Originally Posted by shiprek
If you have a clamp on ampmeter you can use this to determine what amperage you are drawing. For example, a 1500 watt heater should pull approximately 12.5 amps +/-. The resistance of your heater should read roughly 9.6 ohms depending on its age and condition. A 14/3 extension cord is close to the maximum allowable current (15 amps max.). If you are dealing with a very long extension cord you also have to deal with voltage drop will will cause more current draw and possibly melt your extension cord. If you have not tripped a breaker (or blown a fuse) on your household circuit I suspect you extension cord is too small (wire size) or too long (50-100 feet?) If this is also being used outdoors/in a garage it should be on a GFCI. Hope this helps.
A few clarifications. First, a clamp on amp meter would need to be clamped around only one wire in the extension cord, either the positive or neutral, but not the ground. If you clamp it around the entire extension cord, it will read zero as the current in the positive and neutral will cancel each other out to the meter. Second, since the heater is a resistive element, additional resistance and voltage drop in the extension cord will lower the total current being drawn.