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I am curious, and I've found a bit of information online, but maybe its not clicking in my head... when does the neutral line in a 3 phase system carry voltage.

i do understand the answer is quite simple (When the legs are unbalanced) but for whatever reason, its not making sense to me.

why is it important that the loads are balanced on all legs?

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    \$\begingroup\$ From Wikipedia "There are two basic three-phase configurations: delta and wye (star). As shown in the diagram, a delta configuration requires only 3 wires for transmission but a wye (star) configuration may utilize a fourth wire. The fourth wire, if present, is provided as a Neutral and is normally Grounded. The "3-wire" and "4-wire" designations do not count the ground wire used above many transmission lines, which is solely for fault protection and does not carry current under non-fault conditions." more here en.wikipedia.org/wiki/Three-phase_electric_power \$\endgroup\$ – Tyler Apr 1 '16 at 0:38
  • \$\begingroup\$ If you draw a graph of three sinewaves at 120deg intervals, then a fourth graph of the sum of the other three, you should see that the sum is zero when the amplitudes are equal. Thus there is no neutral current when the phases are balanced. Conversely . . . . \$\endgroup\$ – peterG Apr 1 '16 at 0:58
  • \$\begingroup\$ It carries voltage (in any meaningful quantity) only if something is really wrong, since it's the 0V centre point. It carries current at moments of imbalance between phase currents. It's important to get current and voltage right, to avoid killing yourself when working with mains and above. \$\endgroup\$ – Asmyldof Apr 1 '16 at 0:58
  • \$\begingroup\$ It can also show a few volts with respect to ground if one or more of the three phase conductors are supplying 120v loads of meaningful amperage (which would be returned through the neutral), in addition to or instead of 3-phase machinery (which returns through the phases). Do you have a lot of computers? Domestic window air conditioning units? \$\endgroup\$ – Chris Stratton Apr 1 '16 at 3:54
  • \$\begingroup\$ the neutral would have to compensate to balance the system if the three phases are not balanced. It has to carry the weight of the imbalance. \$\endgroup\$ – old_timer Apr 1 '16 at 14:26
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"Carry voltage" technically is a very loose term.

Voltage is relative, meaning it must be measured relative to another point, such as ground or another conductor.

All conductors have resistance and obeys ohms law. When current flows there will be a voltage drop throughout the length of the conductor. This happens in all conductors disregarding the systems involved.

In well balanced 3 phase system the neutral current will be very minimum compared to the phase currents. That explains why voltage drop is almost zero in that wire.

Imagine a thin straight pole is poked into earth perpendicular to ground and tip of the pole is attached to 3 ropes and pulled from three equally spaced (120 degree) directions. The pole tip will stay stationary even when pulled with great force. That's what happens to the neutral in well balanced 3 phase system.

But when a rope a slightly slag or exert more force than the other two, the pole tip will move. Thus, imbalance 3 phase system will have neutral current and the proportional voltage drop along the wire.

In the real world, there is no perfectly balanced 3 phase system. That is why most of the time when neutral voltage is measured against the local earth, there will be a voltage, mostly around 6 volts for the 415 volt 3 phase systems.

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