Welding cable size calculator
Which cross-section a given current and cable length need — and how much arc voltage a long, thin run is quietly costing you.
How it is worked out
The calculation uses the full circuit: both the welding lead and the return
lead, because the current goes down one and back along the other. Voltage drop
is ΔU = 2 · ρ · L · I / S, with ρ = 0.0175 Ω·mm²/m for copper.
The permitted drop is normally taken as 4 V.
The symptom to recognise
A machine that welded fine on the bench and feels soft at the far end of a long extension is almost never faulty. It is the cable: the drop happens in the copper instead of at the arc, and the arc gets what is left. The usual combination is a 200 A machine, 30 metres of 25 mm² cable and a puzzled welder.
Two things that cost as much as an undersized cable: a return clamp on painted or rusty steel, and joints made with tape rather than proper couplers. Both add resistance exactly where you cannot see it. If the cable warms up noticeably along its length, it is undersized — a correctly sized welding cable stays close to ambient.
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