How to choose a welding machine
Four numbers decide whether a machine suits the work: maximum current, duty cycle, supply requirement and what the arc control offers. Everything else on the box is marketing.
Maximum current
Work backwards from the thickest single-pass weld you expect to make: about 40 A per millimetre. Thicker material is welded in several passes, so 5 mm plate does not need 200 A — but a machine at the very top of its range all day is a machine that spends half its time in thermal shutdown.
The current calculator gives the figure for the electrode diameters you intend to use, which is a more honest starting point than the plate thickness alone.
Duty cycle — the number that gets exaggerated
"200 A" on the front of the box usually means 200 A at 20 % duty cycle: two minutes of welding in every ten. What matters is the current at 60 %, and that figure is on the rating plate rather than the box.
Duty cycle also depends on ambient temperature — the plate figure is measured at 40 °C, and a machine in a hot workshop in August will do worse. The duty cycle calculator converts between currents and shows what supply the machine really needs.
The supply
The input current is not the welding current: a 200 A single-phase inverter draws around 30 A from the wall, which a standard domestic circuit cannot provide. Before buying, work out the input current and check what the building can give. A machine that can only be run at half output is a machine half bought.
Two practical points. Use a type C breaker, because the inrush of an inverter trips a type B. And if the machine will run off a generator, allow about 30 % margin — see why your welder trips the breaker.
Which features earn their price
Worth paying for. Hot start and arc force on an MMA machine — they turn a difficult basic electrode into an easy one. A proper wire feed unit with metal drive rolls and adjustable pressure on a MIG. HF start and a foot pedal or slope control on TIG. And AC output if you will ever weld aluminium, because without it you cannot.
Worth less than the price suggests. Synergic programs, which are useful when they match your wire and gas and misleading when they do not. Digital displays showing preset rather than measured values. And "IGBT" on the front panel, which describes a component every modern inverter contains.
The parts nobody photographs
The leads, the electrode holder, the earth clamp and — on a MIG — the torch are what you actually hold, and cheap machines save money there first. A machine supplied with 2 metres of thin lead and a stamped-tin clamp will feel weak regardless of what is inside it: the reason is in the cable size calculator.
Check spares availability too. Contact tips, liners and nozzles are consumables, and an orphaned machine whose torch parts nobody stocks is scrap long before it stops working.
Frequently asked questions
How many amps do I actually need?
Roughly 40 A per millimetre of the thickest material you will weld in one pass. For a home workshop up to 5 mm, a 160 A machine is enough; for 10 mm structural work, 200 A; beyond that you are into three-phase territory. Buying 250 A "to be safe" on a 16 A domestic supply buys you a machine you cannot run at full output.
Is a cheap inverter worth it?
For occasional light work, often yes. What you give up is duty cycle, the honesty of the rating plate, arc control features and the availability of spares in three years. What you must not give up is a CE mark and compliance with EN 60974-1 — that is the safety standard, not a marketing one.
Multi-process or single?
A combined MIG/TIG/MMA machine is genuinely useful if you will use all three. If you will really only use MIG, a dedicated MIG set at the same price will have a better wire feed unit — and the feed unit is what determines whether the machine is pleasant to use.
Updated: