We have a high-current industrial machine rated for 3-phase at 230 V; the nameplate literally says "PHASES: 3Ph + PE", "VOLTAGE: 3X230VAC", and "FULL LOAD: 92A".

There are only three input terminals (neglecting the PE, which is attached to the machine's metal casing), so it must be a three-wire delta input (please correct me if that's inaccurate).

There is a 600 V 3ph service coming into the building (also only 3 conductors), and a transformer is needed to reduce the voltage for the machine. I believe a good match would have 600 V delta-input, and 230 V delta-output, but it seems delta-delta transformers are difficult to find, and that delta-input-wye-output transformers are far more common, at least in the size we need (using "standard" sizes, 45 kVA or larger, based on 230 * 93 * sqrt(3) ~= 37 kVA).

In any case, a [used] transformer was selected, and I'm not sure it's properly suited. The vector diagram on the nameplate indicates that it's a delta-input-wye-output, and the nameplate also claims "H.V.: 600V" and "L.V.: 220V". Because there is generally a bit of wiggle room with voltages, and because the transformer has taps (on the input side), we aren't too worried about under-voltage (i.e. 220 V isn't a showstopper, despite the target device being rated at 230 V), and the machine's manual suggests anywhere from 208 to 240 V is acceptable.

We had the transformer hooked up to the 600 V service, and took some measurements while it was not yet connected to the machine. On the input side (coming from the utility company), we measured ~615 V on all line-to-line combinations (with only a few volts of difference between the different phases), so the input is a little high, but that seems common when there's no real load, and as mentioned, there are taps, so we can adjust if needed.

What surprised us was the output side... line-to-line, we measured ~450 V, and line-to-neutral, we measured ~260 V (again, with a small bit of variation between the phases, but nothing significant). I can't see how this concurs with the nameplate stating "L.V.: 220V".

Another detail: the transformer's nameplate also has an unlabeled field stating, verbatim, "X1-X2-X3---- 220V" (four hyphens and four spaces before the "220V")... and I don't know what to make of such a marking. Is that suggesting the line-to-neutral voltage is 220V? Or line-to-line?

Finally, we measured the voltage between [output] neutral (X0) and ground (tied to the casing), and although one might expect it to be zero (if neutral -- X0 -- and ground are actually connected), we measured ~1.4 V.

We stopped at this point, because if the machine wants 230 V line-to-line, and we were measuring ~450 V on the transformer's outputs, there's a good chance we could damage the machine if it were connected.

I'm bewildered by the transformer nameplate values, versus the measurements we took. Why would the output we measured exceed the nameplate value by nearly 20% (line-to-neutral) or 105% (line-to-line)? How likely is it that the nameplate is inaccurate, or the transformer is internally miswired or defective? Also, what's the deal with the small-but-non-zero voltage between X0 and ground?

Last, but perhaps most important, supposing we had a transformer that was a better match (e.g. one rated with output "230Y/133"), would we be able to simply ignore the neutral output lug (X0) and connect the three lives (X1, X2, X3) to the machine, and be done with it? I believe so, but with the proviso that the phase loads need to be balanced. The machine has some sub-devices that use a lot of current (heating elements) but according to the schematics, they seem to be wired in a balanced fashion.

Yep, it's a wall-o'-text, but thanks in advance for any insights about using a wye-output transformer with a machine that has no neutral terminal, and how to be sure we have chosen the right transformer.

And yes, this existing question -- Driving a Delta Load with a Wye -- seems to suggest the answer to my question is simply "yes"... but I was hoping to get a bit more detail.

Addendum: I forgot to mention that each of the transformer's coils has two terminals on the secondary side, so the first coil has X1 and X4, the second coil, X2 and X5, and the third, X3 and X6. We measured line-to-line and line-to-neutral for the second set of terminals, with the hope that they represented an alternate output voltage if used instead of X1,X2,X3 ( kinda like output-side taps, not sure if that's even a thing ), but I can't recall the values. :-/ I think they were the same as or similar to the first set.

UPDATE: I had been suspecting this, but finally, I've received confirmation: the nameplate ( which was damaged, and got replaced ) is not accurate; it isn't a 600-to-220 ( line-to-line ) transformer after all ( confirmed with OEM ). And that explains why we were having a problem with it! Once again, thanks to everyone who commented / answered.

  • 2
    \$\begingroup\$ Welcome! I couldn’t comprehend everything in your wall of text but my answer is nonetheless: Please call an electrician. \$\endgroup\$
    – winny
    Commented Oct 12, 2022 at 6:19
  • 1
    \$\begingroup\$ The overvoltage on output could be due to just lack of loading. If you have some less important 3-phase powered device, you could test if the voltage drops under load. \$\endgroup\$
    – jpa
    Commented Oct 12, 2022 at 14:17
  • \$\begingroup\$ @winny -- thanks, we've got one involved, although i'm not sure how comfortable he is with three-phase; he wasn't able to give us a certain answer about compatibility based on the nameplate, and the shop owner is "invested" in the existing transformer; i'm simply trying to improve my understanding of 3ph. \$\endgroup\$
    – bernz
    Commented Oct 13, 2022 at 1:30
  • \$\begingroup\$ @jpa -- yeah, i'm a little familiar with that phenomenon, although i don't fully understand it ( eg. is "unloaded phantom over-voltage" present in every sufficiently "large" circuit? can its value be predicted? etc ). unfortunately, we don't have a spare 3ph load with which to test, although your suggestion is good, thanks. \$\endgroup\$
    – bernz
    Commented Oct 13, 2022 at 1:30
  • \$\begingroup\$ Your input is 2.5% over nominal; your output should be 2.5% over nominal, too, irrespective of loading. Output voltage is a function ONLY of the ratio of turns of wire around the core between the primary and secondary (Y-D or D-Y adds a 1.732 factor; however that should already be accounted for in the nameplate voltages). Your output that's 18% higher on the secondary could result from 1) shorted high-side windings, 2) variations in load between the readings on the high and low sides, or 3) fluctuations in the high-side voltages between your two readings. \$\endgroup\$
    – pdtcaskey
    Commented Oct 19, 2022 at 3:45

4 Answers 4


Since each phase winding on the secondary side of the 'used' transformer has two terminals, it should be possible to 'delta-connect' the secondary side to obtain 3 x 220V supply for the machine.

