13
\$\begingroup\$

My understanding is that in the US (and Canada?) residential units receive split-phase 120V/240V power meaning the voltage between two phases is 2x the line-to-neutral voltage. Here in Europe residential units receive 3-phase 230V/400V power meaning the voltage between two phases is only 1.7x the line-to-neutral voltage.

Is there a reason for the European arrangement? If Europe also used split phase power then we could get one of these:

  1. 200V/400V power meaning most appliances would be safer due to the lower line-to-neutral voltage while power-hungry appliances (like stoves) could still get the same 400V as before.
  2. 230V/460V power meaning power-hungry appliances (like stoves) could get 60V higher voltage without there being a need raise the line-to-neutral voltage so most appliances would be just as safe as before.
  3. Some intermediate option between these.

To a layman like me 2x just seems better than 1.7x. Surely no one has an AC motor at home that would actually need all 3 phases? Is there some reason for delivering 3-phase power to households I'm not seeing or what is the reason for the European arrangement? Is it just an artifact of history that's too costly to change now?

\$\endgroup\$
3
  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Commented Jun 30, 2022 at 4:52
  • 1
    \$\begingroup\$ Most 240 V single-phase things in the US are either also rated to run on 208 V (120·√3), or come in 208 V versions, because apartments commonly use a three phase supply (120 V L-N, 208 V L-L) to the whole complex, then two of those phases to each apartment instead of split-phase. \$\endgroup\$
    – Hearth
    Commented Jul 3, 2022 at 17:08
  • 1
    \$\begingroup\$ Worth mentioning that hardly anyone wires motors directly to 3-phase these days. You’ll use a VFD which converts whatever you have to 3 phase at an arbitrary frequency. \$\endgroup\$
    – Navin
    Commented Jul 28, 2022 at 6:05

6 Answers 6

22
\$\begingroup\$

Three phase power is better in almost every way except the costs of the wiring to your premises, because it needs more conductors. In Europe thinner conductors due to the voltage, so the comparison is not direct.

Three phase power for residential in the Netherlands is 4 wires (L1-L2-L3-N) and an earth rod per house. This gives you 230V L-N and 400V L-L. Good for almost 17 kW with a typical 3x25A fusing on just 6mm2 cabling (10 AWG).
This is the default for new houses, and the wiring is there since 1960, they just never installed the other two fuses.
There is a HV-LV transformer per suburb, not per house.

Compared to 17 kw single phase which would give you a 1x75A service. That's a range where wiring and protection circuits become significantly expensive.

It also provides you with the possibility of running three phase motors, which is especially beneficial with high performance HVAC systems (which we use also for heating) or people with small workshops in the garage. Some big houses also have pools and flow-through shower heaters or sauna's that reap the benefits of this. These things require lots of power.

Also the possibility to put those heavy loads such as induction cooktops or PV inverters on all three phases balances the local grid preventing voltage sags.

I forgot the most high power load: EV-charging.

We can get up to 3x80A, but most people are on 3x25A or 3x40A. Which costs you 250 or 1000 euros per year. More than enough power, just don't turn everything on at once.

\$\endgroup\$
11
  • \$\begingroup\$ You said: "Some big houses also have pools and flow-through shower heaters or sauna's that reap the benefits of this. These things require lots of power." I think this part isn't really relevant to my question? With split-phase you can also get a lot of power by using two live wires with opposite phases (and the voltage would be 2x the L-N voltage rather than merely 1.7x). However, I guess if the HVAC systems, which you also mentioned, actually benefit from motors using all 3 phases of mains voltages then that's a good answer to my question. \$\endgroup\$
    – QuantumWiz
    Commented Jun 28, 2022 at 12:50
  • \$\begingroup\$ Why this is better? Because of reduction of phase split circuits? \$\endgroup\$ Commented Jun 28, 2022 at 14:21
  • 2
    \$\begingroup\$ Newer HVAC (especially larger, high end) systems tend to use variable speed compressors and fans (since they are much more efficient), so you probably would not specifically choose three phase for a new HVAC if the installation was small enough that the power company would let you use single phase. Otherwise you're just paying for more wiring that the DC power supply in the unit won't really benefit from. \$\endgroup\$ Commented Jun 28, 2022 at 14:46
  • 7
    \$\begingroup\$ @user1850479 3-phase is beneficial in variable-speed motors as well, you use smaller capacitors and the PFC part is somewhat easier. The same goes for EV chargers. \$\endgroup\$
    – fraxinus
    Commented Jun 29, 2022 at 11:28
  • 1
    \$\begingroup\$ @QuantumWiz Even if it goes to a 3phase bridge rectifier, the resultant ripple is much less; it never goes to zero. \$\endgroup\$
    – Aaron
    Commented Jun 30, 2022 at 12:43
18
\$\begingroup\$

120V is not a practical voltage for the more powerful household appliances, which is why 120V countries also get 240V split phase. Since most houses there have both 240V and 120V, manufacturers can sell powerful appliances in 240V and not bother to make 120V versions.

