Plasma Cutter Amps Chart: Amps and Air by Thickness

Plasma cutter amps chart by thickness

No maker publishes amps for a thickness as a rule. Each one rates its own machines, so the chart gives the smallest machine in each maker’s range whose rating covers the thickness, in mild steel. Where the makers differ, both figures are shown. Everlast’s rule of thumb for most brands sits beside them: 20 A to 1/8 in, then 10 A for each extra 1/8 in.

Every machine has two thickness ratings, and they are far apart. The clean-cut rating (Miller and Hobart say “rated”, Hypertherm “recommended”) is what it cuts at a steady hand speed. The sever rating is the most it can part at a crawl, with a rough edge. Size a machine by the clean-cut columns.

The amps are the machine’s top output, not a dial setting: on a bigger machine, turn down to the amps your cut chart gives for the plate and the tip fitted. Tips are sold by amp rating (Hobart lists 30 A and 40 A tips for one torch), so match the tip to the amps. For Everlast’s rule, this chart counts 3/16 in as a full 1/8 in step. These ratings belong to the machines named: another brand at the same amps may cut less or more, so its own cut chart wins.

Protect your eyes: OSHA’s minimum lens shade for plasma cutting at these amps is shade 8 where the arc is clearly seen. The welding shade chart has the full table. For the outlet and breaker, take the input amps in the machine chart below to the welder breaker size chart; for field work, the generator size calculator sizes the generator. The welding cost calculator prices a job.

Plasma cutting throws sparks and hot metal well away from the cut, and the fume from galvanized, painted or stainless metal is harmful: clear the area of anything that burns and cut with good ventilation. Never cut a closed container. Switch the machine off before you change the tip or electrode, as its manual says.

Clean-cut and sever ratings by machine, with air

The makers’ own figures for mild steel, with the hand travel speed each rating is taken at (ipm is inches per minute). Hypertherm lists two recommended thicknesses, a faster one (20 ipm on most models) and a slower one (10 ipm). Piercing in the middle of a plate has a lower limit than an edge start: 3/16 in at 30 A, 5/16 in at 40 A and 7/16 in at 60 A on Miller’s sheets.

Miller and Hobart call their travel speeds approximate. The Hypertherm 45, 65 and 85 A machines ask for 450 scfh of air, which is 7.5 cfm. Miller and Hypertherm quote standard cfm (scfm).

Stainless steel and aluminum

Miller gives a clean-cut rating in each metal for three machines. Its sheets and Hobart’s add that aluminum and other metals that carry heat away fast can cut as much as 30% less than mild steel. Hypertherm’s capacity pages give one figure per machine, with no separate stainless or aluminum rating; its manuals have a cut chart for each metal.

What size air compressor for a plasma cutter

The cutter’s manual gives the air it uses, and compressor makers advise a margin on top. Two of them put a number on it: Quincy Compressor adds 25% to the tool’s air, and Chicago Pneumatic sizes a piston compressor at 1.5 times it, noting that a single-stage piston compressor normally has a 50% duty cycle. The chart applies both to each maker’s air figure. The tank is Quincy’s rule of thumb, at least 5 gal per cfm.

Chicago Pneumatic says to size a compressor by its cfm at the pressure you need, not by horsepower. Everlast, which makes plasma cutters, says most need about 5 to 6 cfm at a minimum, that a compressor able to keep up is usually around 20 gallons or more, that small pancake compressors are not enough for any but the smallest cutter, and that the air line wants a separate dryer. Hypertherm says dirty, oily air is the root cause of many common problems: the air must be clean, dry and oil-free.

Plasma cutting on a 120 V outlet

On 120 V the makers hold these machines below full output: Miller’s and Hobart’s 40 A machines drop to 27 A by themselves. These are the makers’ figures for mild steel. Miller adds that a 30 A machine on a 120 V, 20 A breaker loses 20% of its cutting capacity.

Running a CNC table? This page sizes the machine and its air. Feed rate, kerf and pierce height depend on the torch, the consumables and the height control, so take them from the cut chart in your machine’s manual.

Quick answers

Your machine’s own manual and cut chart win over every figure here. Thicknesses are in inches with millimetres beside them; air is in standard cubic feet per minute (1 cfm is about 28.3 L/min) and pounds per square inch.

• Rated and sever cuts, travel speeds, air and input amps: Miller’s Spectrum 375 X-TREME, 625 X-TREME and 875 spec sheets and Hobart’s AirForce 12ci, 27i and 40i spec sheets. Compiled in our own words.
• Stainless and aluminum ratings: the “Max Thickness” lines on Miller’s product pages for the same three machines.
• Recommended and severance capacity, air and input amps: Hypertherm’s Powermax comparison, its Powermax30 XP, 30 AIR, 33 XP, 65 SYNC and 85 SYNC specifications and its Powermax45 SYNC operator manual.
• What rated and sever cuts mean: Miller’s guide to choosing a hand-held plasma cutter and the notes on the spec sheets.
• The rule of thumb and the compressor advice for plasma cutters: Everlast’s articles on choosing amperage and on air compressors.
• Compressor margins and tank size: Quincy Compressor’s sizing guide and Chicago Pneumatic’s “Understanding Compressed Air” and “How Do You Size a Piston Air Compressor?”. Applying them to a plasma cutter’s air is our own arithmetic.
• Choosing “the smallest machine whose rating covers the thickness” is our own reading of the makers’ ratings, not a published amps rule.