CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
A technician is running 24-volt thermostat wire for a distance of 200 feet to a furnace. Using a standard wire-gauge distance chart (18 AWG rated for runs up to 100 ft, 16 AWG for 100–200 ft, 14 AWG for 200–300 ft) to keep voltage drop under 5%, which wire gauge should be installed for this run?
- A20 AWG
- B12 AWG
- C18 AWG
- D16 AWG
Show answer & explanationAnswer & explanation
Correct answer: D. 16 AWG
At 200 feet, 18 AWG wire's resistance causes excessive voltage drop (calculated drop exceeds 5% of 24V, or about 1.2V), so the chart calls for stepping up to 16 AWG, which keeps resistance—and thus voltage drop—within acceptable limits for reliable low-voltage control operation.
Why the other options are wrong
- A. 20 AWG has higher resistance than 18 AWG and would cause even greater voltage drop at this distance.
- B. 12 AWG is unnecessarily heavy and not standard practice for typical thermostat wiring.
- C. 18 AWG is only rated for runs up to 100 feet before voltage drop becomes excessive.
Low-Voltage Wire Sizing
Thermostat/control wire gauge must increase with run distance to limit voltage drop, since thinner wire has higher resistance per foot.
- 18 AWG: up to ~100 ft
- 16 AWG: ~100–200 ft
- 14 AWG: ~200–300 ft
- Excessive voltage drop causes erratic relay/thermostat operation
Memory trick: Longer run, thicker wire—resistance adds up like tolls on a highway.