How to size neutral line in three phase system with single phase load

Dear all.
I am to install a three phase system for load consisting mainly single phase. I have divided the single phase loads in to three and fed each group with one hot from the three hots. But each group use the same return line the neutral, the problem is, shouldn't I tripple the thickness of the neutral because it cary all the phases currents? To worsening my problem standard cable neutral thickness is even smaller than the phase. I have attached sketch of this
If the idea is that you have a distribution panel that has the 3 phases coming in and single phase circuits going out then if I understand you correctly, you are correct. The neutral of a single phase circuit should be the same size as the active (or hot). When all the single phase circuits combine and connect to the incoming 3 phase then at that point, no, the neutral is usually sized smaller than the phase conductors because the neutral has only to carry the unbalanced portion of the loads on the three phases. If the 3 phases all had identical loads the neutral current would be a theoretical zero Ampere. The balance is never perfect so the neutral is included where loads can be single phase. Unless you had one phase fully loaded and the other 2 with no load at all there should not be a problem.
Needless to say that in this distribution panel that takes the 3 phase to make a bunch of single phase circuits that the single phase circuits should all have a circuit breaker each which is sized and of the type to suit the cable and installation conditions of each of the single phase circuits. The main switch and main circuit breaker (if you use a breaker at that point) should be 3 or 4 pole types and not 3 individual switches or breakers.
Not knowing enough about the regulations in your part of the world nor the application I am not sure if this is required or kosher but I would consider an MEN (Earth to Neutral) link at the panel that breaks the 3 phase into single phase circuits.
Hope that helps in some way and isn't an insult to your intelligence :)
 
If the idea is that you have a distribution panel that has the 3 phases coming in and single phase circuits going out then if I understand you correctly, you are correct. The neutral of a single phase circuit should be the same size as the active (or hot). When all the single phase circuits combine and connect to the incoming 3 phase then at that point, no, the neutral is usually sized smaller than the phase conductors because the neutral has only to carry the unbalanced portion of the loads on the three phases. If the 3 phases all had identical loads the neutral current would be a theoretical zero Ampere. The balance is never perfect so the neutral is included where loads can be single phase. Unless you had one phase fully loaded and the other 2 with no load at all there should not be a problem.
Needless to say that in this distribution panel that takes the 3 phase to make a bunch of single phase circuits that the single phase circuits should all have a circuit breaker each which is sized and of the type to suit the cable and installation conditions of each of the single phase circuits. The main switch and main circuit breaker (if you use a breaker at that point) should be 3 or 4 pole types and not 3 individual switches or breakers.
Not knowing enough about the regulations in your part of the world nor the application I am not sure if this is required or kosher but I would consider an MEN (Earth to Neutral) link at the panel that breaks the 3 phase into single phase circuits.
Hope that helps in some way and isn't an insult to your intelligence :)
You would never put a breaker in the neutral. That could leave the single phase circuits dead but both the phase and neutral lines would be "hot" as a result. As for Earth to Neutral connections, these must only be at the service entrance panel to the building or the secondary of step-down transformers. Otherwise, currents will flow in the "Ground" conductors which is bad for many reasons. This reminds me of the old 3 pin sockets and plugs used on 240 volt US dryers and stoves, which present an extreme shock hazard should the pin that is both neutral and ground (or wire from it) be open. You then have 120 volts on the frame of the appliance through the fan, light, timer or any other 120 volt load in it. All of these need to be replaced with the approved 4 pin cord and plug.
 
In a balanced star connected system, neutral current is always zero.
The OP says his loads are all sized equally.
The local service company should be consulted as to policy of sizing such a supply neutral.
Max.
Consult the local wiring rules would be more pertinent. In Australia the relevant standard for the scope of this project is AS3008. Arbitrarily reducing the size of the neutral conductor might seem sensible under normal load conditions but several important considerations are ignored in that thinking: in a hard fault condition the Neutral could then be sized too small to carry and survive the fault current, the touch voltage on any accessible earthed conductive part could be excessive and the installation would be non standard so you would need notes for the panel to explain to anybody else how the panel meets regulatory requirements and thereby explaining what needs to be done / maintained to retain that compliance. Even then you could still be done for a non compliant installation.
 
