# Getting 120v Single Phase from 3 Phase

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.
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(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
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wrote:

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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Harold and Susan Vordos wrote:

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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wrote:

104V.
wired
phase
place
from
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.

it
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.
two
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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Harold and Susan Vordos wrote:

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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George wrote:

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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John
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.
Jerry
wrote:

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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.

not
just
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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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Harold and Susan Vordos wrote:

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.
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snip----
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.

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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.
Harold
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Harold and Susan Vordos wrote:

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.
http://www1.mscdirect.com/CGI/NNPDFF?PMPAGE676&PARTPG=NNLMK32
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.
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Pete C. wrote:

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.
Don
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Don Murray wrote:

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.
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mandatory.
machines
store
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delta
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.

trouble
still
it's a

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.

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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.

the
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.
Harold
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Harold and Susan Vordos wrote:

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.
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