Measure your voltage at the work site while it is under load and divide wattage by that. Voltage will dip under load, especially with long/small conductors like extension cords, or anything else that causes resistance (so if your outlet in the house is 121v, you might have 105 at the end of a 14g extension cord that is pulling 10 amps).
Heat also causes resistance. As your conductor heats up, resistance goes up more, which heats the conductor more, and so forth until something melts or the breaker goes. This is one reason to use an extension cord bigger than your wiring.)
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You'll also get more accurate info with a dumb meter on the leg in question than you will with a clamp-on device, especially one that is trying to guess current on individual runs from the service entry...