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Preface: I'm a beginner.

Do positive and ground need the same wire thickness? Is it safe is use ground wires that are thinner than the positive?

I think I understand that for a given amount of current, length, temp., etc., one should use a certain wire thickness to avoid heat issues due to resistance. Is that only for positive or for both positive and ground?

My thinking is:

When current leaves the supply and goes to the load (supply --> load), it does some work W (e.g. light up an LED). Does doing "work" mean it loses some energy J such that on the wire from load to ground (load --> ground) it carries lower energy and is unable to heat up compared to the wire from supply --> load.

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  • \$\begingroup\$ If the same current is passing through two wires then they need to be the same diameter. Is that what you're thinking? A simple loop where 1 A goes in and then 1 A returns? If so, same diameter. If not, then different diameter may be acceptable. \$\endgroup\$ Oct 19, 2021 at 1:56
  • \$\begingroup\$ Yes. I'm thinking just a simple loop (like connecting a battery to an LED). So the heat energy given off from resistance is not affected by the work done at the load? So if I used undersized wires for a given current, I should measure the temperature for the wire supply-->load and load-->ground? (in a theoretical setting) \$\endgroup\$ Oct 19, 2021 at 1:58
  • \$\begingroup\$ The wire resistance reduces the voltage seen by the load, thus resulting in less power being transferred to the load. The load does the same to the wires, which will get MUCH hotter if you short them together with no load! So yes, the work done on the load does affect the wires (quite drastically!), just not if you assume a constant current, when in reality if you short the wires you will get MUCH more current. \$\endgroup\$ Oct 19, 2021 at 3:08
  • \$\begingroup\$ The same rules apply to both wires. The minimum acceptable wire size for positive and ground will be the same in your scenario. Either wire or both wires can be larger than the minimum. And there is no requirement that they both be the same size. Aesthetically, they should be the same size. It will seem odd and cause people to scratch their heads if they are not the same size. \$\endgroup\$
    – user57037
    Oct 19, 2021 at 3:09
  • \$\begingroup\$ @Ishouldchange Your title made me consider, only for the smallest flicker of time, talking about the fact that protons (positive) are almost 2000 times more massive than electrons (negative.) Sorry, my mind goes that way at times. ;) I think I see where your mind is at. But it is really about the voltage difference across the wire and the current in the wire that matters. And the difference dropped across the ground wire is no different than what is dropped by the supply side, given the same wire type and same length of wire. The physics isn't any different at the two opposite ends. \$\endgroup\$
    – jonk
    Oct 19, 2021 at 5:40

4 Answers 4

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There is no necessity to use the same size wires as long as both are adequate for the current each conducts. Going larger reduces heating, voltage drop but weighs more and may be more costly.

In some cases there might be a hidden return path (eg. through a chassis) and any ground wire might be just a return for a control current.

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  • \$\begingroup\$ To add to this, check the AWG wire chart for the maximum current that a wire can hold. You can have a 1A current running through a thick 4 AWG wire for the positive side and a thinner 14 AWG wire on the negative side, and they will both hold that current just fine. \$\endgroup\$
    – thisjt
    Oct 19, 2021 at 3:00
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In a simple series circuit, like a battery, resistor and LED, the current will be the same at all points in the circuit.

The required wire size is determined by the current flowing in the wire, regardless of where the wire is in the circuit. The required wire size may be determined by the heating effect of the current in the wire's resitance, or by the voltage drop caused by the wire's resistance (or by both effects).

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The current is equal at all points in a series circuit (Kirchoff's current law). The same amount of current therefore flows through the positive and ground wires.

Both wires need to be at least thick enough to handle the current the circuit draws, but they can have different thicknesses.

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The current carrying capacity of a cable is dependent on its cross-sectional area.

The positive and negative source cables (also known as 'bus') should both have adequate capacity to safely carry the total current.

Likewise, the positive and negative cables of individual devices should both have adequate capacity to safely carry the current drawn by each one of them.

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