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However, the fuel type itself can be an issue. "Some construction sites mandate diesel fuel only," says Leisner. Since the welder/generator is bolted to a mobile service truck, you may get away with a gas engine. But on other sites, there may be safety concerns with using gasoline.

"A lot of times, the thinking is I want to match the welder fuel to the truck fuel," says Snyder. "When I get re-fueled, I can do it all at the same time. Plus, I don't have the risk of putting the wrong fuel into the welder."

Gouging determines size
Carbon arc gouging is a common practice that often determines the size of the welding unit required. Consider the case of a cracked bucket. "Generally, the procedure is you want to remove the metal down to the sound material at the base of the crack," says Snyder. "You don't just fill the crack in. One good way to do that is with the arc gouging process."

The arc gouging process consists of melting the metal using an arc created with a carbon rod, then using compressed air to blow the molten metal away. "Usually, a separate compressor supplies the air," says Snyder. "However, a few engine-driven welders are manufactured with built-in compressors."

Under-sizing a welding unit for carbon arc gouging ranks among the most common mistakes. This problem is compounded when you try to use carbon rods that are too large for the welder output. "Then you get the call that the machine doesn't work real well," says Snyder.