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.
Four worked examples from the data plate
The same arithmetic as the calculator above, for four typical plates. The I1max values are examples — put in your own.
- Inverter with PFC, 230 V, I1max = 16 A. S = 230·16 = 3.7 kVA; with the ×1.3 reserve, about 4.8 kVA.
- Inverter without PFC, 230 V, I1max = 24 A. S = 5.5 kVA, reserve ×1.8 — about 9.9 kVA. Similar welding output, yet a generator nearly twice the size of the PFC case.
- Transformer, 230 V, I1max = 40 A. S = 9.2 kVA, reserve ×2.5 — about 23 kVA. That is why transformer welders are practically never run off a portable generator.
- Three-phase inverter without PFC, 400 V, I1max = 10 A. S = √3·400·10 = 6.9 kVA, reserve ×1.8 — about 12.5 kVA; with PFC about 9.0 kVA is enough.
A cross-check with our other calculator: according to the duty cycle calculator, an MMA inverter at 200 A draws about 30 A from the mains, which is about 6.9 kVA, or about 9.0 kVA with a 30 % margin. That is the PFC case; without power factor correction the same welder needs about 12.5 kVA.
kW or kVA — reading the generator’s plate
The calculator answers in kVA because a welder loads the generator with apparent power. Generator plates often show both kVA and kW. The difference is the power factor — usually 0.8 for three-phase sets, so 10 kVA is 8 kW. Compare kVA with kVA. If the maker quotes only kW, ask at what power factor.
The second trap is two outputs on one plate: maximum (short-term) and rated (continuous). Welding needs the continuous figure. ISO 8528-1 defines several power classes and catalogues label them differently — “max”, “LTP” and “standby” are for short running, “COP” and “PRP” for long running. Compare the calculator’s result with the latter.
A three-phase generator and a single-phase welder
The 230 V socket on a three-phase generator usually delivers only the power of one phase, not the whole rating on the plate. A “12 kVA” set may therefore give a single-phase welder noticeably less. Makers quote the 230 V output separately — compare that figure with the calculator’s result at 230 V. Some sets have a selector that puts the full output on 230 V.
Eco mode and idle speed
Many generators drop their engine speed at light load (eco throttle, auto-idle). For welding that is a problem: at the strike the load jumps, and the engine first has to pick up speed. The voltage sags, the arc goes out, and an inverter may throw an undervoltage error. Switch eco mode off and weld at full speed — many generator and welder manuals say exactly that.
How to test a generator in practice
- Switch eco mode off, disconnect other loads and let the engine warm up for a few minutes.
- Set the current you will really use and burn a full rod, or run beads as long as your usual work.
- Watch: does the engine bog down on the strike, is the arc steady, does the welder show a supply error?
- Repeat at full current if you intend to use it.
If the engine struggles to recover on every strike and the arc is soft only on the generator, not on the mains, the reserve is too small. Dropping one rod size (for example from 3.2 to 2.5 mm) usually solves it faster than buying a bigger generator.
Common sizing mistakes
- Sizing by welding current. “200 A welder, 2 kW generator” — what counts is I1max, not the number on the case.
- Comparing with the generator’s maximum output instead of its continuous rating.
- Treating the 230 V socket of a three-phase set as the full output.
- Running a grinder and a compressor off the same set while welding — their start-up adds to the strike surge.
- A long, thin extension from the generator. How to work out its losses is shown in why your welder trips the breaker, in the extension lead section.
If you are buying a welder with generator work in mind, look for PFC and a generator-compatibility note on the data sheet — more in the guide to choosing a welding machine.
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.
What size generator for a 200 amp welder?
It depends on I1max on the plate. An MMA inverter at the full 200 A draws about 30 A from 230 V, which is about 6.9 kVA. With PFC (×1.3 reserve) you need about 9 kVA of continuous output; without PFC (×1.8) about 12.5 kVA. Welding at lower current needs less.
Is a 3 kW generator enough for a welder?
Only for the thinnest rods and only with a PFC inverter. On a 2.5 mm rod (about 85 A) an inverter draws about 11 A, roughly 2.5 kVA; with the ×1.3 reserve that is about 3.2 kVA, without PFC about 4.4 kVA. A 3.2 mm rod already needs about 4.5–6.3 kVA.
Should I compare kW or kVA?
kVA — that is what the calculator gives. If the generator quotes only kW, convert with the power factor on its plate (usually 0.8 for three-phase sets: 8 kW is 10 kVA). And compare with the continuous rating, not the maximum.
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