3 ms·

In the US, devices with high power requirements are generally connected "between" phases -- the phase-to-neutral voltage is(depending on your transformer config

In the US, devices with high power requirements are generally connected "between" phases -- the phase-to-neutral voltage is(depending on your transformer configuration) generally 120v, with some configurations generating 208v from 1 of the 3 phases to neutral. Phase-to-phase voltage is generally 220-240v. Loads that span all 3 phases are generally motor loads that will benefit from the rotating magnetic field that 3-phase service provides -- such as pumps and compressors. 3 phase power to residential is rare in the US.

sbierwagen 10y ago
You're confusing two subtly different things. Residential power in North America is usually delivered as "split phase", +120V, neutral, and -120V. (180 degrees out of phase: https://www.youtube.com/watch?v=vOh2OSJ44eE https://www.youtube.com/watch?v=vOh2OSJ44eE) The voltage across the two hot conductors is 240V, which is used for big stuff. You get it by tapping a transformer in the middle of a coil: https://en.wikipedia.org/wiki/Split-phase_electric_power https://en.wikipedia.org/wiki/Split-phase_electric_power Three phase power has three hot conductors, 120 degrees out of phase to each other. In the US it can be delivered at a bunch of different voltages, depending on what transformer the customer installs, but generally I worked with 277/480. (277V hot to neutral, 480V hot to hot. Generally all commercial lighting runs on 277V.) And to be specific, three phase is primarily useful because motors that use it are self-starting, you don't need to capacitor-and-switch arrangement single phase motors use, which frequently break. Less important now that most big motors use VFDs. Also, supposedly due to some frequency voodoo you don't need to use as much cable when running three phrase transmission cable.
- sathackr 10y ago
  Thanks for the clarification and much more detailed explanation. I was aware of the split-phase nuance and have been hit by 277 a couple of not-so-fun times. As to the benefit to motors, it was my understanding that using split-phase or a single phase and a capacitor creates a less-than-optimal magnetic field which results in more noise/vibration and wear on the motor as opposed to 3 phase.
- rplst8 10y ago
  There also exists 120/208 three phase in the US. 120V phase to neutral, 208V phase to phase. It's very common in commercial buildings that don't have big loads.

sbierwagen 10y ago

You're confusing two subtly different things. Residential power in North America is usually delivered as "split phase", +120V, neutral, and -120V. (180 degrees out of phase: https://www.youtube.com/watch?v=vOh2OSJ44eE https://www.youtube.com/watch?v=vOh2OSJ44eE) The voltage across the two hot conductors is 240V, which is used for big stuff. You get it by tapping a transformer in the middle of a coil: https://en.wikipedia.org/wiki/Split-phase_electric_power https://en.wikipedia.org/wiki/Split-phase_electric_power Three phase power has three hot conductors, 120 degrees out of phase to each other. In the US it can be delivered at a bunch of different voltages, depending on what transformer the customer installs, but generally I worked with 277/480. (277V hot to neutral, 480V hot to hot. Generally all commercial lighting runs on 277V.) And to be specific, three phase is primarily useful because motors that use it are self-starting, you don't need to capacitor-and-switch arrangement single phase motors use, which frequently break. Less important now that most big motors use VFDs. Also, supposedly due to some frequency voodoo you don't need to use as much cable when running three phrase transmission cable.

sathackr 10y ago
Thanks for the clarification and much more detailed explanation. I was aware of the split-phase nuance and have been hit by 277 a couple of not-so-fun times. As to the benefit to motors, it was my understanding that using split-phase or a single phase and a capacitor creates a less-than-optimal magnetic field which results in more noise/vibration and wear on the motor as opposed to 3 phase.
rplst8 10y ago
There also exists 120/208 three phase in the US. 120V phase to neutral, 208V phase to phase. It's very common in commercial buildings that don't have big loads.