Help, what is single phase

Thread Starter


Joined May 11, 2004
Hi, all:

I am a programmer who got laid off, now I am trying to be an electrician. I know some of the physics about electronic. But in real life, there is a lot of which I don't know.

For example, I understood that 3 phases has 3 wires. But, why is single phase can have 2 wires, are they synch or asynchronous???

I dunno that you guys can answer this. If I am pulling 3 phases (A, B, C + W) 600 Amp power into a panel, does it means I can have total of 600 Amp breakers on each hot? Or I can only have total of 600 Amp breakers on all 3 hot wires??? (not consider about cont. load, you know, the 125%/80% stuff, or any other variations)


Thread Starter


Joined May 11, 2004
Thank you electricmayhem:

I know how the 3 phases works. What I need to know is that why is single phase has two hots (A, B, +N) what phase they have on those wires.


Thread Starter


Joined May 11, 2004
Hi, Hamster:

Thank you very much, now, that's closer to what I asked. See, in three phase power, it is basically a 3 core generator. Each differs at 120 degree. So, when it turns a complete cycle (1/60 sec), it would generate 3 diff phases of power output. Ok, now, every book I read, talks about 3 phases in detail. But, not one, not one book says why is it called single phase, but tow wires. Are the AC phases on these wires in sync or out of sync?? Are they 120 or 180 degree different???? Are those wires come from some source??? If they are, I can connect two wires together and still give me 120 V??? Or can I use single phase 1 wire setup (120 V) and connect to tow diff breakers and single 220 V outlets to connect that to a dryer????

Yes, I got NEC, been reading that too (1999, because 2002 won't be in effect in a year or two). I read it about 1/4 through now. It only has regulations on that book, none of the basic stuff. So, I started going through some other books, some of them talk about basic stuff, but, ALWAYS skip the single phase stuff. I've bought 3 diff books and went through them now, no luck, so, HELP PLEASE :)


Thread Starter


Joined May 11, 2004
Thank you very much, electricmayhem .....

This helps a lot. It clears some for me now. Sorry to keep bugging you guys. But, I tried the books and the Internet, and I can't find the help on my own. I really appreciated for the time that you guys wasted on me :p

Now, the two wires that comes in, are they in phase? or they lags in some degree???


Joined Apr 28, 2004
lol...oops...i just reported it to th emoderator... my's 3:29 AM and i'm sleepy...sorry guys. but yeah.... they lag 120 degrees.


Joined May 12, 2004
I'm pretty convinced that in an ordinary wall socket (two wires) one lead is zero and the other is alternating around it 230Vpp, but you never know which one it is. That means if you stick a wire in the socket it's only 50 % chance you will die. I reckon that could be called singel phase.



Joined Mar 16, 2004
Sorry I didn’t fully understand your question at first, but now I do. You know more than I assumed and I apologize if I came off harshly. I am just going to start from the beginning and make a few restatements. There are average measurements and peak to peak measurements. For AC distribution the average (rms) value is the common form of measurement.

In a three phase system the average power is raised by introducing an extra two wave forms. Each phase is 120 degrees out just like cutting a pie into 3 pieces. Each line has a potential difference of 120 Volts no relation between these two numbers. Or at least I don’t think there is.

Below link shows a phaser diagram (pie)

From how you stated your questions it makes me feel that you think these wave forms would have a canceling effect on each other. If that were true we would obviously loose all benefit from going to a system like this. Three phase powered devices are designed to take advantage of the three wave forms. A simple example would be an electric motor would have three isolated coils one for each phase to induce a magnetic field. The motor is doing just the opposite job as a generator.

I am going on a gut instinct that the 3 wire (2 hots and neu) single phase system is just that single phase. So the neutral should be zero volts referenced to ground one hot should be 120 volts to ground and the other would be -120 volts.

|-120 v <---------> 0 v <---------> +120 v|
|hot 2________nuetral_________hot 1|

How the breakers terminate on the single phase panel is what determines the voltage on the circuit.

More explanation can be found here.

poly phase

Electrical safety - neat stuff here



Joined Mar 21, 2004
Hi Leo,
Check out this and c you get any thing new other than discussed so far.

As you might know, Electrical supply voltages are different in different contries.
Here in my place Line to neutral is 240V RMS. So, The Phase to Phase will be 240*1.732 = 415V
(240* Square root of 3)
A single phase supply will always have one Phase and one neutral, I dont understand the two hot wires that you are talking about. (are you talking about single phasing in motors? it is basically loosing of one phase and the loosing of power balance in the flux - which finally leads the motor to burn)

In a three phase system the total effective current (The current which ultimately contributes the VA or Watt ) will be shared by each phase in a particular manner.
ie, if you connect a motor which draws a current of 60 A, then each line current seperatly will be 60/root 3 ie,34A. (that tells you how you can connect a breaker.

Suppose you connect your load in star connection and all of them are balanced you can c there will be no effect if you connect the neutral or not.
ie, the return current will be shared by the other two phases for any of the phases.

To understand the three phase well, I would sugest you to draw a three sine waves exactly 120deg appart with different colours and then analyse the rise and fall of voltage seperatly from 0deg to 480 deg.
Best of luck.


Joined Jan 16, 2009
Thank you electricmayhem:

I know how the 3 phases works. What I need to know is that why is single phase has two hots (A, B, +N) what phase they have on those wires.

a house has a single phase service.its made up of 2 hot and a enutral wire .each hot wire will be about 120 to 126 volts,120 volts run lits and all 120 volt thongs like a toaster or hair drier ect and both wires that are 120 volts togather will run a electric stove or water heater,electric heat,ect.


Joined Mar 24, 2008
Wikipedia has pretty good articles on 3 phase and power distribution.

In the USA we use something called Split Phase for home use, which is different. Basically two 120VAC legs 180° from each other called L1 and L2. Taken together L1 and L2 is 240VAC, which is good for stoves, dryers, and other high power home appliences. It's been argued successfully at ACC that this is not called 2 phase.