# Why aren't there any 3-phase domestic power-outlets, or any interest in 3-phase for household (mains) power or anything relatively low-power?

Since I discovered what 3-phase electrical power is on Wikipedia, I have noticed that it's associated with very high-power applications. I have not been able to find anything on the subject of 3-phase power for anything but very high-power applications, and it appears that the only 3-phase power outlets in existence are meant for ultra-high-power industrial equipment. I find this strange because I imagine (lower voltage) 3-phase would be better than single-phase for household power outlets.

The main reason that I think 3-phase would be better, is that it could be rectified into cleaner DC power. 3-phase consistently provides power while single-phase repeatedly goes down to 0 volts before rising back up. 3-phase would naturally make less power-supply noise, so we could have cleaner power with smaller capacitors. I make guitar pedals as a hobby, and power supply noise is quite a nuisance.

It might possibly be safer, as the AC voltage could be lower (in case if two wires are shorted together by a person), but it could still have a higher DC output power with a 3-phase rectifier, and possibly other things.

So why is it that home power outlets aren't 3-phase anywhere in the world, or even interest in it?

• I just spun up a 1hp (750W) three phase motor earlier this evening. Using a single-phase to three-phase converter. Lots of shop tools (mills, lathes, and tablesaws) use 3 phase power. I would not call them very high-power or ultra high-power. – mkeith Jul 24 '16 at 8:31
• Sure there are! Google IEC 60309. All civilized countries have them. How else would we be able to run welders, wood processors, conveyors, pressure washers and what not? – winny Jul 24 '16 at 8:34
• "... it appears that the only 3-phase power outlets in existence are meant for ultra-high-power industrial equipment ..." - I don't think you know what ultra-high-power means :P – marcelm Jul 24 '16 at 11:17
• I suggest a post showing the power input filter you have chosen for your guitar pedals. There are many people that could help with such a thing here, but few that could rewire a nation for 3 phase. – Eric Urban Jul 24 '16 at 12:18
• Are you implying that in your country you do not have 3-phase power for houses? My country is poor (Serbia) but only very old houses do not have 3-phase power supply. It is mostly used for devices which use more than 3KW like heaters, kitchen ovens or similar, but also mono-phase loads are distributed among phases (different single-phase outlets are on different phases). Also it is important to note that voltage between 2 phases is higher than between phase and neutral line by factor of $$sqrt(3)$$ (400V vs 230V in most of Europe). – Darko Jul 24 '16 at 17:29

Cost.

• A three-phase distribution system would require four wires to each house - the three phases and a neutral connection. Single phase only requires two.
• A three-phase fuseboard would be required.
• Four wires plus earth would be required in the domestic wiring.
• Five-pole connectors would be required.

Figure 1. 3-phase connectors - European and North American.

... and it appears that the only 3-phase power outlets in existence are meant for ultra-high-power industrial equipment.

It's usually worthwhile above a couple of kW if three-phase power is available, as it is in most industrial and commercial buildings.

The main reason that I think 3-phase would be better, is that it could be rectified into cleaner DC power. 3-phase consistently provides power while single-phase repeatedly goes down to 0 volts before rising back up.

This is true.

3-phase would naturally make less power-supply noise, so we could have cleaner power with smaller capacitors.

This may not be true. In single-phase and 3-phase the diodes only conduct close to the peak of the voltage waveform when the incoming voltage exceeds the capacitor voltage. This results in pulses of current on each phase and causes odd harmonics on the current waveform.

Figure 2. A VFD drive showing the rectifiers, a filter to reduce noise, the six switching transistors to PWM the current to the 3-phase motor. Source: ECMWeb.

Variable frequency drives (VFD) use that principle to rectify the mains and generate a high-voltage DC supply internally. As you stated, the capacitor value can be much smaller when using a 3-phase supply than it could with a single-phase supply. VFDs are commonly available in the range 1 kW to hundreds of kW.

It might possibly be safer, as the AC voltage could be lower (in case if two wires are shorted together by a person), but it could still have a higher DC output power with a 3-phase rectifier, and possibly other things.

That sounds attractive but the advantage of higher voltage is that currents are reduced and voltage drops and power losses along the wires are reduced. In addition, we still need some high-power loads such as water heaters in immersion heaters, washing machines and dishwashers.

I make guitar pedals as a hobby, and power supply noise is quite a nuisance.

Figure 3. A 3-phase transformer.

Not as much of a nuisance as requiring a three-phase transformer. I don't think anyone makes 3-phase wall-warts! ;^)

So why is it that home power outlets aren't 3-phase anywhere in the world, or even interest in it?

Still cost.

• It would require 4 or 5 wires, depending on the presence of a neutral. Ground is usually the required fourth wire after the three phases. – user2943160 Jul 24 '16 at 20:21
• I took it as understood that neutral would be required for single-phase items. They could, of course, be wired phase to phase. – Transistor Jul 24 '16 at 20:29
• Ah, I simply didn't read your first bullets thoroughly enough. Everything's good. – user2943160 Jul 24 '16 at 21:02
• So it sounds like the fixed cost would increase 25% with the addition of a 4th wire. Is there a reduction in the recurring electric power cost due to energy savings? – nu everest Jan 2 '18 at 18:01

For household, an important consideration is the cost of wiring. If you have two more copper wires to bring to each socket, it becomes more expensive. The energy meter would be much more complex and expensive as well. There would be three times the fuses on the fuse board (which would also have to be much bigger), etc...

Moreover, given the relatively low currents required for household appliances, it is just not necessary.

