Lights dim when the AC starts—not because the utility failed, but because voltage sagged on the branch circuit. Voltage drop is the gap between what leaves your breaker and what arrives at the load after fighting through copper.
Resistance turns volts into heat
Copper is a good conductor, not a perfect one. Every strand has resistance per meter. Current through that resistance obeys Ohm’s law:
Voltage lost in wire = current × resistance (× 2 for out-and-back)
Lost volts become heat in the jacket—warm conduits, tripped breakers, and energy you paid for that never reached the appliance.
Longer or thinner = worse
| Factor | Effect on drop |
|---|---|
| Longer run | Linear increase |
| Smaller AWG / mm² | Much larger increase |
| Higher amps | Linear increase |
A 30 m hop to a garage on 12 AWG at 40 A is a different project than a 5 m kitchen outlet at 15 A.
High drop hurts motors first
Induction motors need adequate voltage to develop torque. Chronic undervoltage causes:
- Higher current for the same mechanical work → more heat in windings
- Shorter insulation life and nuisance thermal trips
- Hard starts on compressors (fridge, heat pump, well pump)
- EV chargers reducing charge rate to protect electronics
Resistive loads (toasters, heaters) still work but deliver less power— a 1,500 W space heater on 110 V instead of 120 V is not 1,500 W anymore.
Thermal risk in the wire itself
Code ampacity tables assume proper installation. Combine undersized wire with continuous load (EVSE, heat strip) and you can exceed temperature ratings even if the breaker never trips—because breakers protect wires from short circuits, not from decades of mild overheating.
Pressure analogy for homeowners
Think of voltage like water pressure and wire like garden hose diameter:
- Skinny, long hose → pressure at the nozzle collapses
- Wider hose or shorter run → sprinkler works as designed
Electric “pressure” is volts; “flow” is amps. You need both at the end of the run.
What to do before you blame the appliance
- Log circuit length and gauge on a sketch
- Measure or estimate steady-state amps
- Run the Residential AC Voltage Drop Calculator
- If >3%, upsize conductor on the next pull—not after the third failed compressor
Related reading
- Residential Voltage Drop Guide — 3% vs. 5% targets
- Green Home Wire Sizing — project planning
Devices “get weak” when volts never arrive. Fix the path, and the same motor often outlives the warranty without a miracle board swap.