Voltage Drop Calculator

Calculate voltage drop over a copper wire run by current, length, and gauge

Calculating voltage drop over a copper wire run — based on current, length, and wire gauge — matters because real wire has resistance, and that resistance causes a measurable voltage loss over distance that becomes more significant the longer the wire run and the higher the current it carries, which is exactly why wire gauge selection matters for longer runs, not just for a wire's rated current capacity.

Why wire gauge affects voltage drop, not just current capacity

Every wire gauge has an associated resistance per unit length — thinner wire (higher gauge number, in the standard AWG system) has higher resistance per unit length than thicker wire (lower gauge number), and that resistance, combined with the current flowing through it and the total length of the run, determines how much voltage is lost between the source and the load. This means a wire gauge that's perfectly adequate for a short run's current-carrying capacity can still cause a problematic voltage drop over a much longer run, even though the wire itself isn't being overloaded in a safety sense — voltage drop and current-carrying capacity are related but genuinely separate considerations when selecting appropriate wire gauge for a specific installation.

A worked example

The identical wire gauge and current can produce an acceptable voltage drop over a short run but an unacceptably large one over a much longer run — the voltage drop scales with wire length (more resistance accumulates over more distance), which is exactly why longer wire runs often need a thicker gauge than the current-capacity rating alone would suggest, specifically to keep voltage drop within an acceptable range at the load end.

How this connects to your other electrical calculations

Once a wire's actual voltage drop is known, that figure connects directly to Ohm's Law — the voltage drop itself is a direct consequence of the wire's resistance and the current flowing through it, the same V=IR relationship applied specifically to the wire as a resistive element in the overall circuit.

Common mistakes

Selecting wire gauge based solely on current-carrying capacity (making sure the wire won't overheat) without separately checking voltage drop for longer runs is a common oversight — a wire that's safely rated for a given current can still cause a functionally problematic voltage drop at the load end if the run is long enough, so both considerations need checking independently for longer wire runs specifically.