3-Phase Explained ( Commerical Electrical )

Hey Guys,

Came accross a nice explanation on 3-phase to help those of you who actually do commerical inspections and would like to REALLY know more about 3-phase. Hope you enjoy !

Most alternating-current (AC) generation and transmission, and a good part of use, take place through three-phase circuits. If you want to understand electric power, you must know something about three-phase. It is rather simple if you go at it the right way, though it has a reputation for difficulty.
Phase is a frequently-used term around AC. The word comes from Greek fasis, “appearance,” from fanein, “to appear.” It originally referred to the eternally regular changing appearance of the moon through each month, and then was applied to the periodic changes of some quantity, such as the voltage in an AC circuit. Electrical phase is measured in degrees, with 360° corresponding to a complete cycle. A sinusoidal voltage is proportional to the cosine or sine of the phase.
Three-phase, abbreviated 3φ, refers to three voltages or currents that that differ by a third of a cycle, or 120 electrical degrees, from each other. They go through their maxima in a regular order, called the phase sequence. The three phases could be supplied over six wires, with two wires reserved for the exclusive use of each phase. However, they are generally supplied over only three wires, and the phase or line voltages are the voltages between the three possible pairs of wires. The phase or line currents are the currents in each wire. Voltages and currents are usually expressed as rms or effective values, as in single-phase analysis.
When you connect a load to the three wires, it should be done in such a way that it does not destroy the symmetry. This means that you need three equal loads connected across the three pairs of wires. This looks like an equilateral triangle, or delta, and is called a delta load. Another symmetrical connection would result if you connected one side of each load together, and then the three other ends to the three wires. This looks like a Y, and is called a wye load. These are the only possibilities for a symmetrical load. The center of the Y connection is, in a way, equidistant from each of the three line voltages, and will remain at a constant potential. It is called the neutral, and may be furnished along with the three phase voltages. The benefits of three-phase are realized best for such a symmetrical connection, which is called balanced. If the load is not balanced, the problem is a complicated one one whose solution gives little insight, just numbers. Such problems are best left to computer circuit analysis. Three-phase systems that are roughly balanced (the practical case) can be analyzed profitably by a method called symmetrical components. Here, let us consider only balanced three-phase circuits, which are the most important anyway.


The key to understanding three-phase is to understand the phasor diagram for the voltages or currents. In the diagram at the above, a, b and c represent the three lines, and o represents the neutral. The red phasors are the line or delta voltages, the voltages between the wires. The blue phasors are the wye voltages, the voltages to neutral. They correspond to the two different ways a symmetrical load can be connected. The vectors can be imagined rotating anticlockwise with time with angular velocity ω = 2πf, their projections on the horizontal axis representing the voltages as functions of time. Note how the subscripts on the V’s give the points between which the voltage is measured, and the sign of the voltage. Vab is the voltage at point a relative to point b, for example. The same phasor diagram holds for the currents. In this case, the line currents are the blue vectors, and the red vectors are the currents through a delta load. The blue and red vectors differ in phase by 30°, and in magnitude by a factor of √3, as is marked in the diagram.
Suppose we want to take two phase wires and neutral to make a three-wire household service supplying 120 V between each hot wire and ground. The neutral will become the grounded conductor, the two phases the hot conductors. Then, the wye voltage is 120, so the delta voltage will be √3 x 120 = 208 V. This is the three-phase line voltage necessary in this case. Note that the two 120 V sources are not opposite in phase, and will not give 240 V between them. On the other hand, suppose we do want a 240 V service. Then this must be the line voltage, and the voltages to neutral will be 139 V, not 120 V. A 120 V three-phase service will give only 69 V from line to neutral. Note that √3 appears everywhere, and that the differences in phase explain the unexpected results.

