7 Facts You Need to Know About Neutral Wire in a 3 Phase Circuit

If you are training electrical apprentices or are an electrotechnology professional, you would’ve heard of the terms neutral wire, neutral conductor or neutral current when discussing 3 phase circuits. In this blog, I will discuss 7 facts that you need to know or explain to your learners about the neutral wire in a 3 phase circuit. This list is by no means exhaustive but covers some of the most critical aspects.

Let's get the basics out of the way

A three-phase system has three branches, one for each phase. If it’s a 3 phase supply or a transformer, these three branches will be windings of the alternator or the transformer.

The load side can have more variations, for example, if the system is a balanced electrical load, then the three branches will be each phase of the load, for instance, windings of a motor. And if the system is an unbalanced electrical load, each phase could be multiple loads.

There are different ways of identifying the phases, and the most popular ways in Australia are A, B and C or U, V and W. Each of these phases have two ends, and they numbered labels, for example, the ends of phase U are U1 and U2

Star of the show

You can connect 3 phase systems in two ways – Star and Delta. We will only discuss star connection in this blog post because delta connection doesn’t require a neutral wire.

Star connection is when one side of each phase is connected to create a star point, and the other side connects to the lines. The sides at the star point should be consistent; that is, they should all be 2 or 1 but never mixed.

Image of the motor terminal block and winding circuit.

Okay. Let’s get the 7 facts straight

Fact 1: You get two voltages from the 3 phase supply because of the neutral wire

In Australia, the most popular 3 phase supply voltages are 400V and 230V. You may have also seen 415V/240V, that’s a different way of saying the same thing. The 400V is the line voltage, and 230V is the phase voltage.

This setup powerful because it allows you to connect a three-phase load in a way that each phase will have either 230V or 400V across them or connect a single-phase load that needs 230V across one phase and neutral

If there were no neutral wire, only the first scenario would have been possible. Another point to remember here is that the line and phase currents in star connected systems are the same

Fact 2: You don't need a neutral wire for balanced loads

Balanced loads are electrical loads with 3 phases, like a 3 phase motor or a 3 phase water heater. These loads are designed in a way that each phase has the same resistance or impedance, so if they have the same voltage across each phase, the current will also be the same.

A balanced system satisfies the following criteria

  1. Current in each line is the same and
  2. Power factor is consistent, which means the phase angle of each current is consistent with respect to their phase voltages

In single-phase, loads the neutral wire provides the return path for the current, and in balanced 3 phase loads, because they satisfy the above criteria, the currents enter and return through lines creating 0A of out of balance current. So, there is no need for a neutral wire.

3 phase star connected balanced load with neutral connected
3 phase star connected balanced load with neutral broken

Fact 3: The current in the neutral wire is the phasor sum of all the line currents

In a balanced system, when all currents and their power factors are the same, the phasor sum of all line currents is 0A. That’s the reason why there is no need for neutral wire in a balanced system. The mathematical calculation can be quite elaborate, so I’ll discuss that in a different post, but here I will show you a graphical method or vector method.

Let’s say the currents are

IA = 5A, IB = 5A and IC = 5A, and the electrical load has a power factor of 1, which means the phase angles are 0, 120 lagging A and 240 lagging A respectively.

Play Video

What happens to the neutral current when line currents are different or of different phase angle? As you can imagine, it won’t be 0A so let’s find out neutral current for the following example

IA = 727mA at 0 degrees

IB = 727mA at 120 degrees lagging A

IC = 1.927 A at 240 degrees lagging A

Play Video

Since the neutral is current is returning to the supply, the phasor will be in the opposite direction. This means that we need to measure the phase angle in the opposite direction.

Fact 4: Neutral wire carries out of balance current in unbalanced loads

A load that doesn’t draw the same current in each line or has a different current phase angle is considered an unbalanced electrical load. This usually happens in all 3 phase installations because there could be several single and three-phase loads in an installation and you can’t control which line draws how much current at a time. In these cases, the currents don’t balance out, and there is some leftover. This is the out of balance current, and it’s one of the purposes of the neutral wire to carry it to the supply. 

Play Video

Without the neutral wire, all sorts of instabilities occur in the system like unstable voltages, unexpected currents and even dangers of electric shock.

Fact 5: A broken neutral wire changes the phase voltages when the electrical load is unbalanced

In an unbalanced 3 phase electrical load, the line currents are different, which causes the neutral current to flow from the star point of the load to the supply star point. If the neutral wire is broken or disconnected, the out of balanced current cannot return to the supply through the star point, but it must return. So, this current takes the path back to the supply through the lines.

