Engineer Guy wrote:I'll chime in on this one...
U.S. power is 3 Phases, 120 degrees 'out' from each other. When you measure across our 2 'Hots', it reads 208 VAC; not 240. I had to dig into this detail when installing a 240 VAC Demand 'Flash' WH at a House. The following WH performance down-rating quoted below from Wikipedia was also noted about this Demand WH in Manufacturer's Documentation imported from the 240 VAC U.K. by 'Grainger':
'In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between phase and neutral. If heating equipment designed for the 240-volt three-wire single phase system is connected to two phases of a 208 volt supply, it will only produce 75% of its rated heating effect.'
Wikipedia: Single-phase Electric Power
I don't think Engineer Guy is clear with his description of what type of power we have here.
We have two distinct systems. Single phase, which is actually two 120 volt phases producing 120 volts to neutral and 240 volts between them. They're both 120 volts but 180 degrees out of phase with each other.
The other distinctly separate system is 120 volt three phase which he correctly states is 120 volts to neutral (but a neutral lead isn't used with three phase motors) and 208 volts between phases as they are 120 degrees out of phase (versus 180 out of phase in single phase).
Doubting my version (as Engineer Guy is pretty darned knowledgeable) I just measured the 240 outlet in my garage (lathe, mill, 2 welders, and table saw use 240) and measured 243 volts with a Fluke meter. I also verified with a friend who is a supervisor at a hydroelectric plant on the Columbia river.
A big advantage of three phase motors is that they are self starting w/o any extra windings, capacitors, centrifugal switches, etc. to start them turning like single phase motors.
My lathe and milling machine were designed to run on three phase but residential areas are rarely provided with three phase power. As the motor on the mill would be difficult to impossible to change out and the lathe would be a major PITA to change the motor on I built rotary phase converters using another three phase motor (as a rotary transformer), a time delay relay, a second relay for control of the phase converter starting, a starting capacitor to start the rotary transformer, and finally some motor running capacitors to even out the current on the three legs. See pic in album.
It was interesting to learn that all outlets are switched in the "land down under" and that the hot is in fact 240 single phase.
Cheers,
Gus