Here's the existing wiring of the 'used' transformer.

enter image description here

It's to be rewired as shown below.

enter image description here

  • \$\begingroup\$ thanks, this is what the shop owner is hoping for ( a possible reconfiguration that could yield the required output voltage, line-to-line, with three live wires )... and interestingly enough, this transformer has not only [secondary-side] X1, X2, and X3, but a second set of terminals, X4, X5, and X6 -- could these be a path toward configuring the output to behave delta-style? would we have to disconnect the neutral bus bar that [appears to] tie the three windings together ( forming the central triple-connection of the wye pattern )? ( will edit question to add info re the extra terminals ). \$\endgroup\$
    – bernz
    Commented Oct 13, 2022 at 1:43
  • 1
    \$\begingroup\$ Terminals X4, X5 & X6 are to be disconnected from the bus and the windings X1-X4, X2-X5 & X3-X6 are to be connected in delta. \$\endgroup\$
    – vu2nan
    Commented Oct 13, 2022 at 5:37
  • \$\begingroup\$ My answer has been updated. \$\endgroup\$
    – vu2nan
    Commented Oct 14, 2022 at 2:39
  • \$\begingroup\$ okay, great, this makes sense to me, and i had heard other people ( not online ) allude to the idea of it being "misconfigured" or that it might be reconfigured, and this sounds like what they were talking about. i will of course confirm this with whatever electrician the shop engages to do the final hook-up. this is the insight i was really missing, thanks. \$\endgroup\$
    – bernz
    Commented Oct 14, 2022 at 12:20

As the answer you linked to says, there's no problem connecting a delta-wound machine to a wye-wound supply. People do it all the time connecting machinery to a public electricity supply. Ignore the neutral if you don't need it.

But you need to be very clear what voltage you have and what your machine needs. Connecting a 220V phase-to-phase machine to a 220V phase-to-neutral supply won't end well. The phase-to-phase voltage is always √3 (about 1.7) times the phase-to-neutral voltage.

  • \$\begingroup\$ thanks, and yes, that's the tricky part. we mostly only have the nameplate to tell us what's up, though i understand the previous owners of the machine said that it's 230 V line-to-line ( or "phase-to-phase", as you put it ), nothing involving neutral, which means our three live cables need to have 230 V between any two, and the transformer's line-to-line is well above that, despite its nameplate stating "X1-X2-X3---- 220V". we'll proceed with caution. :-) \$\endgroup\$
    – bernz
    Commented Oct 13, 2022 at 1:34
  • \$\begingroup\$ while i appreciate your answer, and wish i could select two answers as "the" answer, the rewiring suggestion from 'vu2nan' was the critical info i was missing. i apologize for not having included the mention of the second set of output terminals ( somehow 'vu2nan' intuited their existence ) in the original posting of the question. \$\endgroup\$
    – bernz
    Commented Oct 14, 2022 at 12:25

I think what you might be facing is a difference in naming convention between Delta and Wye configurations.

In a Delta configuration, if I say I have a 220V three phase transformer secondary, then the convention is that it means 220V phase-to-phase On the other hand, in a Wye configuration, saying that the secondary is 220V (your label states "L.V.: 220V") likely means that the phase-to-neutral is 220V.

I think your measured 260V is the 220V adjusted for your slightly high 615V input voltage.

Unfortunately I think this difference in naming convention means you have an unsuitable transformer for the task. But before scrapping it, I would recommend following jpa's advice from a comment, and verify that the high phase-to-phase reading you are getting is not an anomaly. But rather than a "less-important" load as jpa suggests, I would suggest a multi-meter with a low-impedance mode.


If the voltages between A, B, and C phases are within tolerance for your machine (NEMA standards allow for +/- 10% from rated voltages and utility standards are typically +/- 5% at the interconnection between the utility and the end-user), it does not matter how those voltages were generated.

There are other practical considerations, though, for the transformer designs - most notably the effects of losing one of the three source phases.

A 230V-LL machine should work OK with 207-253V. Utility variations should result in 209-231V for the transformer output if the building's system is 600V nominal, but there will be additional voltage drop from there to the machine terminals that depends on the building wiring. Unless the machine is next to the main switchgear or if your building's system voltage is lower than 600V, you're taking a chance that you could drop below 207V.

As I noted elsewhere, I'd be very wary of that measured secondary output of your transformer. With its input 2.5% higher than nominal, the transformer output should be 225.5V, not 260 - that's 18% and well outside reasonable measurement-error tolerance. It would be interesting to see what the voltages are under load and no-load conditions and making sure that the loading doesn't change (at least much) or the incoming voltage doesn't change between input and output readings. If there is error based upon delta/wye, the factor would be either 1.732 or 0.866. Nothing close to 1.18 either way.


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