You would never put a breaker in the neutral. That could leave the single phase circuits dead but both the phase and neutral lines would be "hot" as a result. As for Earth to Neutral connections, these must only be at the service entrance panel to the building or the secondary of step-down transformers. Otherwise, currents will flow in the "Ground" conductors which is bad for many reasons. This reminds me of the old 3 pin sockets and plugs used on 240 volt US dryers and stoves, which present an extreme shock hazard should the pin that is both neutral and ground (or wire from it) be open. You then have 120 volts on the frame of the appliance through the fan, light, timer or any other 120 volt load in it. All of these need to be replaced with the approved 4 pin cord and plug.
I never suggested a breaker in the neutral Mr. I did say a 3 pole switch, which you may note is the correct number for a 3 phase supply. I did not suggest a 4 pole switch to include the neutral but even if I had, it is in fact in my part of the world, perfectly reasonable and totally ok depending on the installation conditions. And no, MEN links in downstream distribution panels are mandatory in some installation conditions.
 
In summary it would appear that all four wires should be the same gauge.
In Australia, if the phase conductors are greater than 2.5mm sqr, the neutral conductor is smaller than the phase conductors. If the cables are all single conductors so 3 for phases and one for neutral (and another fior Earth if applicable) then you have the option of sizing the neutral the same as the phases but if you use a standard multi conductor cable with greater than 2.5mm sqr actives in it, the neutral will be smaller than the actives.
 
I never suggested a breaker in the neutral Mr. I did say a 3 pole switch, which you may note is the correct number for a 3 phase supply. I did not suggest a 4 pole switch to include the neutral but even if I had, it is in fact in my part of the world, perfectly reasonable and totally ok depending on the installation conditions. And no, MEN links in downstream distribution panels are mandatory in some installation conditions.
Scroll up a few and see that you did say "3 or 4 pole . . " Not allowed in the US. Neither is connecting Neutral to Ground except as I stated.
 
Scroll up a few and see that you did say "3 or 4 pole . . " Not allowed in the US. Neither is connecting Neutral to Ground except as I stated.
So I did. My bad. But not totally bad, In Australia it is acceptable in certain conditions to use a 4 pole (or 2 pole for single phase) provided that the switch breaks both the active(s) and neural so a high impedance neutral should never happen without a high impedance active as well. This is exactly what the isolation switch now mandatory (in Aus') on cookers and hobs does.
Not surprisingly, we have the same (or exceptionally similar) MEN system here as you do in the USA and I expect you also have the same rules there as here about the MEN regulations pertinent to a distribution to another (secondary or sub) installation at some distance (think shopping malls or out buildings on farms) which are sufficiently displaced from the original installation to warrant a local ground stake, local equipotential connections to other services and to complete the installation, another Multiple Earth Neutral (or MEN) link.
The OP is talking about cables of 120mm sqr so its not a big stretch to believe this is a substantial installation and a MEN link would be part of it.
I know what you mean about the resulting currents in the pipes of adjacent services. If done properly though, it may be a current but with very limited potential. In practice, here at least, it does not seem to be a problem. It was a bigger problem before the MEN system was retrofitted to the very first suburbs to be electrified. In those suburbs, working on a water or gas manin could be lethal for all the wrong reasons.
 
I agree. After seeing how the 240 volt systems work in Europe, I realized how foolish the US (and your) system is. By grounding the center tap of the transformer, but never having any 120 volt loads, there is no need for a neutral at all. There is still only 120 volts to ground, so there is not really any worse shock hazard. Twice the energy can be delivered with the same wiring. You do need two pole breakers on single phase circuits of course.
So I did. My bad. But not totally bad, In Australia it is acceptable in certain conditions to use a 4 pole (or 2 pole for single phase) provided that the switch breaks both the active(s) and neural so a high impedance neutral should never happen without a high impedance active as well. This is exactly what the isolation switch now mandatory (in Aus') on cookers and hobs does.
Not surprisingly, we have the same (or exceptionally similar) MEN system here as you do in the USA and I expect you also have the same rules there as here about the MEN regulations pertinent to a distribution to another (secondary or sub) installation at some distance (think shopping malls or out buildings on farms) which are sufficiently displaced from the original installation to warrant a local ground stake, local equipotential connections to other services and to complete the installation, another Multiple Earth Neutral (or MEN) link.
The OP is talking about cables of 120mm sqr so its not a big stretch to believe this is a substantial installation and a MEN link would be part of it.
I know what you mean about the resulting currents in the pipes of adjacent services. If done properly though, it may be a current but with very limited potential. In practice, here at least, it does not seem to be a problem. It was a bigger problem before the MEN system was retrofitted to the very first suburbs to be electrified. In those suburbs, working on a water or gas manin could be lethal for all the wrong reasons.
 
