For a 100 amp subpanel, the appropriate wire size depends on the wire material (copper or aluminum) and the length of the run due to voltage drop considerations:
- Copper wire: Typically, #4 AWG copper wire is used for a 100 amp subpanel if the run is short (less than about 50 feet). For longer runs around 80 to 100 feet or more, #1 AWG copper wire is recommended to reduce voltage drop and ensure safe ampacity
- Aluminum wire: For aluminum conductors, #2 AWG is generally the minimum for 100 amp service at shorter distances, but for longer runs (around 100 feet or more), #1/0 AWG aluminum wire is advisable to handle the load safely and minimize voltage drop
Additional considerations:
- The National Electrical Code (NEC) 83% rule allows some flexibility for service conductors but typically does not apply to subpanel feeders, so it's safer to use the larger wire sizes.
- If the subpanel is fed from a 200 amp main panel, the wire size must be adequate for the subpanel's 100 amp breaker rating, not the main panel rating.
- Voltage drop is a key factor; for runs over 100 feet, upsizing the wire (e.g., using #1/0 aluminum instead of #2) is recommended.
- Common cable types for subpanel feeders include SER cable for aluminum or individual conductors like THHN/THWN in conduit for copper
Summary Table
Wire Material| Typical Minimum Size for 100 Amp Subpanel| Recommended Size for
~100 ft Run
---|---|---
Copper| #4 AWG| #1 AWG
Aluminum| #2 AWG| #1/0 AWG
Always consult local electrical codes and consider hiring a qualified electrician to ensure compliance and safety.
