Using the given unit resistance of 7.77 Ω per 1,000 ft for 18 AWG copper, what is the loop resistance for a 5,000 ft run?

Using the given unit resistance of 7.77 Ω per 1,000 ft for 18 AWG copper, what is the loop resistance for a 5,000 ft run?

• A. 39.0 Ω
• B. 77.7 Ω
• C. 155.4 Ω
• D. 7.77 Ω

Answer: (B)

The main idea is that resistance depends on length, and the loop resistance accounts for both directions of the circuit. For 18 AWG copper, the unit resistance is 7.77 Ω per 1,000 ft. A 5,000 ft run has a one-way resistance of 5 × 7.77 = 38.85 Ω, but the loop includes a return path, effectively doubling the length to 10,000 ft. So the loop resistance is 10,000/1,000 × 7.77 = 10 × 7.77 = 77.7 Ω.

The main idea is that resistance depends on length, and the loop resistance accounts for both directions of the circuit. For 18 AWG copper, the unit resistance is 7.77 Ω per 1,000 ft. A 5,000 ft run has a one-way resistance of 5 × 7.77 = 38.85 Ω, but the loop includes a return path, effectively doubling the length to 10,000 ft. So the loop resistance is 10,000/1,000 × 7.77 = 10 × 7.77 = 77.7 Ω.