I agree. After seeing how the 240 volt systems work in Europe, I realized how foolish the US (and your) system is. By grounding the center tap of the transformer, but never having any 120 volt loads, there is no need for a neutral at all. There is still only 120 volts to ground, so there is not really any worse shock hazard. Twice the energy can be delivered with the same wiring. You do need two pole breakers on single phase circuits of course.
Not to be contrary but I quite like the US system. It is quite clever really to distribute 2 phase power so appliances run on 115V but larger items can still get 230V and the potential w.r.t earth is only 115V (neglecting surges and lightning etc). But the biggest bonus in the US system is the 60Hz mains frequency. It turns out that if you want to put a human heart into fibrillation by electric shock, 50Hz is particularly good at doing that but 60Hz really is not.
The down side with the US system is getting access to 3 phase power because it is (as I understand it) not as readily available and an expensive connection to get.
 

MaxHeadRoom

Joined Jul 18, 2013
19,165
Having lived for the first half of my life with the UK system, I don't really find any advantage with the N.A. 1phase/split phase system.
In the UK it was customary to wire every so many homes on each phase, in one case on installing off-peak storage heaters in each room of a house, I was required by the service Co. to spread them across 3 phases, for which they easily supplied into the residence.
Max.
 
Having lived for the first half of my life with the UK system, I don't really find any advantage with the N.A. 1phase/split phase system.
In the UK it was customary to wire every so many homes on each phase, in one case on installing off-peak storage heaters in each room of a house, I was required by the service Co. to spread them across 3 phases, for which they easily supplied into the residence.
Max.
The North American system by virtue of the connection to earth of the neutral between the two actives is in fact a 2 phase system and not just a split phase as such. It needs only single pole breakers because each phase is a valid phase and neutral distribution.
You haven't actually said what you don't like / do like. The phase load balancing you are used to in the UK also happens in NA, but on 2 phases in domestic distribution and 3 phases in industrial installations.
We have wandered a little of topic here I guess. I'm just curious about your problem with the NA 2 phase system. It seems pretty good to me.
 
The North American system by virtue of the connection to earth of the neutral between the two actives is in fact a 2 phase system and not just a split phase as such. It needs only single pole breakers because each phase is a valid phase and neutral distribution.
You haven't actually said what you don't like / do like. The phase load balancing you are used to in the UK also happens in NA, but on 2 phases in domestic distribution and 3 phases in industrial installations.
We have wandered a little of topic here I guess. I'm just curious about your problem with the NA 2 phase system. It seems pretty good to me.
As I said, the "NA 2 phase system" is inferior to the European system in three respects: First, the confusion of neutral and ground results in potentially dangerous and damaging currents in the ground wires and building plumbing, steelwork, etc. Second, for the same size wiring, only half the energy is delivered. In Europe, hair dryers, toasters and other high-wattage appliances can plug in to the standard outlets and work in less time. Third, there is no need for the "Neutral" pin in high wattage receptacles as is the case with dryers and stoves in NA; the older, dangerous practice of using one pin for both just is not an issue on the European system. Finally, its not really a "2 phase system" at all; its a split or center tapped single phase system. Both halves are in phase.
 

MaxHeadRoom

Joined Jul 18, 2013
19,165
The term '2 phase' is a misnomer for the centre tapped secondary of a 1ph transformer, as almost all sites also refer to it as split phase or 1ph.
2 phase traditionally requires 3 conductors other than at 180°.
Max.
 
The term '2 phase' is a misnomer for the centre tapped secondary of a 1ph transformer, as almost all sites also refer to it as split phase or 1ph.
2 phase traditionally requires 3 conductors other than at 180°.
Max.
No, that would be 3 phase with one phase missing leaving 2 phases but not 2 phase. And the configuration of the distribution transformer is not really the point which is the voltages and phase angle between the actives. By definition, 2 phase is 180 Deg, 3 phase 120 Deg and 4 phase would be 90 Deg and so on. 2 phase is not a misnomer any more than 3 phase is. 2 phase just happens to be very easily produced from a single phase but that does not change what it is. And this discussion will boil down to "tomaytoe" or "tomartoe" eventually.
Time to let this off topic side branch die me thinks.
 
