What size wire should be used for a secondary distribution box

The wire size for a subpanel depends on the amperage rating, distance from the main panel, and conductor material, with common sizes ranging from 8 AWG for 40 amps up to 2/0 AWG for 200 amps.

Determining Wire Size

The correct wire size is determined by the maximum expected load of the subpanel, not just the sum of branch circuit breakers. This involves performing a load calculation that accounts for continuous loads (operating for 3 hours or more) and non-continuous loads, applying a 125% factor to continuous loads as required by the NEC, and considering demand factors for realistic usage .

Ampacity and Material

• Copper conductors have higher ampacity than aluminum, so a smaller gauge can carry the same current.
• Aluminum conductors require a larger gauge to handle the same amperage safely . For example, a 100A subpanel located 80 feet from the main panel typically requires #4 AWG copper or #1/0 AWG aluminum to account for voltage drop and safe operation .

Distance and Voltage Drop

Longer runs increase resistance and voltage drop, which can reduce performance and damage equipment. NEC recommends keeping voltage drop under 3% for branch circuits and 5% for feeders. For long distances, it is often necessary to increase wire size beyond the minimum ampacity to maintain proper voltage .

Practical Wire Size Guide

Additional Considerations

• Always size wires to handle 125% of continuous loads.
• Consider future expansion and high-demand equipment like welders or EV chargers.
• Ensure compliance with local electrical codes and NEC standards.
• Use a professional wire size calculator for precise sizing based on load, distance, and voltage drop . Selecting the correct wire size ensures safety, efficiency, and code compliance, preventing overheating, voltage drop, and potential fire hazards.