To feed a standard 100-amp subpanel, use a 100A main breaker and 3 AWG copper wire (or 1 AWG aluminum). For a 60-amp subpanel, use a 60A breaker with 6 AWG copper (or 4 AWG aluminum). This assumes 75°C terminations, 30°C ambient temperature, and THHN conductors in conduit.
- Conductor Material: Copper (unless aluminum is explicitly stated)
- Temperature Rating: 75°C column (NEC Table 310.16)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Up to 3 current-carrying conductors in PVC or EMT conduit
- Voltage: 240V split-phase residential
The Core Sizing Matrix: Breakers, Wire, and Distance
Sizing the main breaker for subpanel installations requires matching the overcurrent protection device (OCPD) to the conductor ampacity, while simultaneously verifying that voltage drop remains under 3% for the total run length. The table below provides the exact specifications for the most common residential and light-commercial subpanel feeds.
| Subpanel Rating | Main Breaker Size | Copper AWG (75°C) | Aluminum AWG (75°C) | Max Run for <3% VD (240V) |
|---|---|---|---|---|
| 60A | 60A | 6 AWG | 4 AWG | 122 ft |
| 100A | 100A | 3 AWG | 1 AWG | 146 ft |
| 125A | 125A | 1 AWG | 1/0 AWG* | 186 ft |
| 200A | 200A | 3/0 AWG | 4/0 AWG | 234 ft |
*Note on 125A Aluminum: 1/0 AWG aluminum is rated for 120A in the 75°C column. NEC 240.4(B) permits the "next size up" rule, allowing a 125A breaker to protect a 120A conductor, provided the calculated load does not exceed 120A. If your actual load is exactly 125A, you must use 2/0 AWG aluminum.
Why We Don't Just Use the Next Size Down
A frequent question on the workbench is: "Why can't I use 4 AWG copper (rated 85A) on a 100A main breaker for subpanel use if my actual calculated load is only 70A?"
The answer lies in the fundamental purpose of the breaker. The overcurrent protection device (OCPD) is installed to protect the wire, not just the connected load. According to NEC 240.4, conductors must be protected against overcurrent in accordance with their ampacities.
The only exception is the aforementioned "Next Size Up" rule (NEC 240.4(B)). If your calculated load is 88A, and the standard wire size (3 AWG) is overkill while the smaller size (4 AWG at 85A) is slightly under, you are permitted to use the 4 AWG wire with a 90A breaker. However, you can never use a breaker larger than the wire's ampacity (subject to standard breaker sizes) just because the current load happens to be low.
Derating and Variables: What Changes the Math
The baseline table above assumes ideal conditions. In the real world, environmental and installation variables force you to adjust your wire gauge. Here is a decision framework for when you must upsize your feeder conductors.
| Variable | Trigger Condition | Required Action |
|---|---|---|
| Conductor Bundling | 4 to 6 current-carrying conductors in the same raceway (e.g., pulling two 240V circuits to the subpanel). | Apply 80% derating factor to the 90°C column ampacity. (e.g., 3 AWG THHN drops from 100A to 80A; must upsize to 2 AWG for a 100A feed). |
| High Ambient Heat | Conduit routed through an attic or on a roof where temperatures exceed 30°C (86°F). | Apply temperature correction factors from NEC Table 310.15(B)(1). At 50°C (122°F), multiply ampacity by 0.75. |
| Aluminum Lugs | Using aluminum wire on older breakers or panels not explicitly marked "AL/CU". | Do not connect. Upgrade the breaker to an AL/CU rated model, or use copper pigtails. Always apply Noalox anti-oxidant paste to aluminum terminations. |
Aluminum vs. Copper Interchangeability: Never treat aluminum and copper as direct 1:1 substitutes. Aluminum has a higher coefficient of thermal expansion and lower conductivity per cross-sectional area. A 1 AWG aluminum wire is required to carry the same 100A load that a 3 AWG copper wire handles. Furthermore, aluminum requires specific torque values and anti-oxidant compound to prevent high-resistance connections that lead to melted lugs over time.
Voltage Drop Calculations and AHJ Sign-Off
Ampacity ensures the wire won't catch fire; voltage drop ensures your equipment actually works. The NEC recommends (via Informational Notes in Article 210 and 215) that feeder voltage drop not exceed 3%, and the total combined drop (feeder + branch) not exceed 5%.
For a 240V subpanel, a 3% drop equates to a maximum loss of 7.2 volts. The distances listed in the core matrix table are calculated using the standard single-phase voltage drop formula: D = (VD × CM) / (2 × K × I), assuming a copper K-factor of 12.9. If your trench run to a detached garage subpanel exceeds the distances in the table, you must upsize the wire, even if the breaker ampacity technically allows the smaller gauge. For exact calculations on complex runs, use the Southwire Voltage Drop Calculator to verify your specific parameters.
When to Involve an Engineer or the AHJ
While standard residential subpanels (60A to 200A) fall well within the scope of a competent DIYer or journeyman electrician, certain scenarios mandate professional engineering review and explicit Authority Having Jurisdiction (AHJ) approval:
- Parallel Conductors: If your subpanel requires 400A or more, you will likely need to run parallel sets of conductors (NEC 310.10(H)). This requires exact matching of wire lengths, materials, and routing, and must be explicitly approved by the local inspector.
- Extreme Distance Runs: If your feeder run exceeds 300 feet, voltage drop mitigation may require upsizing the wire by 3 or 4 AWG sizes. At this point, the cost of copper becomes prohibitive, and transitioning to underground aluminum URD cable or specialized conduit pulls requires a formal load calculation and engineered voltage drop study.
- Torque Verification: Modern NEC (110.14(D)) strictly requires that terminations be torqued to the manufacturer's specifications using a calibrated torque screwdriver or wrench. Inspectors in many jurisdictions now require proof of torque tools on site. Never guess the tightness of a 200A main lug; under-torquing causes arcing, and over-torquing strips the aluminum bus bar.
Always pull a permit for subpanel installations. Local codes may have amendments regarding grounding electrode systems (like the requirement for a grounding rod at detached structures) that supersede general NEC guidance.
