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220V and 240V, Why Both Numbers Are Used
They usually describe the same thing. The numbers are historical, and nominal voltages in the US have drifted upward over the last century.
| 220 volts | 240 volts | |
|---|---|---|
| US nominal today | Not standard, legacy term | 240 V single phase, 120 V to neutral |
| Where 220 came from | Older US standard, and much of the world today | Current ANSI C84.1 nominal |
| Acceptable range | not applicable | 228 to 252 V under ANSI C84.1 Range A |
| On a transformer nameplate | Rarely seen on new US gear | 240/120 on single phase, 208Y/120 or 480Y/277 three phase |
| Three phase equivalent | 208Y/120 in commercial buildings | 480Y/277 in commercial buildings |
| Where you still see 220 | Older drawings, imported equipment | Current US nameplates and specifications |
| Utilisation voltage | Equipment rated 208 or 230 | Equipment rated 240 |
Which you actually need
On US equipment built in the last few decades the number you want is 240, or on three phase commercial work 208Y/120 and 480Y/277. If a specification says 220, it is almost certainly legacy wording for a 240 volt circuit, but confirm before ordering because the equipment will be labelled with the real figure.
The practical risk is ordering a transformer to a number lifted off an old drawing. ANSI C84.1 sets the nominal system voltage and the acceptable range around it, and equipment is built to those nominals, so specifying 220 on a new US order invites a clarification at best and a wrong unit at worst.
On three phase commercial work this rarely comes up, because the numbers that matter are 208Y/120 and 480Y/277. It surfaces most often on single phase and on imported machinery, where a nameplate says 220 and somebody assumes a transformer is needed when the existing 240 volt supply is already within tolerance.
Working from a specification that uses one of these terms loosely? Send it over and we will tell you which equipment it is actually calling for, before you order the wrong thing. Get a quote or call 786 220 2801.