General discussion


3 phase power

By grantk ·
3 phase power as a description of the AC
supply seems wrong. Does 3 phase only have
2 live sources? If so, why is it not referrred to
as 2 phase?
In common household supply of 230/240v AC
with 1 live, 1 neutral and 1 earth should it not
be called single phase supply?

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by TheChas In reply to 3 phase power

My first question, is what prompted you to post this kind of question here on this forum.

After all, most of the peers here are in IT, not AC power positions.

As to your question, 3 phase power is rather complex.

First, standard 120 VAC is SINGLE phase power.

Standard houshold 240 VAC is sometimes refered to as 2 phase power.
With 240 VAC, you have 2 hots (each at 120 VAC in respect to neutral) neutral, and ground.
The 2 hot lines are phased such that when combined the voltage doubles.
If the 2 hot lines were in phase, combining the lines would either result in zero voltage, or a parallel current path.

3 phase power does have 3 distinct AC waveforms that are 120? out of phase with each other.
Each single phase to neutral has a nominal potential of 120 VAC.
In the US, most 3 phase power connections have 3 hot lines, neutral, and ground.

Because of the way sine wave voltages add, when you combine 2 phases of the 3, you only get 208 VAC.

Now, to complicate matters further, there are 2 types of 3 phase in use in the US.
and Wye

I don't know 3 phase well enough to provide details of the differences between the 2. Just that you cannot connect a device wired for one type to power of the other type.

Another fun fact of 3 phase power is that you need to wire up devices with the proper phase rotation sequence for the device to work properly.
This is VERY critical with motors.

Hope this helps.

It just occoured to me that this issue is even more complex.
Many countries outside of the US have 240 VAC single phase AC mains.

To better understand 3 phase, you need to have a power line technician show you what the voltage looks like on an oscilloscope.
When connected properly to a true 3 phase AC source, the scope display will show 3 seperate AC sine waves. They will be in sync with each other, but separated by 120? or 5.5 mS at 60 Hz.


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3 phase outside USA

by grantk In reply to Wow

Thanks Chas for that detail. I got to this site from a querie through google.
In NZ and Australia the national grid is 230/40 volts. Local industry use "3 phase" power source for large electric motors.
What prompted my question was that a local home appliance manufacturer of electric ranges has the option of single phase and double phase power sources. This made me wonder whether for NZ situation "3 phase" terminology was actually incorrect.

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