What are the different voltages: 110/115/117/120/125/220/240?

 

One thing where things might get a bit confusing is the different numbers people bandy about for the voltage of a circuit. One person might talk about 110V, another 117V or another 120V.

 

These are all, in fact, exactly the same thing… In North America the utility companies are required to supply a split-phase 240 volt (+-5%) feed to your house. This works out as two 120V +- 5% legs. Additionally, since there are resistive voltage drops in the house wiring, it’s not unreasonable to find 120V has dropped to 110V or 240V has dropped to 220V by the time the power reaches a wall outlet. Especially at the end of an extension cord or long circuit run. For a number of reasons, some historical, some simple personal orneriness, different people choose to call them by slightly different numbers. This FAQ has chosen to be consistent with calling them “110V” and “220V”, except when actually saying what the measured voltage will be. Confusing? A bit. Just ignore it.

 

One thing that might make this a little more understandable is that the nameplates on equipment often show the lower (ie: 110V instead of 120V) value. What this implies is that the device is designed to operate properly when the voltage drops that low.

 

208V is *not* the same as 240V. 208V is the voltage between phases of a 3-phase “Y” circuit that is 120V from neutral to any hot. 480V is the voltage between phases of a 3-phase “Y” circuit that’s 277V from hot to neutral.

 

In keeping with 110V versus 120V strangeness, motors intended to run on 480V three phase are often labeled as 440V.

 

 

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