Same circuit, different decade
In North American residential work, 220, 230 and 240 volts all describe the same thing, the voltage between the two hot legs of a standard single phase service. The nominal figure was raised over time as the grid was standardised, from 110/220 to 115/230 and finally to the current 120/240. Older equipment, older electricians and older documentation still use the earlier numbers out of habit. A dryer outlet described as 220 is a 240 volt outlet.
When the difference is real
Outside North America it is not the same number. 220 volts is a genuine nominal utilisation voltage across much of Asia, South America and parts of Europe, usually at 50 Hz and single phase from a 380Y/220 system. So a machine labelled 220V from a European or Asian supplier may want a different voltage and a different frequency from a US 240 volt circuit, and assuming they match is how equipment gets damaged.
What the tolerance actually allows
ANSI C84.1 sets service voltage tolerance at plus or minus 5 percent for normal operation, so a nominal 240 volt service can legitimately measure anywhere from 228 to 252 volts. Equipment is built to cope with that range. Reading 232 volts at an outlet does not mean you have a 230 volt system, it means you have a 240 volt system sitting within tolerance.
Where it touches transformers
On a transformer nameplate the secondary is stated as the design figure, normally 120/240 for single phase or 480Y/277 and 208Y/120 for three phase. If a specification asks for 220 or 230 volts on a US job, confirm what is actually meant before ordering, because the two possibilities, a standard 240 volt unit or a genuine imported 220 volt requirement, are different transformers.
Common questions
Is 220V the same as 240V
In North America, yes. 220, 230 and 240 volts all refer to the same standard single phase circuit between two hot legs. The nominal figure was raised over the decades and the older numbers persist in habit and older documentation. Outside North America 220 volts is a genuine separate nominal voltage, often at 50 Hz.
Why do people say 220 when it is 240
Because it used to be. North American nominal voltage went from 110/220 to 115/230 to the present 120/240 as the grid was standardised. The older terms stayed in everyday use, particularly for dryer and range circuits.
Can I plug 220V equipment into a 240V outlet
If the equipment is North American, yes, they are the same circuit. If it is imported and genuinely built for 220 volts at 50 Hz, check the frequency requirement first, because a voltage match does not fix a frequency mismatch.
What voltage should I actually measure at a 240V outlet
Anywhere from about 228 to 252 volts. ANSI C84.1 allows plus or minus 5 percent on service voltage for normal operation, so readings in that band are within specification.
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