Generator sizing for a welding machine
A 160 A machine does not need a 160 A generator — it needs the power it draws from the supply, plus a reserve for the fact that it draws it in steps. Worked out below from the rating plate, because that is the only honest source.
Where the numbers come from
The apparent power of the machine comes straight off the plate:
S = U₁ · I₁max on a single-phase supply and
S = √3 · U₁ · I₁max on three-phase. That is the power the machine
really pulls out of the socket — not the welding current printed on the case
in large figures.
Converting "welding current → supply draw" through efficiency and power factor spreads the answer almost twofold, because both depend on how the machine is built. This calculator therefore does not ask for welding current at all.
The reserve: why three figures
| Machine type | Reserve | Why |
|---|---|---|
| Inverter with PFC | ×1.3 | The corrector smooths the draw; the load is close to linear |
| Inverter without PFC | ×1.8 | The input rectifier draws current in spikes the set has to cover |
| Transformer | ×2.5 | Low power factor and a hard surge at every strike |
These factors are rounded experience and should be read as such. The manuals for the machine and for the set take precedence: some makers state the minimum source rating outright, and then there is nothing to discuss. Where two numbers disagree, take the larger.
AVR is not optional
A set without automatic voltage regulation holds its voltage only under a steady load. Welding is a stepped load: strike, short circuit, pause. Without AVR the voltage jumps with them, and the input electronics of an inverter take those jumps on themselves.
The second thing worth asking about before buying is waveform distortion. Inverter generators give a cleaner waveform and treat a welding machine more gently than plain synchronous sets, though they cost more.
Three things from the field
Duty cycle counts twice. A set sized with no margin heats up along with the machine, and a hot set loses output. If you plan long runs, take the reserve from the top of the range rather than the bottom — the duty cycle calculator helps here.
The lead from the set loses volts too. Ten metres of undersized extension can undo the power margin you paid for. Exactly how much — cable voltage drop.
Earthing a generator is its own subject. Working in the field off a set does not suspend shock-protection rules, and in many configurations it changes them — see electrical safety.
Frequently asked questions
Where do I find I₁max?
On the rating plate of the machine. It appears twice: I₁max is the supply current drawn at maximum welding current, I₁eff the value averaged over the duty cycle. For sizing a generator you take I₁max: the set has to cope with the peak, not with the average.
Why must the generator be bigger than the machine?
Because a welding machine is a non-linear, pulsed load. It draws current in steps, in time with striking and short-circuiting droplets, and a generator answers that rhythm with a dip in voltage and speed. The reserve covers those steps; without it the voltage sags, the arc dies, and on an inverter it ends in electronics.
Can you weld off a set without AVR?
With a transformer machine, awkwardly, yes. With an inverter, no. An unregulated set throws voltage spikes whenever the load changes, and the input electronics of an inverter do not forgive them; the repair usually costs more than the difference in price of a set with AVR.
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