As I said, the "NA 2 phase system" is inferior to the European system in three respects: First, the confusion of neutral and ground results in potentially dangerous and damaging currents in the ground wires and building plumbing, steelwork, etc. Second, for the same size wiring, only half the energy is delivered. In Europe, hair dryers, toasters and other high-wattage appliances can plug in to the standard outlets and work in less time. Third, there is no need for the "Neutral" pin in high wattage receptacles as is the case with dryers and stoves in NA; the older, dangerous practice of using one pin for both just is not an issue on the European system. Finally, its not really a "2 phase system" at all; its a split or center tapped single phase system. Both halves are in phase.
It is arguable that the 230/400 3 phase systems have the same issue with currents flowing in other utilities and unrelated infrastructure. This is the other reason for the MEN system and equipotential bonding done at every major distribution panel at the LV end of the distribution. Your point about the doubled currents for the same loads is absolutely correct as is your hint that many appliances in NA are lower power than in the EU et al. I don't follow what you mean by "one pin for both". A 115V outlet in NA is unpolarized unless it has an Earth pin. Assuming either conductor on a 2 pin plug is neutral is taking a 50/50 chance of electric shock. And we may have to agree to disagree about it being a 2 phase system but I cannot agree with "both halves are in phase". That is simply not true. They are 180 deg out of phase and hence there are two phases of the mains voltage with a Neutral associated with both. So it's not A, B, C & N or L1, L2, L3 & N but L1, L2 & N. 2 phases, not 3. ;-)
 
It is arguable that the 230/400 3 phase systems have the same issue with currents flowing in other utilities and unrelated infrastructure. This is the other reason for the MEN system and equipotential bonding done at every major distribution panel at the LV end of the distribution. Your point about the doubled currents for the same loads is absolutely correct as is your hint that many appliances in NA are lower power than in the EU et al. I don't follow what you mean by "one pin for both". A 115V outlet in NA is unpolarized unless it has an Earth pin. Assuming either conductor on a 2 pin plug is neutral is taking a 50/50 chance of electric shock. And we may have to agree to disagree about it being a 2 phase system but I cannot agree with "both halves are in phase". That is simply not true. They are 180 deg out of phase and hence there are two phases of the mains voltage with a Neutral associated with both. So it's not A, B, C & N or L1, L2, L3 & N but L1, L2 & N. 2 phases, not 3. ;-)
The 240 volt outlets common in the US used for dryers and electric stoves have only three connections; two are the 240 volts, with 120 volts to ground, and the third is connected to the frame of the appliance, yet carries the neutral return of the lamp and timer which are 120 volt loads, and the wire from the outlet goes back to the main panel and s treated as a neutral. I believe that now, finally, this has been changed with a revision of the electrical codes, but my house, which is only 20 years old, has them. Again, should the "neutral/ground" wire open anywhere, the frame of the appliance becomes energized with 120 volts to any grounded object like water pipes or a wet floor since current then flows through the lamp and timer through the unfortunate person touching the frame.
 
The 240 volt outlets common in the US used for dryers and electric stoves have only three connections; two are the 240 volts, with 120 volts to ground, and the third is connected to the frame of the appliance, yet carries the neutral return of the lamp and timer which are 120 volt loads, and the wire from the outlet goes back to the main panel and s treated as a neutral. I believe that now, finally, this has been changed with a revision of the electrical codes, but my house, which is only 20 years old, has them. Again, should the "neutral/ground" wire open anywhere, the frame of the appliance becomes energized with 120 volts to any grounded object like water pipes or a wet floor since current then flows through the lamp and timer through the unfortunate person touching the frame.
OMG So the fact that the rest of the NA system is so good and clever is clearly a fluke! Nobody could be responsible for something so good and then do something so amazingly stupid as combine the earth and neutral like that. I did not know that was done. My opinion of NA electrical codes has taken a hit and not looking too good for it now.
There is always a chance of a conductor breaking especially in a power lead / flex but even then, if protective earth is run independently there still needs to be a second fault before anyone can get hurt. By combining the earth and neutral then a single fault renders the appliance live and also draws attention to it because it is not working and does so without any indication of the danger.
At least an RCD would still work properly so there is some hope for NA and higher voltage appliances without mortality. :)
 
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