In Germany and Switzerland, where three-phase power was originated and developed, it is known as Drehstrom, “rotating current” for this property of constant power. Ordinary AC is called Wechselstrom, or “change current.” Nikola Tesla, the discoverer of polyphase currents and inventor of the induction motor, employed two-phase current, where the phase difference is 90°. This also can be used to create a rotating magnetic field, and is more efficient than single-phase, but is not quite as advantageous as three-phase. Two-phase power was once rather common in the United States, where Tesla was important in the introduction of AC, but has now gone completely out of use.
Two-phase can be supplied over three wires, but there is no true neutral, since the phases are not symmetrical. However, it is always easy to double the number of phases in a transformer secondary by making two secondary windings and connecting them in opposing phases. Four-phase does have a neutral, like three-phase, but requires four wires. In fact, three-phase is more economical than any other number of phases. For applications like rectifiers and synchronous converters where DC is produced, it is most efficient to use six-phase AC input, which is easily produced from three-phase in a transformer.
If you are transmitting a certain amount of power single-phase, adding one more conductor operated at the same line voltage and current and using three-phase will increase the power transmitted by 72% with only a 50% increase in the amount of copper and losses. The advantage is obvious. Under certain conditions, transmitting a certain amount of power by three-phase only requires 75% of the copper of single-phase transmission. This is not the major advantage of three-phase, but it does play a factor.
Three wires are usually seen in high-voltage transmission lines, whether on towers or poles, with pin or suspension insulators. Some high-voltage lines are now DC, since solid state devices make it easier to convert to and from AC. The DC lines are free of the problems created by phase, as well as eliminating the skin effect that reduces the effective area of the conductors. It is not nearly as easy to manage long-distance electrical transmission as might be thought.

I have been installing 3 phase systems for more than 20 years and do not know that much about it.:smiley:

A little more than basic info.

How 'bout this, single phase is two hots and three phase is 3 hots.:cool:

You really want to have fun explain why the two hots at a 240/120 service are only single phase and not ‘two’ phase.:mrgreen:

lol…I could TRY but man I can imagine the FLACK i would get…and It would be quoted as…

“Paul, We are HI’s not Electricians…lol…” well guys I am a HI also and I find this stuff soooo FUN to mess with.

Man…I just love my job…

I did a seminar last week to the apprentice program on Delta/Delta & Delta/Wye Transformers…and how to balance the transformers and development of the panel schedule…

Man…Bob…you could hear a PIN drop when I started talking about 3rd and 5th harmonics and so on with Wye transformers and well it was a boring class…I put them to sleep with all the technical mumbo jumbo.

Oh…I forgot…when one of the guys yelled out…how do you size the neutral on a 120/208 Wye system…I went into the formula ( A(2) + B(2) + C(2)) - (AxB + BxC + AxC) and squared rooted on down( the number in (2) is squared to 2…I am sure you are familiar with it just cant seem to use the right things here as the keyboard wont allow it…lol

Anyway…we did this and had to explain why it is different when calculating a Neutral on a Delta versus a Wye…and so on…

Anyway…I then stopped and said…Ok guys…if we have a 480V primary Line Voltage…what is the Primary Phase Voltage…and over HALF said…240V…I knew it was going to be a LONG day !

I don’t think even Pam Anderson would keep me awake if she was talking harmonics.

I once sat in on a large meeting of electrical testing contractors, electrical engineers from different firms, a bunch of other contractor reps etc. The meeting was about how we where going to chase down possible 166th order harmonics I just nodded a lot and asked what panels they needed open.:roll:

lol…I think on issues of THOSE harmonics even I would have walked out of the room. lol…The only POSSIBLE way to achieve that level of harmonics is the panel schedule for one is totally screwed up and the well…I cant concieve that level of harmonics as I am sure the neutral would simply MELT from the terminal block…and I can almost BET they are replacing transformer phases like they do their underwear.

Dude…My harmonic rises just thinking about sitting in a room with Pam Anderson, Even IF tommy lee was married to her.

Hey Thanks Paul, Now I understand - :mrgreen: think I will have a RR drink now;)

Hey Kev,

It is kinda one of those things that if you are a HI and you have a broad understanding of the basics of electrical theory and what 3 phase is and a little of how it works it can only increase knowledge and knowledge is KING.

Can you imagine the feeling it gives you to explain something to someone and they go…YEAH…I get it…now maybe not ALL of it but they dont need to understand all of it…lol…you know what I mean.

Like Harmonics…heck industry experts still do not understand the entire effects they have but as a HI who puts is thermal temp gun on a neutral wire in a 3 phase WYE panel and wonder why the heck the neutral is charred or discolored need only slap a amp probe on that sucker…not because you have to ( ie: SOP ) but because you WANT to…You just GOTTA know and guess what…if you are doing commercial 3 phase inspections…You just may NOW know something the actually electrician who wired it did not.

