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What size copper wire is needed for a secondary distribution box

The appropriate copper wire size for a subpanel depends on the amperage, distance, and load, with common residential feeders ranging from #4 AWG to 3/0 AWG for 100–200A service.Determining Wire Size

The correct copper wire size for a secondary distribution box (subpanel) is determined by several factors: 1. Load and Amperage: Calculate the maximum expected load of the subpanel, including continuous loads (operating 3+ hours) and non-continuous loads. NEC requires sizing conductors for continuous loads at 125% of the load current to prevent overheating (NEC 210.19(A)(1)), . 2. Distance and Voltage Drop: Longer feeder runs increase resistance and voltage drop. NEC recommends a maximum 3% voltage drop for branch circuits and 5% for feeders. For example, a 200-foot run to a detached garage may require upsizing the wire to maintain voltage within acceptable limits . 3. Conductor Material: Copper has higher conductivity than aluminum, allowing smaller wire sizes for the same amperage. For example, a 200A service typically requires 3/0 AWG copper for a 100-foot run, while aluminum would require 250 kcmil . 4. NEC Guidelines and Derating:

  • Ambient temperature above 86°F (30°C) requires derating according to NEC Table 310.15(B)(2)(a).
  • More than three current-carrying conductors in a conduit require additional derating: 4–6 conductors → 80%, 7–9 → 70%, 10–20 → 50% .
Typical Residential Subpanel Wire SizesSubpanel AmperageCopper Wire Size (AWG)Distance Consideration100A#4 AWGUp to 50 feet125A#2 AWG50–100 feet150A#1 AWG50–100 feet200A3/0 AWG100+ feet

These sizes assume standard 120/240V single-phase service and typical residential loads. Always verify with a load calculation and consider voltage drop for longer runs .

Practical Tips
  • Use copper conductors for indoor or short runs; aluminum may be more cost-effective for long outdoor feeders.
  • Ensure all connections are rated for the conductor type.
  • Use a reliable NEC feeder wire calculator to confirm sizing based on your specific load, distance, and installation conditions . By following these guidelines, you can ensure your secondary distribution box operates safely, efficiently, and in compliance with NEC standards.
What size copper wire is needed for a secondary distribution box

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