a German made universal cylindrical grinder, which cannot be strapped for 240 volts, or anything within reason. The only time the transformer will be energized is when I use the machine. There will be no idling consumption otherwise, so I won't be paying for making heat. Regardless of your opinion, I feel I've done a damned good job of addressing the problems at hand. Your bias against delta service seems to be a much greater stumbling block for you than my choices are for me. Very strange, Pete. However, I think if you visited my shop and observed how nicely it all has come together, I think you might find yourself agreeing that I did the right thing. :-)


Reply to
Harold and Susan Vordos

Yep, which is just one more reason to avoid them if possible. I did!!


Reply to
Harold and Susan Vordos

Interesting. In my former area commercial rates were lower per KWH than residential rates. As long as you could manage your loads well enough to avoid problems due to peak metering you could get better rates by hanging your house off of your commercial service.

That induction furnace of yours seems to be the biggest issue no mater which way you go. Such a massive single load makes it difficult to optimize for peak metering.

As for phase converters, for manual machines I'd use inverter drives instead since their costs have become quite reasonable and they don't have an idle current issue.

For CNC machines in many cases you could modify them to run from mostly single phase power and a smaller inverter or rotary phase converter. Not real practical for multiple machines in a larger shop, but a workable route for a couple machines in a home shop.

It's likely that the potential issues diminish as the equipment gets newer.

If you've only got one huge load then 480 service is probably not worthwhile.

Stepping up for the one problem grinder makes sense, just as adding boost transformers makes sense if it's only a machine or two that need them.

Remember, I'm not biased against delta service, I'm biased against wild leg delta service, two different things.

I'm sure your shop is quite nice, it sounds like it's well equipped.

Were I planning a new shop with the full list of equipment to be in it, their specs, etc. I might well decide on a service configuration that included delta service for the machines. I would just avoid a service configuration that included a wild leg.

Pete C.

Reply to
Pete C.


I certainly get that idea-----even PUD was somewhat surprised that I wanted the delta service.

I don't agree on the boost transformers, not when I'm as pleased as I am to have the service I have. As I stated, I don't consider buying boost transformers to be a bargain, nor would I like feeding them on a continual basis, in spite of the fact that we pay only $.0445 kwh for our power. I have yet to discover one thing about my service that displeases me, and I've had 3 phase delta since 1967. One of us is nuts! :-)

You keep saying that, as if there's other types. I've heard of a corner grounded delta (but don't understand how it relates to voltage), but as far as I know, they all have a wild leg if you shoot for 120V. Is there something I don't know or understand? The moment you introduce the neutral, seems to me you're bound to have only two phases that yield 120V unless your wired wye.

OK---fill me in, then. What type of service would it be? Strictly 240 V? That's what I have now. I do *not* use my 3 phase for single phase service*, although when I wire my mill permanently, I will have single phase for the power feed and work light, but only there. I'll do that so I can keep the wires away from the bottom of the machine. I have the box overhead, almost directly over the motor on the mill. 12' ceiling. You can do that when you have a 5 wire system. Everything else is strictly 3 wires, plus ground. Do you still have heartburn over my wiring?

*The same phases that constitute my A & C phases in the 3 phase panel feed my single phase panel, although through separate lines and conduit, from the pole. Want to see a pic?


Reply to
Harold and Susan Vordos

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