The "cleaner" DC rectification you mention can be obtained with a single phase just by using bigger capacitors anyway. And the transformers for one phase are much simpler to manufacture than for three phases, and you need less diodes to rectify it.

From a safety point of view, three phases provide no improvement. It wouldn't allow using lower AC voltages (why would it?). And it just multiplies the risks of miswiring a lot.

Last thing: how would you deal with wiring a simple lamp? Having some complex circuitry to spread the load over the three phases? Choosing one single phase for each lamp, trying to balance the number of lamps equally for each phase? Can you imagine how complex it would be to wire a house, then?

Honestly, I think it's a good thing that we have to deal with only one phase.

• It is very handy for motors, though. No starter capacitor needed. reversing is easy. – mkeith Jul 24 '16 at 8:40
• Universal single phase motors work well enough for household appliances. They are not more expensive to produce. Reversing is as easy. Varying the speed is easier. – dim Jul 24 '16 at 8:46
• True enough. I think for people who have workshops, three phase might make sense. Not so much in the house. – mkeith Jul 24 '16 at 16:02
• @mkeith: Yes, that used to be a big problem. VFDs (variable frequency drives) have come to the rescue for moderate loads. They can take a single phase in, generate a three-phase out and give a soft start so that the lights don't dim every time you start the lathe. – Transistor Jul 24 '16 at 20:33
• How to wire a lamp 3-phase? Easy; use a 3-phase bridge rectifier. Powering a lamp with DC is better as it wouldn't have the 100hz/120hz flicker caused by AC. If it's LED, it would need to be rectified anyway. – Electric-Gecko Apr 26 '18 at 4:10

I think our power distribution system here in New Zealand is engineered similarly to most of the first world.

• Three-phase 50Hz power is delivered to a transformer that typically supplies a couple of streets.
• Phase to neutral voltage is 240V, and the phase to phase voltage is 440V. (I'm aware that some countries, notably USA, use 110V/60Hz rather than 240V/50Hz)
• Each house is hooked to one phase and neutral.
• Premises with very high power requirements can opt to have a three-phase connection. This is basically to keep the load on the phases balanced. Unbalanced loads upset transformers and create a difference current in the neutral wire (there is no neutral wire current for a perfectly balanced load). Also, heavy load customers are likely to be using large electric motors, and there are advantages of 3-phase motors compared to single phase.

So what are the downsides of a three-phase connection?

• The familiar single-phase 3-pin plug (phase, neutral, earth) becomes a 5-pin plug (3 phases, neutral and earth).
• More expensive in-house distribution due to the extra wires required.
• More expensive metering. I seem to remember seeing 3 separate meters in premises that had 3-phase power, but that was a few years back.
• Simple appliances (e.g. Toasters, hair dryers, heaters) become a lot more complex. Generally a 3-phase version would need three elements.

You are right that the rectified 3 phase power is smoother than single phase, and smoothing capacitors would be smaller. But the ripple would be at 300Hz, which is a more audible frequency which could be a downside.

I don't think many power distribution authorities would be interested in using a lower voltage for safety reasons. They want to keep the voltage high to minimize losses and to keep wire sizes down. Our 240v network would need to go down to below 100v to be completely safe.

• Generally, 'safe' AC is limited to 60V peak-to-peak, I think. – user2943160 Jul 24 '16 at 20:23
• Wait; But doesn't the high distribution voltage go down when it reaches your house, through a transformer in each house or one per city block? – Electric-Gecko Jul 25 '16 at 4:06
• It makes sense to me that some appliances (toasters, heaters) don't make sense 3-phase. If regular household power outlets were 3-phase, then these appliances would only use one of them. I realise that this would cause unbalanced loads. But supplying each house one of 3 phases would also cause unbalanced loads, as some houses use more power than others. So I don't see how 3-phase outlets would make the problem worse. – Electric-Gecko Apr 26 '18 at 4:25
• @electric-geco The answers you have list the pros and cons quite accurately. You may have a different view and that is your right – kiwiron Apr 26 '18 at 7:10

In our country we have the possibility to choose whenever you want 1 phase (1x25A) or 3 phase (3x20A) domestic input netork. The price is higher for 3x20A, then you ussualy spread those 3 phases equaly over your house, or connect 3 phase load. Note that kitchen top cooker has the possibility to connect it on 3 phase network, because it is rated 7kW power. When high currents come into play, there are no suitable domestic outlets as they are rated to 16A. Usually there is a connection box, where there is a fixed connection between house and load wires.

• Saying "in our country" is completely useless unless you specify where you are. We can't read minds. – Someone Somewhere Sep 21 '18 at 10:43

So why is it that home power outlets aren't 3-phase anywhere in the world, or even interest in it?

It is available in many places in the world as standard delivery, even to residences. Here in North America most utilities will allow you have it if you ask, but most of the time they will require that you pay for the extra wiring it will take to get to the nearest 3 phase source, and that is almost always cost prohibitive, i.e. thousands or even tens of thousands of dollars. It also typically comes with something else that residential users don't have to deal with, a Demand Meter and possibly a Power Factor meter, both of which mean penalties if predetermined values are not adhered to. When people look into it, 99.99% of them don't go through with it.

3ph is best for the network (better balance on loads across phases), it's also useful in electric motors (1ph cannot start a motor without some some 'aid' -> capacitor, while 3phase can self-start an asynchronous motor).
Here (Italy) 3 phase, since little time, is available at no extra cost than 1ph (just 27€ to replace the meter for a household), formerly it had a monthly fee that was higher. Anyway almost all houses here have 1ph service because of 'traditional higher cost'. Only few new building designed for heat-pumps and electric stove begin putting 3ph.