Why do the primary windings of some power transformers have so many different values for US "mains voltage"?

Thread Starter

Dolmetscher007

Joined Mar 21, 2019
34
I need to purchase a couple of small power transformers. I've always noticed that a lot of transformers have primary windings for 115V, 117V, 120V, and maybe even more. Some of them have all of the above in one single transformer, while some are labeled with just one as their primary voltage. Yet, they are sold as "mains voltage" to some other voltage. I totally understand why the secondary windings could be for any number of voltages, but the primary winding... I would think... wouldn't really change, right? Yet... it definitely seems to.

What am I missing?
 

Audioguru again

Joined Oct 21, 2019
3,173
You are missing the resistance of the wires between the high voltage to 120VAC electricity transformer in the street and your house.
My high voltage transformer in next door to my house so my "120VAC" measures 124.5VAC right now when most people are not using much power. It drops to 123VAC sometimes. Nothing burns out but my kettle heats faster than a kettle far away from civilization.
 

shortbus

Joined Sep 30, 2009
8,568
I've always noticed that a lot of transformers have primary windings for 115V, 117V, 120V, and maybe even more.
Would you give a link to such a transformer? Never seen it myself. Now then some transfomre makers do list the primary voltages you say on their transformer, but that is probably only because they've not updated their data sheets since those voltages were the standard. But like I said have never seen one with taps on the primary for such close values of voltage, makes no sense and would cost much more to do.
 

Tonyr1084

Joined Sep 24, 2015
5,969
As Audioguru again says, there's line drop in voltage. My parents house, if memory serves, was typically 117 volts (AC) whereas at work the power at the outlet was 112V. At a home in Connecticut, downstairs voltage was around 119V and up two stories in the attic the voltage was 114V. AND when I plugged something in up in the attic the voltage would drop even further. I once had a TV that its power supply went short. So I took it up stairs for repair. I plugged it in and turned it on. Every light up stairs went to about half brightness. The line that ran all the way up to the attic was under sized old knob and tube wiring. I ran a dedicated 12 gauge wire and got full power.
 

Tonyr1084

Joined Sep 24, 2015
5,969
I have never seen one with taps on the primary for such close values of voltage, makes no sense and would cost much more to do.
I've seen multiple primary taps, but they've been for (generically speaking) 120VAC and 240VAC. I'm sure shortbus has likewise seen such transformers, as have many of us. But like shortbus said, to tap a transformer for a difference of 1 or 2 volts doesn't make sense. Line voltages can vary quite a bit.

More modern power supplies list an AC voltage range of 100 to 240 VAC. Those are not transformer type supplies, they're switching supplies; meaning they turn the line on and off rapidly depending on the needed output voltage, and they can work from a range of voltages from 100VAC (seen in Japan and other places) to 240VAC.
 

MaxHeadRoom

Joined Jul 18, 2013
23,092
I need to purchase a couple of small power transformers. I've always noticed that a lot of transformers have primary windings for 115V, 117V, 120V, and maybe even more. Some of them have all of the above in one single transformer, while some are labeled with just one as their primary voltage. Yet, they are sold as "mains voltage" to some other voltage. I totally understand why the secondary windings could be for any number of voltages, but the primary winding... I would think... wouldn't really change, right? Yet... it definitely seems to.

What am I missing?
Mains transformers are not precise devices, plus most residential service supplies can vary, especially peak and off-peak times.
You also have the fact that once N.A. was 110/220, now standard at 120/240, but many appliance manuf. still use 110v on there specifications.
If in NA, use 120v as the guide to your particular spec. But not worth worrying too much over.! :p
Max.
 

