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Single and Triple Phase? Answered

Ok, so I thought I knew how the power phases worked and then I got myself confused.  Our house has three phases and it offers power at 120v, 240v, and 480v (or at least can I think).  All three wires on the power poles go to transformers and then to the house and so on.  I have seen houses that just use one which I think I understand unless they try to get a dryer then what do they do.  I am going to ask a little more than phases here to just to make sure I get this.  In you wall outlet one is from a line on the power poles correct? and the other is ground which works because at the generator site the electricity is grounded on one side.  Now before I get ahead of myself, how can there be single and triple phases in a 120v outlet and what exactly are they.  Now, at a 240v outlet you have two hot wires coming from the power lines right, and then one ground.  Now I think all the lines on the poles come down to 120v, so how do you get 480v from three 120v lines.  3*120 does not equal 480v.  I am confused.  Also will having triple and single phase even effect how things work.  Like we have a 120v single phase compressor and it works just fine at our triple phase hose.  So it doesn't really make any sense at all.


In the USA, power is delivered to most houses as the secondary of a transformer, which is centre tapped, so if you put a meter on it, you'd see 120 - 0 - 120 V. Your outlets are connected from 120-0 and (usually upstairs) 0-120 V.

Your 240 V stuff is taken directly across the transformer.

This looks interesting, but what exactly do you mean across the transformer. Do you mean from one transformer to another (we have three.)

Here's a diagram of what you probably have, done with three transformers.


three phase.jpg

Yes, if you look at my diagram below, you can MAKE a system using three transformers. Can you see how ? I can modify my sktch to explain if you like, but you can strap three together to do it. Transformers are very versatile things !!

In US practice (AFAIR), the neutral is usually tied to a big A ground rod, with the ground at the transformers


I am a journeyman electrician. This should solve all of your questions. 480 volts is only a 3 phase service. It's actually 277/480. You don't multiply 277 x 2 to get your 480, because it is 3 phase. Instead you multiply 277 x 1.732 (square root of 3) = 480

your house is not 3 phase. It is 120/240. You will not be able to get 480 from it. The other phase is a neutral.

Another response to your question, I have heard some electrician say they have a hard time understanding the two triple phase transformers. Y transformer gives you a 110 volts on the one leg, 208 volts on the second leg and first and 277 volts with the three legs used in commercial applications. The other one I don't know what it is called gives you 240 volts 480 volts and 880 volts and I could be wrong because I haven't worked around industrial much and I am not a electrician but was a superintendent for thirteen years and picked up only a small amount of what triple phase is and enough to know it is very complicated and confusing even for those who do it for a living. Single phase is very simple and easy to understand but for safety you need to know what you are doing or have someone who does know what they are doing to give you advice and guidance. A transformer is actually simple at a simple step down or step up. In a step down and keep in mind voltage is always consistent when rated at any given point where amperage is not and has only one limit and that is it soarce. In a step down you taking a higher voltage and stepping it down as the name infers, step up same thing you are stepping the voltage up. You have two windings that set next to each other but not so close that the electricity can jump. The first winding is exactly in relationship to the ratio of the voltage change meaning when the power goes through the first winding that has two times the amount of length in wire as the second one then the voltage is cut in half, the amperage capacity is doubled because the wattage soarce is not effected. The opposite is true in a step up transformer and in triple phase it is very complicated but goes along the same lines and the length of windings is in direct relationship to the voltage on each leg but with triple phase only one leg come in so how they change the phases I have no idea. It doesn't come from the power plant.

Single phase is used in residential because triple phase is very dangerous using high voltage and high ampereage and around children is fatal. Triple phase uses higher voltage because there is less resistance in higher voltage lower ampereage. It is relative, wattage is the unit in which electricity is rated and measured in. Wattage is voltage times ampereage and is a constant in every situation. In motors single phase a five horse power motor is the limit because it more cost effective to change to triple phase than increase the copper gauge in the windings of a motor to allow for the heat created by the higher ampereage. Lighting in commercial and light/heavy industrial is 277 volt because it uses less ampereage less heat that burns up lighting. The other advantage to triple phase is that each leg is out of phase less. In a full rotation of 360 degrees a circle basically a leg is in phase once so the flow of electricity is in phase three times as opposed to twice and once with single phase. Devices run more efficantly and so saves energy, saves money. There is so much to electricity that unless you are educated on it or supervised by someone who is stay away from it unless you like the pretty fire created by watching all your possessions go up in smoke or better you like getting shocked and not in a good way.

OK, Here is an answer which is logically correct, but may NOT be current US practice: certainly it WILL give you what you want.

See how the tappings work ? You can get a domestic supply from any pair of them.


three phase.jpg

I get this whole thing with coils in the generator and so on, but I mean how does the phase actually affect things in your home like wall outlets?

. Your 120 outlets will be single-phase. You do not run all three phases to the outlet. If you do have three-phase incoming, it's a good idea to put 1/3 of your 120 load on phase 1, 1/3 on phase 2, &c, just as it's a good idea to balance the load on each side of 220.


As Nacho points out there are VERY few places where a domestic load needs to be split in three phases, but a farm setting its more likely, because of the size of your loads. Its very bad practice to mix phases within a property - because that would mean you could have 360 V between adjacent outlets.


.  I doubt if you have 3-phase/480V power run to a residence. It's possible, but not very common.
.  You probably have an arrangement much closer to the 120-0-120 steveastrouk outlined on Mar 25, 2011 at 2:50 AM.
.  If you do have 3-phase/480 incoming, then it is being transformed somewhere to get the 120 for your receptacles. Same steveastrouk's comment, but you provide the transformer.

I am not really residence, were on a farm/ranch. We do have 3-phase, but not necessarily 480v. We can put in 480v outlets, and so can some other places, but there aren't any more than 3 wires anywhere in the US so is one 240v or is 480v special.

480V is an odd one, because your phase voltage would be 480/ root(3) or 277 Volts. 380 is believable.


Well, I am not sure if we can actually get 480v, it wouldn't be 380 either, it would be 360 ish. I just wondered because there are welders that run at 480v so I am confused.

Yeah, 360's highly likely - I bet you guys are on the end of long wires from the network aren;t you ?

yea, but there is a plant withing 75 miles ish

. How close you are to a generating plant is unimportant, it's how far you are from a local distribution point ("the network", as steve puts it), the voltage at that point, and the condition of the lines from there to you.

Ok, think I get what you mean, like a substation that steps down the voltage, we are within 10 miles, they are all over.

. Take your voltmeter and get readings for phase-to-phase and phase-to-ground on all legs.

. Yes. Very carefully. If you are not used to working around high voltage, get some help from someone who is.

We have a farm/ranch, so we run welders, big fans, augers, and more.

Really old houses do. My folks have incoming three phase, but only one phase is wired.