New York State Master Electrician Practice Exam 2025 - Free Electrician Practice Questions and Study Guide

The maximum run length of an 18 AWG fixture wire tapped onto a 20A branch circuit is based on the ampacity of the wire and the acceptable voltage drop over the length of the circuit. In residential or commercial wiring practices, the general rule for branch circuits is to minimize voltage drop to maintain the performance and safety of connected devices.

For 18 AWG wire, its ampacity is typically rated for a maximum of 10A for continuous service. However, since it is tapped onto a 20A circuit, it is imperative to ensure that the distance does not cause excessive voltage drop or overheating.

The National Electrical Code (NEC) suggests that the voltage drop should not exceed 3% for branch circuits feeding lighting and receptacle outlets to ensure efficient operation and safety. For a 20A circuit with a typical voltage (for instance, 120V), the voltage drop should ideally remain within 3-5 volts.

By calculation, an 18 AWG wire is suitable for runs up to around 50 feet on a 20A branch circuit without posing a risk of drawing too much current or experiencing unacceptable voltage drop. This makes a distance of 50 feet a reasonable choice, as it balances ampacity, wire