After determining the process(es) you want, you then need to determine output power requirements. There are two factors that determine output power. The first is material thickness (and the second is the electrode diameter and type, covered in section 5). As a general rule of thumb, you need 1 amp of power for every .001" of mild steel. For example, to weld 1/8" (.125in) mild steel, you would need approximately 125 amps. Stainless steel needs about 10% less power, while aluminum requires about 25% more. Remember that for thin material you need a machine with good "low end" performance. For thicker material, you'll need a machine with a high duty cycle (see section 6).

Amperage for a Rated Plasma Cut on Mild Steel
To provide a standard for measuring cut quality and cutting speed, Miller uses the terms "Rated cut." A Rated cut is the smoothest cut possible at a minimum cutting speed of 10 inches per minute using a hand-held torch.

Mild steel thickness	1/8"	3/8"	1/2"	5/8"	7/8"	1"	1-1/4"
Rated Output of Unit (Total power = Amps x Volts DC)	12 A at 110 V	27 A at 90 V	40 A at 140 V	50 A at 110 V	55 A at 110 V	55 A at 110 V	100 A at 120 V