I have an office in a building supplied with 3 phase electricity in the United States. There are several standard household 120V outlets in my office, each connected to a dedicated, 20 amp circuit with an 80% rated breakers. Assuming I will plug in an appliance continuously with a power factor of 1, am I correct in assuming that each receptacle on one of these dedicated circuits will support up to a maximum of 120V x 20A x 1PF x sqrt(3) x. 80 = 3325.5 watts? Or is it standard to somehow downgrade the electricity to single phase 120V when the 208V comes through the panel, effectively eliminating the square root of 3 term in my calculation above,leaving a maximum wattage of 1920? Thanks.

1 Answer 1


No. You don't multiply by the square root of 3 unless you're drawing all 3 phases in a delta configuration, and then, you multiply the pole-pole voltage (208) not the pole-neutral voltage (120).

When dealing with a single phase of wye or split-phase, it's exactly what it says on the tin:

120V x 20A x PF1.00 x 80%. 1920W.

It's not "downgraded somehow", 3-phase wye service is simply three single-phase supplies that happen to be out of phase with each other 120 degrees. Just as split-phase is two single-phase supplies that happen to be 180 deg out of phase with each other.

  • Ahhh. This is what I was missing. @Harper, you are sooo knowledgeable about all this stuff and have been a huge help. If I wanted to learn more about electricity and the kinds of questions I've been asking, are there certain books or other learning material you'd recommend? Thanks again for all the help and guidance. Oct 27, 2017 at 4:22
  • So just to clarify. Let's say I had the outlet changed by an electrician to a 208v outlet. Would the max continuous wattage be 208V x 20A x 1PF x sqrt(3) x 0.80 = 5764 watts? Or is it just 208V x 20A x 1PF x 0.80 = 3328 watts? Oct 27, 2017 at 4:31
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    Just 208. You don't get to do the sqrt(3) unless you do all 3 phases and use them in delta configuration. Oct 27, 2017 at 4:42
  • Got it, @Harper. Is using all 3 phases and using them in a delta configuration relatively straightforward for an electrician to do when changing this out? Oct 28, 2017 at 1:05
  • @StatsStudent why on earth would you want to do that? You're kind of giving us an XY problem here. It's not homework since you say you have the office. But you're asking about a lot of theory with no explanation of what you are trying to do. Perhaps a new question would be appropriate. Oct 28, 2017 at 1:24

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