# What is "three-phase" AC power? **URL:** \ **Category:** Factual Questions\ **Created:** [October 28, 2004, 3:28pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677 "2004-10-28T15:28:35Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Bricker](https://avatars.discourse-cdn.com/v4/letter/b/977dab/32.png) [@Bricker](https://boards.straightdope.com/u/Bricker)\ **Post date:** [October 28, 2004, 3:28pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/1 "2004-10-28T15:28:35Z")
Help me sound intelligent as I get ready to attend a meeting on how much money we are going to spend on building an auxiliary server room. What is “three phase” AC power and why is it different from regular AC power. Are there other phases? ---
**Author:** ![Electronerd](https://avatars.discourse-cdn.com/v4/letter/e/8e8cbc/32.png) [@Electronerd](https://boards.straightdope.com/u/Electronerd)\ **Post date:** [October 28, 2004, 3:38pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/2 "2004-10-28T15:38:07Z")
Check out this article: [http://science.howstuffworks.com/power.htm](http://science.howstuffworks.com/power.htm) If you scroll down to the bottom of the first page you’ll see a list of all the pages in the article. One of them pertains to three phase power. ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [October 28, 2004, 3:38pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/3 "2004-10-28T15:38:26Z")
- - - [http://www.google.com/search?hl=en&q=three-phase+power&btnG=Google+Search](http://www.google.com/search?hl=en&q=three-phase+power&btnG=Google+Search) … - In particular: [http://people.howstuffworks.com/power3.htm](http://people.howstuffworks.com/power3.htm) A three-phase plug has three voltage wires and one ground wire, where “normal” outlets are single-phase power, having only one voltage wire and a ground wire. Generally three-phase power is used for power transmission on the utility grid, and for running larger types of electric motors. ~ ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [October 28, 2004, 3:45pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/4 "2004-10-28T15:45:56Z")
The power that comes out of the common wall outlet is single phase AC at 110V. When you need more power than can be safely supplied by a common wall outlet, the usual solution is to use single phase AC at 220V. This is often used for electric ranges. When single phase AC at 220V is insufficient, like for power-hungry industrial equipment, you often see three phase AC at 440V. It can carry substantially more power than single phase AC at 220V. It also requires more wires, four total, one for each phase and one for the neutral/ground. Adding phases and/or increasing the voltage increases the power handling capacity of an electric circuit. ---
**Author:** ![Small\_Clanger](https://avatars.discourse-cdn.com/v4/letter/s/9fc348/32.png) [@Small\_Clanger](https://boards.straightdope.com/u/Small_Clanger)\ **Post date:** [October 28, 2004, 3:54pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/5 "2004-10-28T15:54:22Z")
Three phase AC is like three separate single phase (read nomal AC outlet) each 120 degrees out of phase. It is a more efficient way of supplying the juice. Some high power devices (big motors, LASERs) run directly off a 3-phase supply. You would expect a 3-phase supply to a whole building (there is one where I am now) The individual phases will be fed off to different bits of the building to spread the load. I’ll refrain from adding yet another link. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 28, 2004, 3:58pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/6 "2004-10-28T15:58:59Z")
Here’s the deal: an electrical system or component can be designed to run off “single phase,” or it can be designed to run off “three-phase.” Three-phase is usually (always?) more efficient. Why? Because single phase has a drawback: 120 times a second, the input voltage to the device is **zero**. Zero input voltage = zero input power. This sucks. And for a significant amount of time, the input voltage is very _close_ to zero. Low voltage = low power. This also sucks. If you want a device to be powered from single phase, and you want the device to produce useful output power _all the time_, then the device _ **must** _ have a means of storing energy. This is done with capacitors, inductors, mechanical inertia, etc. There must be a better way. There is: polyphase power. The most common being three-phase. With three-phase power, the device _always_ has power going to it. This decreases (and can even eliminate) the need for the device to employ energy storage devices. The device that profits most handsomely from three-phase power is the **motor**. Trying to start and run a motor from single-phase can be difficult because (as mentioned above) the motor’s input power is zero 120 times a second. So energy storage mechanisms must be employed (capacitors, shaded poles, etc.). A three-phase motor always has input power, and thus is more efficient. ---
**Author:** ![Saltire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saltire/32/553_2.png) [@Saltire](https://boards.straightdope.com/u/Saltire)\ **Post date:** [October 28, 2004, 4:01pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/7 "2004-10-28T16:01:13Z")
I’m sure this is covered in the links others have offered, but I’ll just state it once here in the thread. Single-phase AC constantly varies from +120V to 0V to -120V and back again 60 times a second (in the US, at least). AC stands for Alternating Current, you know. If you graph it, it makes a sine wave. Three-phase power has 3 hot wires, each carrying single-phase AC. If you graph all three together, though, you’ll see that the sine waves are each 120 degrees away from the other two. Thus, with three-phase power, at least one wire is delivering something close to peak voltage at all times. Thus, you can get more power (watts) out of the 3 hots than you could from 3 hots that were all in phase with one another. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [October 28, 2004, 4:08pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/8 "2004-10-28T16:08:26Z")