Thread Starter

Dolmetscher007

Joined Mar 21, 2019
34
Would you give a link to such a transformer? Never seen it myself. Now then some transfomre makers do list the primary voltages you say on their transformer, but that is probably only because they've not updated their data sheets since those voltages were the standard. But like I said have never seen one with taps on the primary for such close values of voltage, makes no sense and would cost much more to do.
Thanks @shortbus
Judging by everyone's reaction, looks like I either phrased my question incorrectly (absolutely possible/likely), or... this issue is more complex than I thought. To get right to answering your question @shortbus, here is a link to a web page for a transformer company called Magnetic Electronics, Inc. That link should take you to a page with a dozen or so transformers listed; you might have to scroll down some to see the list and their values. I have many other links with many other transformers, all with this same issue, of having primary winding voltages that are different from 120V or 240V. But I think that link should show you what I'm talking about.

For Clarification:
  • US electricity: I know that the electricity found in a typical North American home wall outlet does not serve up precisely 120V at all times, from outlet to outlet, and home to home. I'm sure that if you went around measuring the outlets all across your neighborhood, or maybe even your own home, you could find a pretty broad range of voltages. @Dodgydave says there's a +/- 6% variation. Sounds likely to me. But if you click on that link above, and you look at the primary winding values for those Power Transformers, you will see values like: 100/120/220/230/240V on one... then just 117V on another... then 100/120/200/220/240V on another. Why list out specific values rather than just ranges?
  • International electricity: I also know a lot of electronics products end up in other countries that have different standards than we do in North America. But still... Why would a transformer company list 117V or 115V as it's primary winding value, rather than just say something like ~115-120V, with the same thing for Europe etc. ~200-240V? I don't get the laying out of specific, irregular numbers on the actual labels for the transformers.
 
100/120/200/220/240V
Except for 200, I know about the others. 100V is some parts of Japan, however 208 is a US voltage. It's derived from a 3 phase Y distribution system. 277 single phase is also a US voltage. It's derived from a 3 phase Y system and it used for lighting.
Why would a transformer company list 117V or 115V as it's primary winding value, rather than just say something like ~115-120V
The engineer needs to account for the differences.

The average idiot would probably do better with 110 and 120 V.

208 and 240 is a large difference. In our case when we moved from 240 to 208, the heating elements had to be changed. You might not notice it on a toaster, but will on a diffusion pump.

We had a German outfit doing research in our building that used https://www.nabertherm.com/produkte/labor/en furnaces. They waned to use a German furnace, which I think was 240. They got the power they needed.
 

Thread Starter

Dolmetscher007

Joined Mar 21, 2019
34
Except for 200, I know about the others. 100V is some parts of Japan, however 208 is a US voltage. It's derived from a 3 phase Y distribution system. 277 single phase is also a US voltage. It's derived from a 3 phase Y system and it used for lighting.


The engineer needs to account for the differences.

The average idiot would probably do better with 110 and 120 V.

208 and 240 is a large difference. In our case when we moved from 240 to 208, the heating elements had to be changed. You might not notice it on a toaster, but will on a diffusion pump.

We had a German outfit doing research in our building that used https://www.nabertherm.com/produkte/labor/en furnaces. They waned to use a German furnace, which I think was 240. They got the power they needed.
This is awesome @KeepItSimpleStupid! I had no idea that there were electrical standards around the world other than 120V and 240V. And I also never thought that the US might also have some other standards for different "sectors" like industrial 3-phase stuff etc. Super interesting. Thanks.

ALSO
The taps can be used to adjust the secondary voltage. If you built a project using a 12V transformer and now wish the secondary was 11V you can change the primary tap.
Ahhhhh...!!! So, you're saying... If your transformer has just one single input voltage, and one single output voltage, if you need to alter something later, because of some re-calculation or component substitution down stream, having a 117V or a 115V primary would let you "nudge" the voltage down a bit. Right?
Still seems like you'd just use a variac for that, and I still don't know why they'd only go down to 115V and 117V, but I've never seen one go "up" like to, 123V, or 125V.
 

