Duty cycle is the number of minutes out of a 10-minute cycle a welder can operate. For example, the Thunderbolt XL creates a 200 amp DC output at 20 percent duty cycle. It can weld continuously at 200 amps for two minutes, and then must cool for eight minutes to prevent overheating.

Duty cycle and amperage are inversely proportional. Operating at 90 amps, the Thunderbolt has a 100 percent duty cycle, meaning you can weld without stopping. This inversely proportional rule is true of all Miller machines but does not apply to all machines made by other companies.