I thought that your average house had two phases, rotated 180 degrees, which were generally used seperately, and you could combine them to get 240v, right? Wouldn’t that be “two-phase?” ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [October 28, 2004, 4:11pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/9 "2004-10-28T16:11:50Z")
> [@Bricker](#): > > Help me sound intelligent as I get ready to attend a meeting on how much money we are going to spend on building an auxiliary server room. What is “three phase” AC power and why is it different from regular AC power. Are there other phases? Hopefully the prior replies will give you a quick crib on three phase power. OTOH it is far better to keep you mouth shut and let people think you are ignorant or stupid that to open your mouth and confirm their suspicions. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 4:19pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/10 "2004-10-28T16:19:06Z")
Just to round out the discussion, there are two basic 3-phase load (or generator) configurations. One is the Delta, in which the phases are fed to loads arranged in a triangular shape, with the inputs at the vertices like [this](http://www.tpub.com/content/et/14093/img/14093_55_2.jpg). Note that no fourth wire is present, as it is not needed. Such a system may employ a fourth earth ground wire as a safety feature. The other scheme is the Wye circuit, in which the phases are fed to one end of loads connected to a common point, like [this](http://www.tpub.com/content/et/14093/img/14093_55_1.jpg). This circuit normally employs a fourth neutral wire, but need not, depending on whether the loads are expected to be balanced or not. If the loads are balanced, then approximately zero current flows through the neutral. ---
**Author:** ![Uncommon\_Sense](https://avatars.discourse-cdn.com/v4/letter/u/f9ae1b/32.png) [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)\ **Post date:** [October 28, 2004, 4:24pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/11 "2004-10-28T16:24:25Z")
> [@Bricker](#): > > Help me sound intelligent as I get ready to attend a meeting on how much money we are going to spend on building an auxiliary server room. What is “three phase” AC power and why is it different from regular AC power. Are there other phases? Whatever you do, don’t use the term ‘two phase’. Doesn’t exist per-say. You may use two of the three phases to run a motor, heater, power supply, UPS unit, etc, but it is still considered single phase. 120 volt single phase and 230 volt single phase are common terms you will hear tossed around. 120 volt single phase is using one of the phases and the nuetral as the return. 230 volt single phase will use two of the phases and sometimes the nuetral. The typical systems in a building might be the following; 120/208 three phase 120/230 three phase 277/480 three phase whereas the first number before the / = voltage to ground and the number after the / = the voltage between the hot phases again the first number before the / = the single phase voltage and the number after the / = the phase to phase (single phase still) voltage and the line to line to line (three phase) voltage You can sound even cooler if you toss around the fact that you will probably need a battery backup for the servers in the form of a UPS unit. What this does is ‘conditions’ the incoming power to remove voltage spikes or uneven sine waves while maintaining constant stable power to the servers via a bank of batteries. These batteries can come on-line the instant the power goes out and will maintain power to the servers as long as it is designed to (usually about 30 minutes). This way you have a power conditioner and a means of power backup. Some places even have a generator in case the power is down longer than 30 minutes. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 28, 2004, 4:25pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/12 "2004-10-28T16:25:47Z")
> [@friedo](#): > > I thought that your average house had two phases, rotated 180 degrees, which were generally used seperately, and you could combine them to get 240v, right? Wouldn’t that be “two-phase?” Not really. You _can_ think of it that way, but that’s not a particularly useful classification. What you really have is a single, center-tapped 240 V phase–that is from line to line, there is 240 V, and between each line and the neutral CT, there is 120 V. Some homes, on the other hand, are fed with two phases of a three-phase system, with the line to neutral voltage being the normal 120 V and the line to line voltage being SQRT(3) times that, or 208 V. You may see some high-power equipment, like air conditioners and stoves labeled “INPUT: 208/220 VAC”–and that’s why. ---
**Author:** ![Uncommon\_Sense](https://avatars.discourse-cdn.com/v4/letter/u/f9ae1b/32.png) [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)\ **Post date:** [October 28, 2004, 4:40pm UTC](https://boards.straightdope.com/t/what-is-three-phase-ac-power/271677/13 "2004-10-28T16:40:11Z")
> [@friedo](#): > > I thought that your average house had two phases, rotated 180 degrees, which were generally used seperately, and you could combine them to get 240v, right? Wouldn’t that be “two-phase?” No, In the sense that when you cut an apple in half you don’t have two apples. [Single Phase](http://www.wordiq.com/definition/Single_phase_electric_power) [Two Phase](http://www.wordiq.com/definition/Two_phase)