Thread Starter

Dolmetscher007

Joined Mar 21, 2019
34
Some of these replies seem to be thinking the OP is talking about a measured difference in voltage from the nominal. OP is talking about taps specifically labeled with these slightly different voltages. I've also seen them.

This isn't an exact example but it's close. It's a Hammond tube transformer:

https://www.hammfg.com/electronics/transformers/classic/300?referer=787
100% correct @Veracohr

Hammond is precisely one of the companies that makes a lot of these. My main focus, when it comes to electronics, is vacuum tube based guitar amplifiers. I don't know if that helps anything or sheds some light on things. I could be that this practice of multiple primary taps has some function within the guitar amplifier world that isn't common in other sectors of electronics.
 

metermannd

Joined Oct 25, 2020
189
As for the voltage delivered to a residential customer, it does depend on where you live. Across most of the USA, 240/120V split-phase is default, but if you happen to be a customer of Consolidated Edison (NYC metro area), you're likely going to get 208/120V 3-wire (from 4-wire Y) by default and have to ASK if the company will provide 240/120V service.
 
This is awesome @KeepItSimpleStupid! I had no idea that there were electrical standards around the world other than 120V and 240V. And I also never thought that the US might also have some other standards for different "sectors" like industrial 3-phase stuff etc. Super interesting. Thanks.
400 V is Japan

https://en.wikipedia.org/wiki/Mains_electricity_by_countryThis is awesome @KeepItSimpleStupid! I had no idea that there were electrical standards around the world other than 120V and 240V. And I also never thought that the US might also have some other standards for different "sectors" like industrial 3-phase stuff etc. Super interesting. Thanks.

Generally there is also 50 and 60 Hz. generally 50 hz transformers will work on 60 Hz

In some parts of the US, 3 phase is available for residential too. It's rare, but it happens.

This https://en.wikipedia.org/wiki/Three-phase_electric_power article can drive you nuts too.

We had a 40 kW generator at work that needed to provide all sorts of power. 120, 208 single phase. 208 3 phase, something around 460 3 phase for ventilation.

Exhaust fans which were 3 phase was the primary thing that needed backed up aand they were 460 or so 3 phase..
Then you needed lighting at 277 single phase.
Then 208 3-phase for laboratory motors.
Then 120 single phase for the normal stuff like gas alarms and the RFID lock system.
Unfortunately, emergency lighting used their own batteries except for the critical area that kept the florescent lighting on.
We had to provide UPS power for ~2 minutes for a safety PLC to continue to have power. The UPS that operated off a generator has to be special because the frequency variation is larger than the utility. Eventually we outgrew the generaror and convenience items had to be removed from backup. A convenience item is when you loose power for more than 5 minutes, it costs you a day to bring the system back online.

We had equipment that we essentially plugged into the wall and it used 208V 3 phase at 90A. Essentially a whole house.

We had two Japanease SEM's (Scanning Electron Microscopes) that required 100 V.

In my area and lifetime, the US was 220/110 and changed to 240/120. the US system is split phase, but called single phase bcause that's what the power company provides. Something around 10 kV that fed to a 240 center-tapped transformer. The center-tap is ground. Niagra falls generate voltage at way different frequency. A friend had a company that, at one time was supplied with 2-phase power.

400 Hz is common in Aircraft. The components are lighter. I read an interesting story how 400 Hz came about. The higher the frequency, the smaller the transformer for the same amount of power.
 
Last edited:

Ian0

Joined Aug 7, 2020
2,192
400V is European phase-to-phase voltage, which gives 230V phase to neutral.
When European mains voltage was standardised on 230V in 1995, Britain (which was previously 240V) became 230V +10%-6%, and the rest of Europe (which was previously 220V) became 230V +6%-10%. In other words, nothing changed.
Primary tapped transformers are used on low-voltage filament lamps, because lamp life varies inversely as the twelfth power of voltage, so gives a 3:1 variation in lamp life between 220V and 240V.
 
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