< question completely reworded since people thought I was looking for appliance repair instructions >

Many "Power Transformers" are rated at 115V or 230V. It seems to be the most popular rating. See this list for an example: http://www.digikey.com/product-search/en?pv1393=61&FV=fff4000c%2Cfff8012f&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

Here is a specific example: http://www.tamuracorp.com/clientuploads/pdfs/engineeringdocs/3FD-3XX.pdf

However, mains voltage is 120V (per leg) in the US. I'm confused that these are all rated 5V BELOW the expected mains voltage.

Is it expected that these transformers are for use with mains inputs? If so, why is the rating 5V below the mains voltage? If not, why is the rating so close to the mains level?

  • \$\begingroup\$ Related: electronics.stackexchange.com/q/57739/2028 \$\endgroup\$
    – JYelton
    Jun 24, 2014 at 4:52
  • \$\begingroup\$ This is a case where the power company may be liable for replacing your appliances. \$\endgroup\$
    – Matt Young
    Jun 24, 2014 at 12:12
  • \$\begingroup\$ In the USA, voltages have been creeping upwards for years. Back in the 1960's, standard voltage was 110 volts. Over the years labeled voltages have crept from 110 to 115 to 117, and the now 120 volts. \$\endgroup\$
    – Marla
    Jun 24, 2014 at 19:04
  • \$\begingroup\$ Moreover, if 115 V (+ tolerance) doesn't drive it into saturation at 50 Hz, it should withstand 138 V at 60 Hz. \$\endgroup\$
    – greybeard
    Aug 22, 2023 at 18:00

2 Answers 2


Up until couple years ago in Netherland (and many other EU countries) we used to have 220V(AC), others used 230 or 240V(AC). One of the good things that was brought by EU is the unification of mains power. Over several years the mains voltages have been slowly changed to 230V (+/- 10%) across mainland EU (not sure about UK). Thus a substantial part of the world is now at 230 or 120V.

Many electronic devices don't mind too much about that increase by less than 5% and incandescent light bulbs have a somewhat reduced life time but are cheap.

Not sure what countries use 240V(AC), but do realize you are talking about a less than 5% error which is within mains power tolerance.

  • 1
    \$\begingroup\$ UK was nominally 240V+/-6%, now harmonised to EU 230V+/-10%. This meant no change for the vast majority of the UK, though a few locations may have lost a few volts. Old (1960s and earlier) vacuum tube equipment often has a voltage selector for 220,230,240V because some locales had non-standard voltages. \$\endgroup\$
    – user16324
    Jun 24, 2014 at 8:55

And... now we see even higher mains of 125-128v! I saw 131v in Austin recently. I've witnessed some transformers with the 115v primary spec get too hot to touch on 124v, and that's just being connected, no load. I sent the manufacturer a q and am waiting to hear back on their opinion of this labeling vs reality problem. My main deal now is really safety. I don't want a customer to have a fire!


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