What a 13A or 16A supply actually lets a welder do
A single-phase machine plugs into a standard supply, and that supply is what sets the ceiling on what the machine can do. Most workshops and vans run off a 13A socket, and some fixed workshop circuits offer 16A. Either way, the wall socket is the real limit.
Why the amperage on the machine is not the amperage you get
A welder rated at 200A output needs considerably more current on the input side to produce it, because the transformer or inverter is converting mains current into welding current, not multiplying it for free. On a 13A supply, that conversion runs out of headroom well before the machine's stated maximum. The number printed on the front of the machine is what it can produce under ideal conditions, often on a 16A or higher supply. It is not what you will get from a domestic socket.
In practice, this means a single-phase inverter welder sold as a 200A machine will often only sustain something closer to 140 to 160A continuously from a 13A supply before it pulls more current than the circuit can safely deliver. Push past that and you trip the breaker.
What this means for your work
For most site repairs, fabrication on lighter material, and general MIG or MMA work, the usable amperage from a 13A or 16A supply is enough. The constraint starts to bite when you are running thicker material that needs sustained high current, or when you want the full rated output for long stretches without derating. If your work sits in that territory, the supply itself becomes the thing you are buying around.
- 13A supply: standard domestic and van sockets, the tightest constraint on usable output.
- 16A supply: common on fixed workshop circuits, gives more headroom before the machine has to back off.
- Machine's rated maximum: the figure on the spec sheet, achievable only where the supply can feed it.
Checking which supply you actually have is worth doing before the machine arrives. The MIG welder buying guide covers how to read the rest of the spec sheet alongside this, and duty cycle explained deals with the related question of how long a machine can hold any given output before it needs to cool.
When single phase stops being enough
Most welding work in the UK runs perfectly well off a single-phase supply, and for a lot of trades that never changes. The real question isn't whether three-phase is better in the abstract, it's whether your working pattern pushes past what a single-phase machine can sustain.
Where the output ceiling sits
A single-phase machine is limited by what the incoming supply can deliver, and that shows up as a maximum output the machine can't exceed no matter how good the inverter inside it is. A three-phase supply spreads the load across three conductors, so a three-phase machine can draw more current overall and hold a higher output without tripping the supply or overheating the lead. If your work sits comfortably below that ceiling, the extra phases buy you nothing.
Duty cycle at the amperage you actually use
Duty cycle, covered in detail in the guide to duty cycle, is the share of a ten-minute period a machine can weld at a given output before it needs to cool. A single-phase machine's duty cycle tends to fall off sharply as you push it toward its maximum output, because that output is already close to what the supply can give. A three-phase machine rated similarly typically holds a higher duty cycle at that same amperage, because it isn't working as close to its own ceiling. For someone doing occasional repairs, that gap rarely matters. For someone running a bead for most of a working day, it does.
Material thickness and the amperage it needs
Thicker material needs more amperage to get a proper weld in one pass, and that is where the single-phase ceiling starts to bite. Thin sheet and general fabrication sit well within single-phase range. Structural steel, thick-wall pipe and heavy plate push the amperage requirement up to a point where a single-phase machine either can't reach it at all, or reaches it only at a duty cycle too low for production work.
When paying for a three-phase supply makes sense
Installing a three-phase supply is a cost most tradespeople outside a fixed workshop struggle to justify. It starts to make sense when the work demands that extra output on a regular basis. If most of your work is structural fabrication at high amperage, day after day, the supply cost is the price of doing that work at all. If it's an occasional job among lighter work, hiring in a machine or subcontracting that piece out is usually the cheaper route.
Single-phase vs three-phase: output, duty cycle and supply
The right column to weigh depends on how the machine will actually be used day to day.
| Supply type | Best for | Duty cycle under load | Portability |
|---|---|---|---|
| Single-phase, 13A domestic socketCheck what the circuit can actually sustain | Light repairs, thin-gauge steel, occasional use | Falls away quickly as you approach the top of the machine's rated amperage | Light enough to carry to a job by hand |
| Single-phase, 16A or 32A supplyConfirm the outlet is actually fitted before assuming it's there | A mobile welder or small workshop doing steady daily work on mild steel | Holds up better through the mid-range but still tails off near maximum output | Still liftable by one person, but heavier than a plug-in inverter |
| Three-phaseConfirm a three-phase supply is actually installed before ordering the machine | Production welding, thicker material, a machine running most of a shift | Sustains output close to its rated amperage without the same drop-off | Usually installed and left in one place |
This is general guidance on how the two supply types behave under load. Duty cycle and usable amperage vary by model, so check the manufacturer's datasheet for the machine you're considering.
Questions about power supply
Can you run a three-phase welder from a generator?
Only if the generator itself produces three-phase output, and most generators used on general site work are single-phase. A three-phase generator also needs to be sized well above the welder's rated current, because the peak draw when the arc strikes is higher than the running amperage, and an undersized generator will trip or stall. If your work already depends on a generator for supply, it's worth checking that against what you need before choosing a welder.
What does a three-phase supply installation cost?
It depends almost entirely on how far your premises are from the network's existing three-phase infrastructure, which is something only your local distribution network operator can quote on. A workshop already near a three-phase run is a different job from one that needs new cable brought in from the road, and the difference between those two cases is large. Get a quote from the network operator before committing to a three-phase machine on the assumption that the supply will follow easily.
Is a phase converter worth it?
A phase converter lets a three-phase machine run from a single-phase supply, but it's an added piece of equipment with its own cost, and it doesn't always deliver the machine's full rated output. Rotary converters generally cope with welding loads better than static ones, though neither is free of running losses. If you're choosing a welder from scratch and don't already have three-phase on site, buying a single-phase machine rated for your actual working amperage is usually simpler than adding a converter to make a three-phase one work.