Every Winter, the Space Heater Audits Your Wiring

A 1500 W space heater is the most demanding thing most receptacle circuits ever serve: 12.5 A, continuously, for hours. It is also the cheapest circuit-testing instrument money can buy — and every winter it finds the weak links.

What the heater reveals

• Honest drop: on a 14 AWG circuit 70 ft of cable-path from the panel, 12.5 A drops about 5.4 V — 4.5%, and the heater delivers ~1370 W instead of 1500. Long runs on old 15 A circuits routinely test worse.
• Bad terminations: a warm plug or faceplate is concentrated resistance — backstabbed receptacles and tired contacts show themselves under 12.5 A within the hour. Warm is a work order; hot is an emergency.
• Shared circuits: the breaker that holds the heater fine until the vacuum starts is telling you the circuit's whole budget, thermal and voltaic, is spoken for.

The triage

Measure the receptacle loaded and unloaded (the multimeter guide covers it). Compare against the calculator's prediction for the circuit's gauge and estimated cable path. Matching numbers mean the circuit is just long and thin — acceptable for a heater, annoying for anything sensitive sharing it. A measured drop far above prediction means a connection to find now.

The fix hierarchy

Move the heater to a closer circuit; replace backstabs with screw terminations; and for the room that always needs heat, a dedicated 20 A circuit — or better, a 240 V heater at half the current — ends the annual audit for good.