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Do positive and negative wires need to be of same length all the time for a circuit to run smoothly? Of course same gauge but only different lengths.

What would be its effect on the system if we say my negative wire is like 5x longer than the positive wire or the other way around? Or it does not really matter?

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    \$\begingroup\$ Assuming we're talking about power supply here, and not something happening at radio frequencies on these wires: As long as the resistance of your wires is low enough, their length, absolute or relative to each other, doesn't matter, as they are essentially "perfect" conductors. \$\endgroup\$ Nov 9, 2020 at 23:13
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    \$\begingroup\$ However, vote to close, because this is really a "I have not even a basic education in electricity. I don't know what Ohm's law is!" question, and that might be a bit too broad for a platform for engineering, where that would be a minimum level of understanding! \$\endgroup\$ Nov 9, 2020 at 23:14
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    \$\begingroup\$ It doesn't mean your question is bad, at all, it just means that (imho) it's not within the scope of electronics design. \$\endgroup\$ Nov 9, 2020 at 23:15
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    \$\begingroup\$ @MarcusMüller you are a little too trigger happy with your closing attitude. I can make up an example where different resistance in the plus and minus connection of a dual voltage supply can lead to an unbalance in the voltages to the system supplied. And yes, all you need to know to reach that conclusion is Ohm's law. From a qualitative point of view, different lengths make a difference. In practice, they usually don't but it all depends on the current (and the type of supply and load). \$\endgroup\$ Nov 10, 2020 at 0:17
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    \$\begingroup\$ @princessbubbles15: Don't bother with Youtube. Find a good text book. I found this free PDF on a quick search. It looks much more reliable than random Youtube videos. It may be at a higher level than you are. \$\endgroup\$
    – JRE
    Nov 11, 2020 at 18:04

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Ideally, the two feed wires (forward and return) should be the same length and also closely placed to each other so that any rapidly changing current flow caused by the load doesn't create a significant magnetic field at some distance. The greater the gap between the two wires the bigger the emitted magnetic field might be and, in some cases can cause other equipment interference (à la EMC).

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  • \$\begingroup\$ I've often seen the two wires loosely twisted together. Is that just a mechanical thing, or does it offer some electrical advantage? \$\endgroup\$ Nov 9, 2020 at 23:20
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    \$\begingroup\$ Yes, it's called twisted pair and is used in many different types of feed cable @AndrewMorton. It reduces external magnetic fields even more than just placing the two wires closely. \$\endgroup\$
    – Andy aka
    Nov 9, 2020 at 23:21
  • \$\begingroup\$ Twisting is obviously a mechanical fixitive of spacing and one that can be manually created in addition to manufactured. It also helps a fair degree when wire runs to a sensitive system may pick up interference by helping to equalize the coupling to both sides. Effects in reducing emitted interference vs. a parallel run may be minor, but it doesn't hurt. \$\endgroup\$ Nov 9, 2020 at 23:26
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When given a choice, making them equal length and routed together is the most desirable, as this allows the wires' electromagnetic fields to cancel out.

As for the real world, it depends.

If the pair is carrying a signal, then you'd not only want them to be the same length, but also, alongside each other to minimize loop area. Coaxial cables do this by adopting a concentric design; other cables like Ethernet and HDMI use twisted differential pairs.

For power, the main concern is that the overall IR drop is low enough that the powered device can work reliably.

When there's module-to-module signals involved an additional concern is the fluctuations in power current creating ground noise between devices. In that case, making the return path low impedance reduces this noise, even if it's at the expense of increasing supply noise.

An example of an unequal power path is a car body:

  • sheetmetal is a low-impedance ground return
  • wiring is supply

The car body and engine block connect the battery and high-power devices and so have very low IR drop; more IR drop is tolerated on the power feed. If needed, the power line feeds can be filtered with local power conditioning / regulation.

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You expressly mention DC application in your tag, so here is a (rather artificial) example where the length of the positive and negative cables can make a difference in a power application.

In the nuclear winter of a post-apocalyptic world, you are asked to to power two 2 ohm resistive heaters in two adjacent rooms of your house using a dual +/- 15 V power supply (mysteriously working thanks to a nuclear generator attached to it :-) ).
You figure that with 15V on 2 ohms you will get some 110+ watts from the heater, enough not to freeze to death in the night.

Problem is, you do not have any cables at hand. But you are a resourceful engineer with a McGyver badge and decide to use the iron rebar to act as ground cable, since there is an exposed reinforced bar that is common to both rooms. As power cables you choose to use iron pipes you had scraped from nearby decaying building. The obligatory paperclip will serve as a means to make contact.
The second problem you face is that you will have to route the pipes from one room to the other so that one heater will be close to the power supply (say the minus voltage one) while the route to the other will be much longer, say ten times longer.

You end up with a resistance for the minus 'cable' of 0.1 ohms and a resistance for the plus cable of 1 ohms.
This is what happens with voltages and currents:

unbalanced power supply and load

One heater will deliver some 100 watts, the other one only 50 watts. You decide to let the person who thinks the length of the cables does not make any difference to sleep in the other room.

If you think this example is too much of a stretch (why not sleep in the same room, or put the loads in series disregarding the ground), imagine a power audio amplifier that requires dual voltage supply.

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Most of the time, it doesn't really matter for a DC circuit.

If one wire is excessively long, its resistance may become a problem. If your circuit is susceptible to electromagnetic interference, then using short wires, run close to each other, may help.

In other cases, it makes no difference at all. Consider a flashlight. The positive terminal of the battery is often pressed right up against the lamp. No wire at all. The negative runs from the lamp, the entire length of the flashlight, to the negative terminal of the battery. It works fine like that.

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