Welding cable voltage drop
A soft arc on a long lead is not the machine being difficult — it is volts lost on the way. Below: how many go at your current, length and cross-section, and how much of that turns into heat.
How it is worked out
One formula: ΔU = 2 · L · I · ρ / A, where L is
the length of one lead in metres, I the current in amps,
A the cross-section in mm², and ρ the resistivity of
the conductor: 0.0175 for copper, 0.0282 for aluminium.
The two in the formula is the return path. Current comes back through the work lead, so it travels twice the distance from machine to job. That is why the calculator asks for one lead, not for the sum of both.
The power lost in the cable is P = ΔU · I. At 200 A with
a 4 V drop that is 800 W — the cable heating instead of the metal
melting. That is where the warm lead after an hour's work comes from: not
a fault, just this arithmetic.
This calculator answers "what does the cable I already have cost me". The opposite question — "what cross-section do I need to stay within a given drop" — is answered by the cable cross-section calculator. The formula and the resistivities are the same, so the two always agree.
What counts as too much
| Drop | What it means in practice |
|---|---|
| up to 2 V | Not noticeable at the arc; the lead has margin |
| 2–4 V | Borderline. The arc still holds, but the cable runs warm |
| over 4 V | Soft arc, poor striking, electrode sticking on the start |
These bands are workshop practice, not a standard. They come from a simple comparison: a covered-electrode arc runs at roughly 25 V, so 4 V lost in the cable is a sixth of the voltage that is no longer where it is needed.
What to do when it comes out too high
A thicker lead works in direct proportion: double the cross-section and the drop halves. It is the one measure that always works.
A shorter run works just as strongly and is often cheaper: moving the machine to half the distance does what doubling the cable does. Surplus cable coiled in a ring does not shorten anything and, on alternating current, adds an inductive nuisance of its own.
The earth clamp is often more to blame than the cable. Contact resistance across rust, paint or a slack clamp fits into no formula and can eat more than ten metres of lead. The clamp goes on cleaned metal, as close to the joint as the job allows.
Aluminium instead of copper gives 60 % more drop at the same cross-section. Aluminium cable is cheaper, but to match copper it has to be two sizes thicker — and the price advantage disappears.
What next
Choosing a cross-section for a given drop — the cable cross-section calculator. How long the machine will run without a break — duty cycle. Running off a generator — generator for a welder. Why a wet lead and a bad earth are more than a soft arc — electrical safety.
Frequently asked questions
How many volts may a lead lose?
Workshop practice puts the ceiling at about 4 V at working current, and under 2 V is better. It is not a requirement of any standard, just the point beyond which the arc visibly changes: with roughly 25 V at the arc, every volt lost is four per cent gone.
Why is the length multiplied by two?
Because the current has to come back. The circuit closes through the work lead, so the metal carries current over twice the distance from machine to workpiece. The calculator asks for the length of one lead and doubles it itself — do not add both leads by hand.
Do joined extension leads change the result?
They do, and usually for the worse. Every connector adds contact resistance that this formula knows nothing about; a tarnished or loose plug can cost more than ten metres of cable. If the arc feels soft despite a correct cross-section, check the connectors and the earth clamp first.
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