In 230-250V land, there is no need for that, so all appliances use 250V. And since few houses have three phase, no-one bothers to make three phase versions of most household products, unless it's justified by the power use of the appliance.

Split phase creates a strange situation in 120V land where you get all the annoyances of three phase power:

  • More wiring
  • Different, incompatible sockets
  • Got to balance the loads

...without the main advantage of three phase, which is that, if the load is balanced, power flows continuously instead of 100-120 times per second. This means, according to Wikipedia:

As compared to a single-phase AC power supply that uses two conductors (phase and neutral), a three-phase supply with no neutral and the same phase-to-ground voltage and current capacity per phase can transmit three times as much power using just 1.5 times as many wires (i.e., three instead of two). Thus, the ratio of capacity to conductor material is doubled.[9] The ratio of capacity to conductor material increases to 3:1 with an ungrounded three-phase and center-grounded single-phase system (or 2.25:1 if both employ grounds of the same gauge as the conductors). This leads to higher efficiency, lower weight, and cleaner waveforms.

(Note your 1.7x factor is wrong).

Basically, three phase would be better for everything, including switching power supplies because it gets rid of most of the big smoothing caps. However it is not practical to use at low power, due to the extra expense in wiring, plugs, sockets, PFC units, transformers, assorted electronics, etc.

However, if you get a big motor, or a big motor with a variable frequency drive (most commonly known as "heat pump") then three phase is a much better option. You just don't run a 10kW electric motor without three phase, and if you do, it's damn ugly.

\$\endgroup\$
15
  • 3
    \$\begingroup\$ You said: "Note your 1.7x factor is wrong". However, I'm pretty sure you're mistaken here. With 3 phases the angle between the phases is 120°. Thus the voltage between two phases should be √(2-2*cos(120°))=√3≈1.73 times the line-to-neutral voltage. 1.7 seems correct (to two significant figures). \$\endgroup\$
    – QuantumWiz
    Commented Jun 28, 2022 at 13:00
  • 3
    \$\begingroup\$ @QuantumWiz 1.7 is wrong because you are using it in the wrong way. See my answer \$\endgroup\$
    – DKNguyen
    Commented Jun 28, 2022 at 13:49
  • 1
    \$\begingroup\$ Yes, you don't have to only look at voltage between 2 wires when you have 3 phases \$\endgroup\$
    – bobflux
    Commented Jun 28, 2022 at 14:29
  • 2
    \$\begingroup\$ VFDs with single-phase input and 3-phase output do exist. Of course they have huge smoothing capacitors inside them. I expect most "inverter drive" appliances that plug into the wall (e.g. fridges) do have these inside. \$\endgroup\$ Commented Jun 29, 2022 at 9:01
  • 2
    \$\begingroup\$ @QuantumWiz Yeah, the typical stovetop actually uses two phases in a L-N setup, so 2x230 V breakered at typically 16 A each. That's exactly the same amount of power as your proposed 460 V L-L setup (assuming the same current rating). It does need an extra neutral wire, but that 460 V setup would likely use one as well anyway (240 V sockets with no neutral are AFAIK getting obsolete in the US as well). \$\endgroup\$
    – TooTea
    Commented Jun 29, 2022 at 9:09
6
\$\begingroup\$

I am not sure why so much focus on the voltage difference. That is missing the point. In three phase power you use all three phase wires to deliver power so you can't ignore that. So using a metric of 1.73 for the line-to-line (or line-to-neutral voltage) for three phase power is wrong because by using 1.73 you implying that all you did was connect a device up to only two phases of a three-phases supply thus forcing it to run as a single phase at the same current as the split phase. You can't ignore the other phase wires and the currents in them.

NOTE: As a layman you will need to read up the voltage and current measurements in wye and delta systems; Namely the difference (and equivalence) between:

  • phase voltage vs line-to-line voltage vs line-to-neutral voltage
  • phase current vs line current

The phase value definitions are the same between wye and delta because they are internal to the wye or delta. However, the structure of the delta or wye may not make these phase values directly measurable from "outside" which is what line values are. Therefore line values have different, often swapped, relationships to the phase values depending on wye or delta.

It is most obvious examining a 4-wire delta. A wye has the neutral point "invisible" and not physically accessible even if you are looking inside the delta; You can only mathematically access it.

The main reason you need to know these is that power calculations need to be consistent in the voltage and currents used. No mixing inconsistent definitions of voltage and current in your power calculation.