Ideally, the star point should be at 0V, and that is the case when the neutral wire is intact but when its broken and because the currents are forced to return via lines, this point shifts to a different voltage.

Play Video

Since the star point is no longer at 0V, the phase voltages on load change because the line voltages from the supply are still at the same levels as before. For example, if one point of a circuit is at 12V and the other at 0V, then the voltage difference is 12V, but what if the second point was at 4V? Now the voltage difference will be 8V instead of 12V.

Fact 6: A broken neutral wire changes the line currents in unbalanced loads

Current cannot exist without voltage and as the Ohm’s law suggests, as the voltage changes, the current changes with it and proportionally. As mentioned in the previous section, the phase voltages are affected by the broken neutral, and this affects the phase currents as well. Since the line and phase currents in star connected systems are the same, the currents in the lines change as well.

The change in voltage and current causes the change in the electric power, and this the reason why the loads work in unexpected ways. In some cases, this could cause a brown-out or overvoltage; either way, it’s bad for the electrical load and bad for the system.

3 Phase star unbalanced circuit with neutral connected
3 Phase star unbalanced circuit with neutral broken

Fact 7: Unexpected voltage at the point of the broken neutral wire

When the neutral wire breaks, the voltage at the star point is not at 0V but a different number. We may never know what this voltage could be because it would depend on the connected loads at that time, which means that this voltage could be close to 0V or much higher.

The higher this voltage, the higher is the risk of electric shock to a person or animal that completes the broken path, and this can be very dangerous.

Conclusion

There you have it, 7 facts you need to know about neutral wire in a 3 phase circuit. As mentioned earlier, this list isn’t exhaustive, and there could be more things you may need to know, but this is a good start.

I hope this post help you or your apprentices get some ideas or refresh what you may have learned in the past. Please add your thoughts and other facts that I haven’t listed here in the comments.

Thanks for dropping by

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About the Author

Husnen Rupani

Husnen Rupani

I help electrical training organisations increase learner engagement by designing innovative training equipment. I have a saying "Electricity - you cannot see, you cannot hear it, but by the time you feel it, it may be too late." My main aim is to turn this black magic that we call electricity into something that people can understand.

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George Zeidan

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28 thoughts on “7 Facts You Need to Know About Neutral Wire in a 3 Phase Circuit”

  1. Hi Husnen, thanks for the detailed article.

    Yesterday, almost all my DC adaptors failed – including Ipad pro charger, Macbook charger, small 12V – 1A/1.25A and 9V – 0.65A adaptors for Echo, router, LED lights in aquarium etc. Before going out, there were sounds of sparks, lights popping out etc. Needless to say, financially it was a huge dent to my pockets, including the CPU that’s no longer working.

    When I called the electrician, he found that the connector for the neutral wire (in my residential complex) where it connected to the neutral of my flat had melted, and the neutral wire was charred. He said the connector was of poor quality and wondered why was a connector put in place at any rate. As a solution, he joined the two neutrals directly. I suspect that should not be a long term solution, and we should fit a good quality connector for the neutrals.

    The important thing I am trying to understand is this: All my AC systems, including the TV, Fridge, Geyser seems to be working fine. But everything DC got fried. Luckily, only adaptors for most, except my CPU, which was on sleep mode. I did not have a UPS, but upon opening the CPU saw some liquid on the cabinet floor. I suspect something happened.

    The surprising bit is, almost all the DC adaptors that are no longer working were not in active operation. They were just connected to the power supply, but no laptop/phone/Ipad or other gadgets were being actively charged. Still, those adaptors no longer work. I suspect, even on a standby connection, the adaptors seem to be drawing current. And when the supply neutral fried, something unexpected happened – an overshooting voltage or current that killed anything drawing even a minuscule current.

    1. Is this expected for DC systems – if a neutral goes pop, a standby adaptor can fry (by stand by I mean connected to the electrical lines but not being used)?
    2. Is it expected that the AC equipment could survive the neutral damage, that DC adaptors did not?

    Another thing I am wondering for now is this: How do I protect my house from such future collapse of the neutral wire. Would a UPS help? My electrician said max. UPS I could get is 3 units whereas my house connection is for 10.

    Your comments would be very helpful.

    Regards,
    Prasant

    Reply
    • Hi Prasant, sorry hear that you had such a mishap, electricity is quite unforgiving isn’t it.

