You need roughly 1 amp of plasma cutting power for every 0.001 inch (1 mil) of mild steel you want to cut cleanly in a single pass. A 40-amp plasma cutter handles up to about 3/8 inch cleanly and severs up to 1/2 inch; a 60-amp unit handles up to 1/2 inch clean and severs up to 3/4 inch; and shops running 3/4 inch plate or thicker all day need 80 amps or more with a CNC table behind it. The right number depends less on the thickest piece you might ever touch and more on what you cut every day and how fast you need to move through it.
What is the difference between a clean cut and a severance cut rating?
A clean cut rating is the thickness a plasma cutter can cut through with a smooth edge, minimal dross and a bevel angle tight enough to weld or finish without extra grinding. A severance cut rating is the thickest material the torch can physically punch through at all, usually with a rough edge, heavy dross and a wide kerf. Manufacturers list both because a shop cutting production parts needs the clean cut number, while a shop cutting demolition scrap or one-off maintenance parts can live with the severance number.
Fab-Line sees this trip up new buyers constantly: a spec sheet advertising “1 inch cutting capacity” is almost always the severance number. If your parts go straight to a weld joint or a paint line, size the machine off the clean cut rating, not the headline number in the ad.
How many amps do I need to cut 1/4 inch steel?
A 40-amp plasma cutter cleanly cuts 1/4 inch mild steel with room to spare and will move through it quickly enough for a one or two person fabrication shop. This is the most common amperage for job shops that primarily work in 10 to 14 gauge sheet up to 1/4 inch plate, since it balances air consumption, duty cycle and consumable cost against speed.
How many amps do I need to cut 1/2 inch or 3/4 inch steel?
A 60-amp plasma cutter is the standard choice for shops that regularly cut 1/2 inch mild steel cleanly and need to sever up to roughly 3/4 inch on the pieces that call for it. Once 3/4 inch plate becomes routine rather than occasional, an 80-amp system keeps travel speed and edge quality where a production schedule needs them, especially on a mechanized or CNC table where a slow cut costs real hourly rate.
Plasma cutter amperage by material thickness
| Amperage | Clean cut (mild steel) | Severance cut (mild steel) | Typical shop |
|---|---|---|---|
| 30 to 40A | up to 3/8 in | up to 1/2 in | Hand-held repair, light fabrication |
| 60A | up to 1/2 in | up to 3/4 in | General job shop, mixed thickness work |
| 80 to 100A | up to 3/4 to 1 in | up to 1.25 in | Production shops, CNC-fed tables |
| 125A+ | up to 1.5 in and heavier | up to 2 in | Heavy plate, structural, shipyard-scale work |
Stainless steel and aluminum cut at roughly the same amperage as mild steel of the same thickness, but expect a wider kerf and more dross on aluminum unless the torch and gas settings are tuned for it.
Does duty cycle matter as much as amperage?
Yes, and it is the number buyers skip past most often. Duty cycle is the percentage of a 10-minute window a plasma cutter can run at full amperage before it needs to cool down, and a machine that is technically rated for the thickness you cut but has a 40 percent duty cycle will bog down a shop running parts back to back all shift. A CNC table feeding a torch continuously needs a much higher duty cycle than a handheld unit used in short bursts for repair work, even at the identical amperage rating.
Should I size for the thickest piece I might ever cut, or for what I cut every day?
Size for what you cut every day, then check that the machine can sever your occasional thick piece even if the edge needs a grinder afterward. Buying an 80-amp system because you cut 1 inch plate twice a year, when 90 percent of your work is 1/4 inch sheet, means paying for consumables, air volume and amperage you rarely use. Fab-Line’s plasma cutting systems are sized by working with a shop on its actual job mix, not its worst-case job.
FAQ
What size air compressor does a plasma cutter need?
Most 40 to 60 amp plasma cutters need a compressor delivering at least 5 to 9 CFM at 90 to 120 PSI, with a dedicated air dryer or filter to keep moisture out of the torch. Undersized or wet air is one of the most common causes of poor cut quality and short consumable life.
Can one plasma cutter handle both thin sheet metal and thick plate?
A single machine can cover a wide range if you buy above your daily minimum, but cut quality on the thinnest material drops as rated amperage climbs unless the unit has a genuinely adjustable low-amperage range. A 60-amp system with fine control down to 15 or 20 amps covers sheet and plate better than a fixed 100-amp unit will.
How much does amperage affect cutting speed?
More amperage at the same thickness cuts faster, which is the real value of buying headroom for a production shop: an 80-amp machine cutting 1/4 inch steel moves noticeably faster than a 40-amp machine cutting the same material, even though both can do the job.
Fab-Line’s plasma cutting systems are specified with a shop’s real material mix, not a worst-case spec sheet, and our team has years of experience walking shops through amperage, duty cycle and table setup as part of every quote. Explore the full line of cutting and fabrication equipment on our services page, or read more on plasma cutting fundamentals from the Hypertherm’s own guidance on evaluating plasma cutter power.
Ready to size a plasma cutter for your shop? Contact Fab-Line or call (888) 317-3615 to talk through your material mix with a member of our team.