You will want to refer to a wye and delta schematic (\$\phi\$ is for phase):

WYE:

\$V_\phi=V_{L-N}\ne V_{L-L}\$

\$I_\phi=I_{L}\$

DELTA:

\$V_\phi=V_{L-L}\ne V_{L-N}\$

\$I_\phi\ne I_{L-L}\$


Thus, the same phase voltages and phase currents, it is 2VI for split-phase vs 3VI for three phase. It is also the same result if you use line currents and line-to-line voltages. (Note how I am using only internal values or external values together. Line-to-neutral is not explicitly mentioned because although it is sometimes equivalent to some of the values used, it is not explicitly always internal or external between wye and delta and I do not want to divide things up by that because it would get confusing).

Therefore, for 50% more copper you get 50% more power carrying ability, less peaky power delivery, and a naturally rotating magnetic field.


"Surely no one has an AC motor at home that would actually need all 3 phases?"

I want a three phase bench grinder for my garage but I can't get one because I don't have three phases. Or a table saw. Power delivery is less peaky in three phase and motors are simpler, run and start more smoothly while being simpler (cheaper and more reliable).

\$\endgroup\$
32
  • 1
    \$\begingroup\$ You said: "by using 1.73 you implying that all you did was connect a device up to only two phases of a three-phases supply thus forcing it to run as a single phase at the same current as the split phase". Well, to me a good example of a power-hungry device at home is a stove top. To my knowledge, each of the hot plates uses just 1 or 2 phases, not all 3. \$\endgroup\$
    – QuantumWiz
    Commented Jun 28, 2022 at 14:50
  • 1
    \$\begingroup\$ My impression was that household items generally used only 1 or 2 phases and e.g. 3-phase AC motors would be mostly restricted to industrial settings. \$\endgroup\$
    – QuantumWiz
    Commented Jun 28, 2022 at 16:44
  • 2
    \$\begingroup\$ Getting the three phases in the right order for motors is necessary. Sure, swap any two phases and the motor will turn the other way, but that is a complexity that most homeowners really aren't up to dealing with. And, no, I would not trust most modern home builders to get it right (or suppliers). \$\endgroup\$
    – Jon Custer
    Commented Jun 28, 2022 at 23:31
  • 1
    \$\begingroup\$ @JonCuster Hmm that is a good point. Then again it's the same thing with home builders getting neutral and hot correct. Homeowners getting it correct doesn't seem like that big a concern. \$\endgroup\$
    – DKNguyen
    Commented Jun 28, 2022 at 23:38
  • 1
    \$\begingroup\$ @JonCuster There are electronic 3 phase generators now, mechanical motor-generator sets are not needed nowadays. \$\endgroup\$
    – Uwe
    Commented Jun 29, 2022 at 1:26
3
\$\begingroup\$

There are a few rural properties in the UK that get a 230/460V 100A supply. It's only offered if a single phase isn't enough, and the nearest supply is only 11kV single phase. Given that three phase overhead lines are normal, it isn't commonly used.

A 230/460V 100A supply is inferior to a 230/400V 100A three phase one. The former can supply 46kW, the latter supplies 69kW. The voltage being a little bit lower isn't that important.

A 230/400V supply is enough for pretty much all houses, offices and light industry. At 69kW for 100A, that supplies as much as most premises will need.

\$\endgroup\$
1
  • 2
    \$\begingroup\$ " The former can supply 46kW, the latter supplies 69kW." But this is because in that example there's more copper used (because of the 3rd live wire). You could similarly raise the 46 kW to 69 kW by making the live wires 50% thicker. \$\endgroup\$
    – QuantumWiz
    Commented Jun 29, 2022 at 7:59
3
\$\begingroup\$

The split phase power is used in Canada and the USA but not in Europe. All electronics like computers, audio and video equipment may be powered with 110 V or 230 V AC. So split phase has no advantage over single phase.

All electric appliances used in apartments and residential homes may be used with a single phase only. Brushless motors for dishwashers and washing maschine may be built as induction motors with a capacitor.

Continuous water flow heaters for showering need much power, over 20 kW. The power distribution company may prefer to connect these as a symmetric three phase load.

Kitchen stoves may be connect to single phase as well as three phase. If used with one single phase circuit breaker, you may not use all cooktops together with maximum power.

Quick chargers for electric cars may require three phases too.

If you got solar panels with more power, a three phase inverter may be required by the provider.

So three phase power is better only for large loads to reduce neutral current and to balance load to all phases.

