Watts ÷ 120 gives amps; a 1200 W load is about 10 A. At 14 AWG / 100 ft that's within rating but voltage drop lands in the marginal band.
Stop and call a licensed electrician or emergency services now if there's smoke, sparks, a burning smell, heat, shock, or water near the problem. Otherwise it's safe to answer the questions below.
The likely readout
Most likely cause
14 AWG at 100 ft, ~10 A: 4.2% voltage drop — marginal (okay briefly, not for motors)
Ranked by fit to your answers
1
Marginal — voltage drop in the 3–5% range
90
SAFE NEXT CHECK4.2% is acceptable for brief, resistive loads but hard on motors over time. For continuous or motor loads, step up to about 12 AWG.
Where to stop. Choosing or replacing an extension cord is homeowner-safe. Manufacturers and fire authorities advise plugging high-draw heating appliances such as space heaters directly into a wall receptacle rather than any extension cord. Anything involving the building's wiring or circuit rating is a licensed electrician's job. This is general information, not a quote and not a substitute for a licensed electrician.
Try the safe next check above. If it doesn't resolve it, or would mean working on wiring or a panel, stop and call a licensed electrician — don't replace parts on a guess. Open the full tool to change any answer for your exact situation, or try a related check below.
Electrical Fault Check provides general diagnostic information only. It is not professional advice, not a quote, and not a substitute for a licensed electrician. Do not work on live wiring. If you see smoke, sparks, burning smell, heat, shock, water exposure, or repeated tripping, stop using the circuit and contact a licensed electrician or emergency services as appropriate.