I don't understand the difference between household 240 and 3 phase 240. Isn't household 240 just two 120 phases, making 240V two-phase? If so, wouldn't three phase just be 360v? How, then, is there 240 and 480v three phase? Does each conductor carry the full voltage?

2 Answers 2


Input Phase Angle and number of transformers needed. For High-Delta, type of transformer.

All voltages below are expressed in RMS Average, not Peak...

I find it really hard to refer to Split-Phase as Residential or "House" Power. It is used in business where you're not running a lot of heavy motorized equipment. 240VAC Split Phase is produced off a single phase input transformer with center tapped secondary, producing for output, a single phase across the 240V outer terminals and two 120V legs with phases 180 degrees apart. Centertap is an effective ground (Neutral) at 0V potential and each leg is +120VAC and -120VAC respectively for the full voltage of 240V.

If you view the waveform on an oscilloscope, you will see a single sine wave (single phase) when measuring between Line 1 and Line 2 (below) at 240VAC RMS. Measuring between Line 1 and Neutral will show a single sine wave at 120VAC RMS, measuring between Line 2 and Neutral will show a single sine wave at 120VAC RMS equal and opposite to the L1-Neutral wave (180 degree phase shift)

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Three Phase has three separate circuits with phases 120 Degrees apart. You need three separate transformers, one for each phase. The primary on each is fed with a single phase and produces an output of a single phase on 208 (Y) or 240 (Delta) VAC. Depending on whether the circuit is Wye or Delta, you can have multiple voltages. Each phase-pair carries the full voltage. On Wye with Neutral, the voltage between the phase and Neutral will be slightly less than three-fifths of (it's one-third of the square root of 3, for electronics geeks) the voltage between each phase leg. On Delta, you only have each phase available with no Neutral.

If we hook up an oscilloscope to each of the line terminals, we see the following waveform, three independent power paths, 120 degrees phase shifted which when applied to a three phase motor, provide a rotating vortex magnetic field which makes 3 phase motors self-starting without the requirement of a start capacitor, start winding or other method of kludging in a phase shifted winding to provide the rotational force to start the motor spinning.

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Wye and Delta typically have one Pole Pig per phase (three transformers). On a Wye 480VAC system, the voltage between each phase and Neutral is 277VAC. Transformer secondary electrical schematics below to show how each transformer set is hooked up to derive the connection scheme. You can hook a single phase motor up on L1-L2, L2-L3 or L3-L1 for single phase current (balance necessary on loading between phases) or apply L1-L2-L3 to the proper therminals of a three phase motor.

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On the US power grid, also is used the High-Leg Delta transformer circuit which allows for multiple voltages. Voltage between each Leg produces 240VAC, voltage between the Center Tap and the high leg gives 208VAC Single Phase and voltage between each low leg and the center tap is essentially a Split Phase circuit with 120V from each respective leg to Neutral (residential power tap - grin). High-Leg Delta power is only available in 120/208/240, and is not put in any longer as Wye circuits are preferred instead for their relative simplicity and improved load balancing

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Depending where you live, yes it usually is. The USA is an oddball country in that respect. L1 to N 240Vrms is phase, voltage L1 to L2 415Vrms (or L3) is line voltag, so amperes and conductor mm² is smaller, lighter and cheaper. Single phase uses phase voltage, 2 phase uses line voltage, split phase uses 2 lines, a neutral and usually a capacitor to simulate a third phase (it's a method of motor wiring) It is possible to use more than 3ø, it's easiest to think of phases like the piston and con rods on the crank shaft of an engine. The more you have, the smoother it turns. The more voltage/power, the more torque

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