Derek Broughton wrote:
Yeah, but the pond's probably a long way from the nearest outlet on a
too-small extension cord. 1250/15 = 83V, which is one heck of a voltage
drop, but perhaps not impossible if the house is on the end of a long rural
run - where voltages tend to fluctuate quite a bit anyway.

C'mon Derek, that dog don't hunt. We ain't dealing with a motor here -- this
is just a big honkin' resistor (11.5 ohms, more-or-less), which isn't going to
change resistance significantly, no matter what the line voltage. 15 amps
through a 12.5 ohm resistor is nearly 2600 watts (which also implies 188 volts
across the resistor). Nope.

Either the breaker / fuse is weak, or there's another 500+ watts of other load
on the circuit.