So if anyone ever wants to know more on how transformers work and how to balance out panels and to compute Phase Current and Phase Voltage and KVA of motors…HOLLAR at BOB…thehehhe…no really if anyone ever did want to know effects of things like this they can always call me…as you can see I like to talk. ( or type )

Thats the things I like to teach…not how to be an electrician but how as a HI you can learn to test just about anything and look GOOD doing it. Most of my seminars are like that…based for the HI not electricians.

No bash Paul, I agree with you . you can never learn to much

I have to admit however…the original post goes well beyond what is normally needed to know when dealing with 3 phase and transformers but rather than ME type it all out this one was close enough.

I may edit it to get rid of the wasted fluff that will only confuse.

It is good info Paul.:slight_smile:

Did you say 3 phase transformer?

Here is a nice one.


Is that in the place that you work Bob…?

Well…can be both 1 Phase and 3 phase transformers…used in both applications actually.

If the transformer break down is lets say A-21, B-23,C-23 then you can use either (3) 25KVA Single Phase Transformers or (1) 75KVA 3 Phase Transformer.

What is the size of the one in the picture…looks large…150KVA +

I am sorry the picture is bad, it is a scan of a photo.

That is a 1000 kva 480-208Y/120 transformer it was about 6000 lbs.

This was a service change I did a few years ago, 4000 amp 208 service replaced with a 3000 amp 480 service. The transformer re-fed the old 208 service.

Fun job with lots of copper scrap.

lol…bet you wish you had those scraps at TODAYS prices…lol.

Now thats a HUGE transformer which is indeed due to the massive motors you all deal with.

Curious question…why did they choose to go with a WYE setup with I would guess very little general lightning and applicance loads, I would think a Delta/Delta 480 - 240/120 would be a more economical setup for them…unless this massive sucker is in a office building which means it is a HUGE one.

I do not run into many Delta in this area at all and I can not think of a single building the company I work for that was not a Wye service.

This building was unusual, the tenant we where working for had the first floor and the service I did only supplied that Tennant.

The upstairs tenant had their own services.

They had one 4000 amp 208Y service located beside mine on the ground floor.

Then they had 13.8 KV run to the roof to supply three more 4000 amp 480Y services on the roof.

It was a large building and the upstairs tenant was some sort of data center.

We do some very large buildings, we did one in PA with seven 3000 amp 480Y services.

I think I can link you to a good picture of PA


It happens Terraserver has a photo taken during constrution, you can see the ell shape of the main building.

Please do so…

On the data center…Can you possibly imagine the harmonics that could take place with all the computer systems being plugged in. I hope the neutral was atleast double sized on that one feeding the data center…thehehehe

I tell you…we have come a long way in knowing the effects of harmonics in the way of heat over the past 10 years…only seems to really present a problem on unbalanced Wye systems…but I have heard some peers say Delta are prone to it as well…but since in deltas the neutral cancels out to balance itself not nearly the issue which is why I think in the NEC all the references to harmonics point to WYE systems only.

I can only imagine the time you spend balancing that out if you had to do the panel schedule on that. I did one nearly that size some 15 years ago and the electrical engineer made ME size out the transformer, panel schedule and balancing when normally it is done on the plans by the engineer…talk about sweating bullets ( i was only 21 years old heading up that job )

COOL bob…but you have NOW damaged me…I can’t stop playing with the Taraserver thingie…I have now looked at all my old girlfriends homes so see if I happen to be in the picture…the moment their father threw me out of the house…

90% of the buildings we build will have 480Y/277 services with the neutral conductor size matching the other conductors.

We will bring 480 delta right to the computer rooms to feed a “K” rated transformer. Once we leave a 480 delta 208Y/120 transformer we will often be running double neutrals for the harmonic issues.

Often the specs will require separate neutral for each branch circuit or if we can run multiwire branch circuits the neutral will be one size larger than the other conductors.

The panels that feed the non-linear loads will be ordered with 200% neutral so a 225 amp panel ends up with a 450 amps neutral bar fed with double neutral conductors.

Yes Terraserver is a big distraction.

I can find my own house, most photos are from about 1995