Reasons
Reasons
When current passes through copper, a magnetic field is produced. If the current passes to the load and then back again, as would be the case when the wires are placed in the same conduit, per the NEC, then the two magnetic fields cancel.
What if they don't cancel? In a single phase case, that would mean you have wiring running willy nilly, which would be dangerous for various reasons. If someone wanted to disconnect a 240V circuit and only disconnect one wire, then the other wire is hot somewhere .... just bad workmanship.
Balancing involves multiple loads and multiple circuits, generally in three phase. I can't make unbalanced really work in terms of the conductor issues in my head, because there is always a neutral (or another hot) carrying the current back again in the other direction and this makes the phases cancel out. If you use a CT based ampmeter and you put the CT on one wire in any loaded circuit and then on the other wire, you get values. If you put it around both wires, then you should get 0.
However, in three phase, you have trigonometry. If you only load circuits on the B and C phases, with 100A each for example, there will be magnetism as though you had a 50A one way wire - if I remember my circuits correctly. If you run another copper wire nearby, current will be induced in it. You can put your CT on that other wire and there will be a noticable current if the other wire is part of a complete circuit. This is a real issue with communication wires. All the wires will pick up some current going the same direction if they are connected to additional wires elsewhere to complete the circuit. So, you will have wires that are designed to carry mA, that may be carrying amps and the signal is totally disrupted. They are supposed to be carrying maybe 10mA north and south in one conduit, but this small current relatively disappears because they are part of a bigger unintentional circuit carrying 2A north with another disrupted circuit acting as the return path south.
Back to single phase. Let's separate the wires.
If you ran one wire of a circuit near a copper bar on one side, and then ran the other side of the circuit on the other side of the copper bar and dropped metal shavings on the sides of the bar, I would expect them to stick.
Also if you run a copper communication wire in between, I would expect you to get static or disruption as previously described.
In short - either you will have to separate the two wires that are acting as part of your circuit and put something in between that can both pick up the induced current and which is supposed to be carrying a signal that is small in comparison, or you can unevenly load a three phase circuit. In either case, you can either measure the induced current on a meter by circling that wire in the middle, or you can try to send small signals that would otherwise cause a switch to open or close or an alarm to go off, and show that the signal is no longer communicated in the case of an unbalanced circuit.
Either way, the results are going to be small - they aren't like to show unless you are trying to operate 4-20mA controls or measure with a meter or get static in some other kind of communication. However, if you need to tell a pump to stop on overflow, and the signal can't get there, it can be a big messy problem.
Hope that makes sense.