      The problem does seem unusual. The only thing that I can think of is a surge from the grid or somewhere. Almost all DC equipment use switch-mode power supplies and other electronics, which are susceptible to surges. Have you tried using a surge protector on the smaller units? You can always try a surge arrestor if you want to protect the whole installation. This is only my opinion, without being in front of the problem, I can only speculate. I’m not sure if UPS will help in this situation.

      Please share how you went since you had it fixed, did anything like that happen again?

      Reply
    • The problem of neutral getting cut occurs once in a while in my house 3 phase power supply. If the neutral gets totally cut, the outcome will be devastating. It was noticed that when neutral breaks, the indicator of power plug glows even if the switch is not ON. Before connecting electronic equipments, I used to ensure that this indicator is not glowing.

      Reply
  2. Worth reading and informative. Keep it up. I would love to see more topics like this particularly about control circuits like start stop switches, automation and some more industrial electrical topics. Thanks

    Reply
  3. woo thanks for the updates . i really love to see how the connection to obtain the neutral terminal from a three phase is obtained

    Reply
  4. Can a floating neutral at the main distribution panel cause nuisance tripping of the distribution transformer fuse? 1 Fuse burned out after a power interruption just upon restoration of power. This happened twice, on two separate occasions, one on line 1 & the other on line 3. After replacement of the burned out fuse, everything seems to be running normal. Voltages are also ok 400V 3ph & 230V L-N. Why is this so?

    Reply
    • Sorry for the late reply Kristian. Floating connections can cause all sorts of problems and at the point of the distribution transformer, there are so many variables. I’m not sure if that could be the cause of the nuisance tripping because I’m not experienced in linesman work. But if the fuses burnt out upon activation, could it be the inrush current? Did you manage to find the answer to your question?

      Reply
  5. mr . husen rupani ji,

    there is in an industary the 3-phase energy meter not working when a swich off . but aii motors are in working . when the swich on ,the e. meter is working , the switch ,where connected , earthing or nutral. pl. clear this.

    thanks
    suresh chand
    roorkee,uttarakhand,india

    Reply
    • Dear Suresh,
      I may need more information but I’ll try to answer the best I can from your explanation (Please correct me if I didn’t understand your question). Most digital meters take power from the supply that they are monitoring so when the supply is interrupted and the power connection to the meter is disconnected, it would stop working. Could you please check if the switch is placed after the meter or before? If it is before the meter, it’s probably disconnecting the meter’s supply as well as the main supply.
      I hope this helps.

      Thanks,
      Hus

      Reply
  6. Hi.
    If I was connecting a 3 phase generator to a distribution board and then running single phase circuits from this point. Would this cause any problems with the generator because there would be unbalanced loads?
    Assuming DB is feed via a 32A 3 phase lead with neutral conductor from the generator. This is a 22kva generator and I would be probably be feeding maybe 15A on two phases and about 5-7A in the other phase.
    Thanks Josh

    Reply
    • Hi Josh,
      The unbalance shouldn’t cause any problem, that’s what the neutral is for. As long as your neutral wire is sized appropriately and is not interrupted, the system should work as expected.
      I hope this helps
      Thanks
      Hus

      Reply
  7. I enjoy your detailed analysis of the subject matter but would like to read the engineering aspect of the subject as well. Thank You.

    Reply
  8. Many a times I have surge in my home electrical circuit. One time it was due to improper switching off the current from the transformer. Another time it was due to neutral failure. Many things in my home damaged as a result of these happenings. The items damaged include my inverter, many fans, many chargers, cctv DVR, even some of the wiring in which the faults are very difficult to identify and time consuming to rectify. Now I have taken steps to install surge protector and voltage relay in my electrical installation. It is really costly.

    Reply
  9. Hi Mr Husnen Rupani,

    I just came across a problem where a 3 phase 4 wire, 415v and 300A switchboard operates with L1 = 133A, L2 = 138A L3 = 145A N = 14A and power factor 0.95 lag. As you explained above, N current is 0. My question is, is this normal and is there a way of calculation to find out the maximum value on the neutral cable allowed?

    Thank you,
    Izuan

    Reply
    • Dear Izuan,
      Could you please clarify the question? Is Neutral current 14A or 0?
      The neutral current will be 0 if the system is balanced, which means all line currents are the same and have the same pf. Another scenario when neutral current can show 0 is when the neutral wire is broken somewhere, this can be dangerous but mostly easy to notice because the loads will behave erratically.

      Reply

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