\$\endgroup\$
5
  • 3
    \$\begingroup\$ In the Netherlands any load over 5.75kVA must be connected to 3 phases according to Dutch law, article 2.33 (in dutch), and use the three phases equally as much as possible. \$\endgroup\$
    – Pelle
    Commented Jun 29, 2022 at 11:36
  • \$\begingroup\$ @Pelle Balancing between the phases would at least seem easier with split-phase since there are only 2 phases to balance between rather than 3, but maybe that's not a huge issue. \$\endgroup\$
    – QuantumWiz
    Commented Jun 29, 2022 at 11:49
  • 4
    \$\begingroup\$ @QuantumWiz: No, it is not, it is even worse! US split phase systems start as 3 phase at the generator and AC transmission lines. Ultimately, those phases must be (close to) balanced. It is only on the final level that the three phases are run as single phase to the house/street transformers, where the phase is split. This means that for a split phase both legs need to be (close to) balanced as well as the the three phases of the distribution transformer. \$\endgroup\$
    – Pelle
    Commented Jun 29, 2022 at 12:07
  • 1
    \$\begingroup\$ @Pelle: A split-phase transformer which can handle 500kVA combined on both phases won't have any problem feeding 250kVA on one phase and zero on the other. It could only deliver half as much power on one phase as it would be able to deliver if both phases were balanced, but if the transformer is sized to handle peak loads a utility would have no reason to care about imbalance. As for three-phase balance, a large number of small customers arbitrarily divided among the three phases will generally present a relatively balanced load, and it's possible to use three-phase step-down transformers... \$\endgroup\$
    – supercat
    Commented Jul 1, 2022 at 17:45
  • 1
    \$\begingroup\$ ...that will present a relatively balanced load to their supply even if the demand side load is unbalanced. If one phase has a higher demand than the others, the transfer of power between phases via the rotating magnetic field won't be as efficient as the transfer between an in-phase primary and secondary, but the system will be somewhat self-balancing. \$\endgroup\$
    – supercat
    Commented Jul 1, 2022 at 17:49
0
\$\begingroup\$

All the answers are very long, but this can be better shown with an exmple:

The North American 100A installation provides 240V * 100A = 24kW.

For an EU installation to provide this, you need much less current: a typical 32A 3-phase installation delivers 400V * 32A * sqrt(3) = 22.2 kW.

For practical purposes, we can consider them equal in power.

Compare: three wires at 32A have significantly lower losses than two wires at 100A. Loss = current².

\$\endgroup\$
6
  • \$\begingroup\$ I don't think this really answers the question because lower current needed in the EU example here is due to higher (line-to-neutral) voltage rather than 3-phase vs 2-phase. Also, I think that $sqrt(3)$ is extraneous. You effectively have it twice in your example since $230V*sqrt(3)≈400V$. \$\endgroup\$
    – QuantumWiz
    Commented Nov 20, 2023 at 14:15
  • \$\begingroup\$ In the US (line-to-neutral) voltage is only 120V whereas in Europe it's 230V which allows for a lower current for a given power. However, that's not what my question was about. I also don't have a question about why a high powered application using two phases doesn't need a neutral wire. That seems pretty clear to me. My question was about 2-phase vs 3-phase. [Edited by a moderator to remove obsolete content.] \$\endgroup\$
    – QuantumWiz
    Commented Dec 7, 2023 at 7:39
  • \$\begingroup\$ @QuantumWiz also, the voltage in a three-phase system may be 230, but is usually 400V, taking advantage of the delta/triangle connection, giving 3 times higher power. \$\endgroup\$ Commented Dec 8, 2023 at 16:36
  • \$\begingroup\$ I know why there is a factor of sqrt(3). The problem I was referring to is that you seem to have included it twice in your calculation. If the line-to-neutral voltage is 230V then the voltage between two phases 120° apart is 230V*sqrt(3)≈400V. However, in your calculation you had "400V * 32A * sqrt(3)" so you have 400V which you get by multiplying 230V with sqrt(3) but after that you're multiplying by sqrt(3) again which I'm pretty sure is not correct. \$\endgroup\$
    – QuantumWiz
    Commented Dec 10, 2023 at 13:57
  • 2
    \$\begingroup\$ @QuantumWiz, you are correct that the \$\sqrt 3 \$ has already been taken into account in the 400 V phase-phase. Michael omitted to mention that he is calculating for 32 A on all three phases. If we used P-N voltages we would get total power of \$230 \times 32 \times 3 = 22 \ \text{kW}\$. If we use the P-P voltages then it becomes \$ \frac {400} {\sqrt 3} \times 32 \times 3 = 400 \times 32 \times {\sqrt 3} = 22 \ \text{kW}\$. [Edited by a moderator to remove obsolete content.] \$\endgroup\$
    – Transistor
    Commented Dec 31, 2023 at 0:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.