Fire Alarm Lesson 5&6 Practice Test

The value is about how much resistance changes with temperature for a copper conductor. For 18 AWG, the resistance at a common reference temperature (20°C) is roughly 6.39 ohms per 1,000 feet. Copper’s temperature coefficient is about 0.00393 per degree Celsius, so increasing the temperature to the 75°C standard used in many electrical tables adds about 21.6% to the resistance. Calculating: 6.39 × [1 + 0.00393 × (75 − 20)] ≈ 6.39 × 1.216 ≈ 7.77 ohms per 1,000 feet. That’s why the unit resistance given is 7.77 Ω per 1,000 feet.