is it 110VAC, or 120VAC?

single phase AC

recently, i took a trip to home depot, and while in the electrical depeartment i mentioned my house was “120” and was corrected by the electrical employee who said it is actually “110”. i thought i was right, so i had to check it out.

from some quick internet research, and with nothing 100% confident, i believe the correct answer is 120VAC outlet voltage for the US, today. it looks like some of the confusion is that the voltage has actually changed over the years, and for some reason the 110 name stuck. maybe it’s easier to say. it looks like the nominal voltage has actually been 100, 110, 115, 117, and 120 as standards changed. also, the current spec has a tolerance rating as well, which could be +/- 10% or some people say +/- 5%. (we could probably look this up in a NEMA or IEC publication, but then we’d have to pay for access). so, within that spec, anything could actually operate at 110 and be fine.

some other information says that the difference is just in the way you describe in, that 110 and 120 are actually the same, only 110 is RMS voltage, where 120 is maximum voltage. more research to be done on that.

even more info says that the power coming in is 120, but the devices are made to run on 110 so they can account for long wire runs with voltage drop, so by the time it actually gets to your device, the power is still fine.

In the end, it seems that the difference is largely semantic and they are basically the same. if i heard someone in the store use the term 110 or 120, i would not correct them.

2 phase AC

Two phase ac is what the US residential houses typically get. this power is at 240V, which is really two 120v ‘legs’ or conductors that are 180 degrees apart in phase. so you can use 2, 120v legs to get 240 or just 1 leg with a neutral to get 120.

3 phase AC

this is typically at 208 or 480V, which is 3 legs. again, you may hear this called 220/230 or 440/460, as these values have increased the same as single phase, and indeed, single phase may just be one leg on a 3 phase system.

208V is just 3 legs that are always 120 between any leg and ground, and 480V is 3 legs that are always 277V between each and ground. these legs are 60deg out of phase relative to each other.

These are typically connected in what is called a “Y” or “wye” pattern. The Y connection type has a 4th wire which is neutral. The other option is a “delta” connection. delta connections do not have a neutral.

Advantages of delta is that the phase/total voltage is equal to each line voltage. Delta will give you higher voltage, where Y will give you higher current. balanced systems are where the voltage and current of each individual leg are all equal to each other, which is desired; unbalanced systems usually mean there’s a problem.


so there’s todays lesson on voltages! only legacy personnel describe it as 110, and anyone who can stick a voltmeter on a power line will know immediately what we’re getting these days.

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