Plasma cutter selection turns on the rated cut vs sever cut pair, where rated cut is the clean separation thickness on mild steel at production-acceptable speed, and sever cut is the maximum thickness the arc can pierce through, usually with dross and lower edge quality [S1][S2].
For a hand-held or light-CNC shop cutting mostly 6 mm mild steel, a 25-30 A inverter power supply covers production work, while a 45-65 A machine is the practical floor for any code that lists 12 mm or 16 mm capacity [S1][S7].
Rated cut, sever cut, and the 80-20 thickness rule
The 80-20 rule states that the rated cut capacity of a plasma power supply should be about 80% of the thickest material you intend to cut, leaving 20% headroom for the arc to run at production speed without bogging down [S2]. A power supply rated 10 mm rated cut is therefore a sensible match for the user who rarely exceeds 8 mm mild steel.
Sever cut is the absolute ceiling and should not be the planning number, because cuts near the sever limit are slow, produce heavy dross, and shorten consumable life, according to multiple OEM sizing guides [S1][S2]. When the workpiece regularly crosses 25 mm mild steel, the practical answer is a 100-200 A high-definition system rather than a bigger conventional cutter [S1][S3].
Amperage, consumable nozzle, and cut charts
Amperage on the power supply must match the consumable nozzle rating, with a 45 A nozzle requiring the system be set to 45 A; under-setting produces low energy density, dross, and incomplete penetration, while over-setting accelerates nozzle wear [S4]. The factory cut chart in the operator's manual lists kerf, torch-to-work distance, initial pierce height, pierce delay, cut speed, and arc voltage per material and thickness, and remains the single most reliable tuning reference on the floor [S4].
Cut height matters as much as amperage: if the torch rides more than about 0.005 in. above the recommended standoff, expect angularity, wider kerf, dross, and possible warping on thin plate, while going closer risks torch collision [S4]. For CNC table work, a height control with arc-voltage feedback is the standard fix for this tolerance problem, and is one of the reasons hand-held and CNC consumable lines are sold as separate kits [S4][S5].
Comparison: 30 A inverter vs 60 A mid-range vs 130 A heavy CNC
Three tiers cover most buyers. A 30 A single-phase inverter typically delivers 6-8 mm rated cut on mild steel, weighs 15-25 kg, and runs on 230 V at around 30 A input, suiting site work, HVAC, and thin plate fab [S1][S7]. A 60 A three-phase machine reaches 12-16 mm rated cut, 25 mm sever, and adds higher duty cycle for production fab shops [S1][S3]. A 130 A class CNC high-frequency start system pushes rated cut to 25-32 mm mild steel with finer kerf and lower dross when paired with a height control and quality torch [S3][S5].
Decision rule: pick the tier whose rated cut is closest to your 80-percentile plate thickness, then verify duty cycle at that amperage, not at the brochure maximum [S2][S3]. The plasma cutter reference consolidates these tier definitions, while buyers working near seawater should cross-check the marine duty spec map at marine plasma cutter selection guide.
Duty cycle, primary power, and air supply
Duty cycle is the percentage of a 10-minute window the power supply can deliver rated output at 40 deg C, and a machine rated 60% at full output is sufficient for most fab work but will trip on a continuous long bead above rated amperage [S1][S7]. Industrial CNC cells typically demand 80-100% duty cycle at the operating amperage, which is why CNC-class power supplies are heavier, three-phase, and use larger internal heat sinks [S3][S5].
Primary power drives input current and breaker sizing: a 30 A single-phase unit needs a 30-40 A 230 V branch, while a 130 A three-phase system draws roughly 50-65 A per phase at 400 V, requiring a dedicated feeder and a disconnect that meets local welding-circuit rules [S1][S3]. Compressed-air consumption scales with orifice size; a typical 30 A torch needs 100-130 l/min at 5-6 bar, and a 60 A torch roughly 150-200 l/min, so the air system and plasma duty cycle should be specified together [S1][S3].
Who a small inverter is for, and who should skip it
A 25-30 A inverter is the right tool for site fitters, HVAC ductwork, auto-body patch work, and any user cutting mostly 3-6 mm steel or stainless with intermittent duty, and it is also the only realistic portable choice where single-phase 230 V is all that is available [S1][S7]. Hand-held plasma also gives the operator a CNC-like cut quality on 1-4 mm sheet when paired with a drag shield and a steady hand [S1].
Skip the small inverter if the workload includes 15 mm-plus structural plate, stainless above 10 mm, or any code requiring bevel cuts above 25 deg, because the pierce delay grows, kerf widens, and consumable cost per metre rises faster than the savings on the power supply [S2][S3]. Skip a generic consumer-grade unit entirely if the shop runs two shifts, because duty cycle below 35% at rated output will throttle production within the first quarter of use [S7].
Mechanical fit: plate size, CNC table, and consumable changeover
For CNC cutting, plate and sheet footprint dictates the cutting envelope: a 1.5 m x 3 m table is the practical minimum for a fab shop, while structural steel and shipyards typically run 2 m x 6 m or larger with a downdraft or water table for fume and slag capture [S5]. The CNC controller must accept the plasma height-control signal (typically arc-voltage based) and the start/stop handshake the power supply expects, which is why many buyers standardise on a single brand of torch and power supply [S5].
Consumable changeover is the hidden operating cost. A 30 A drag-shield setup uses a nozzle, electrode, swirl ring, and shield, while a 60 A fine-cut setup adds a different nozzle geometry, and a 130 A high-definition torch may use a different electrode material and a water-cooled shield; mixing brands generally voids the cut-chart guarantee and is the most common cause of premature consumable failure [S4][S5]. For buyers also weighing related heavy equipment, the construction machinery and equipment reference covers adjacent fab-yard tooling, and the mold base selection map is a useful parallel for shops that pair stamping and cutting cells.
Sourcing, standards, and shortlist logic
Selection checklist before issuing a PO: verify the rated cut on the actual material (mild steel, stainless, aluminum) for the thickness you need to cut; use the sever cut only as a sanity ceiling; check the duty cycle at the operating amperage, not at the brochure maximum; match primary power to the available feeder; confirm airflow and pressure; select the consumable family and consider total cost per metre of cut; and for CNC installs, ensure height-control integration [S1][S2][S3][S4][S5].
Shortlist logic: if the buyer cuts under 8 mm mild steel intermittently, a 30 A single-phase inverter is the correct call; if 8-16 mm mild steel is the daily work, step up to a 60 A three-phase machine; above 16 mm or any code requiring dross-free, square-edge stainless, specify a 130 A high-definition CNC system with arc-voltage height control, factory cut chart, and a matched consumable contract [S2][S3][S5]. Track next quarter: tier-1 OEM cut-chart updates for new nozzle geometries and any 2026 catalog additions to 200 A class inverter power supplies for shipyard and structural work [S3][S5].
Spec-level background on the components involved: linear guide.