For a standard 60-amp subpanel feed up to 50 feet, use 6 AWG copper THHN wire on a 60-amp double-pole breaker. For a 100-amp feed, step up to 3 AWG copper THHN on a 100-amp breaker. Always run four wires: two hots, one neutral, and one equipment ground.
Baseline Sizing Assumptions
Wire ampacity is not a single fixed number; it changes based on insulation type, termination temperatures, and ambient conditions. The direct answers provided above—and the matrices below—are based on the following strict baseline assumptions. If your installation deviates from these, you must recalculate.
- Conductor Material: Copper (THHN/THWN-2 insulation)
- Temperature Column Used: 75°C (NEC Table 310.16). While THHN wire is rated for 90°C, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected component. Modern residential breakers and panel lugs (like Square D QO or Siemens QT) are rated for 75°C.
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Fill: Maximum of 3 current-carrying conductors in a single raceway (no bundling derating applied).
- System Voltage: 240V/120V single-phase, 3-wire plus ground.
Ampacity and Voltage Drop Matrix
Choosing the right wire isn't just about preventing the breaker from tripping or the wire from melting; it is also about ensuring the equipment at the subpanel receives adequate voltage. The NEC recommends a maximum voltage drop of 3% for branch circuits and feeders (5% total combined). Below is a spec-sheet-table for standard residential subpanel feeds using copper THHN in conduit.
| Target Subpanel Size | Min. Copper AWG (75°C Col.) | Wire Ampacity | Max Distance (Under 3% VD) | Required Main Breaker |
|---|---|---|---|---|
| 60 Amp | 6 AWG | 65A | ~115 feet | 60A 2-Pole |
| 100 Amp | 3 AWG | 100A | ~110 feet | 100A 2-Pole |
| 125 Amp | 1 AWG | 130A | ~125 feet | 125A 2-Pole |
The Voltage Drop Reality Check
Let's run the math on a 60-amp feed using 6 AWG copper at a distance of 150 feet. Using the standard voltage drop formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=60A, D=150ft, and CM=26,240 for 6 AWG), the drop is 8.8 volts. On a 240V system, that is a 3.68% drop. This exceeds the 3% NEC recommendation. If your detached garage is 150 feet away, you must bump the wire to 4 AWG copper to maintain acceptable voltage levels for sensitive electronics and power tools. You can verify your specific run using the Southwire Voltage Drop Calculator.
Variables That Force a Wire Size Bump
The baseline sizes above assume a perfect, short, single-circuit run. Real-world jobsites rarely cooperate. Use this decision-tree-table to determine if your specific conditions require upsizing the conductor.
| Condition / Variable | Impact on Sizing | Required Action | NEC Reference |
|---|---|---|---|
| Distance exceeds max VD limit | Voltage drop exceeds 3% (7.2V on 240V) | Increase wire size by 1 or 2 AWG steps until VD < 3% | NEC 310.15(B) (Informational Note) |
| Bundling in Conduit (4+ current-carrying conductors) | Heat buildup reduces wire's ability to dissipate thermal energy | Apply 80% derating factor to the 90°C column ampacity, then size breaker based on 75°C column | NEC 310.15(C)(1) |
| High Ambient Temperature (e.g., attic in summer > 30°C) | Insulation thermal limits are reached at lower current loads | Multiply base ampacity by temperature correction factor (e.g., 0.88 for 40°C ambient) | NEC 310.15(B)(1) |
| Using Aluminum Wire (e.g., SER cable) | Aluminum has higher resistance and lower ampacity per AWG than copper | Use 4 AWG Al for 60A; 2 AWG Al for 100A. Ensure lugs are rated AL/CU and use anti-oxidant paste | NEC 310.16 & 110.14 |
Why Not Use One Size Smaller?
A common DIY mistake is attempting to feed a 60-amp subpanel with 8 AWG copper wire. At the 75°C column, 8 AWG is only rated for 50 amps. If you place a 60-amp breaker on 8 AWG wire, a sustained 58-amp load will overheat and melt the wire insulation long before the breaker's thermal trip mechanism engages. The breaker is there to protect the wire, not the panel. You must strictly adhere to NEC 240.4, which requires the overcurrent device to match or be lower than the conductor's ampacity (with specific next-size-up exceptions that do not apply to standard 60A/100A residential feeds).
When an Engineer or the AHJ Must Confirm
While the matrix above covers 95% of residential detached garage, workshop, and addition subpanels, certain scenarios require professional engineering sign-off or explicit AHJ pre-approval:
- Feeds exceeding 200 Amps: Large outbuildings or multi-family setups requiring 400A+ feeds often trigger utility service upgrade requirements and complex fault-current calculations.
- Parallel Conductors: If your load requires wire larger than 1/0 AWG, NEC 310.10(H) allows paralleling (running two sets of wires per phase). This requires exact matching of length, material, and routing, and inspectors will scrutinize the terminations heavily.
- Utility Interlocks / Solar Backfeeding: If the subpanel is part of a solar PV system or backup generator setup with a transfer switch, the busbar rating rules (NEC 705.12) dictate strict breaker placement and sizing limits.
Subpanel Installation FAQ
Do I need a ground rod when I add a sub panel to breaker box in a detached building?
Yes. Per NEC 250.32, any subpanel located in a separate structure (like a detached garage, barn, or shed) must have its own grounding electrode system. This typically means driving two 5/8-inch copper ground rods at least 6 feet apart and bonding them to the subpanel's ground bar using a minimum 6 AWG bare copper grounding electrode conductor. Crucially, the neutral and ground bars in a subpanel must remain isolated (separated). Do not install the green bonding screw or strap that comes pre-installed in the subpanel; that is only used for the main service disconnect.
Can I use aluminum SER cable instead of pulling copper THHN in conduit?
Yes, and it is often much cheaper. Aluminum Service Entrance (SER) cable is widely used for indoor subpanel feeds where physical damage is unlikely, eliminating the need to buy and bend EMT or PVC conduit. However, aluminum and copper are not interchangeable by size. To safely feed a 60-amp panel with aluminum SER, you must use 4 AWG aluminum (rated 65A at 75°C). For a 100-amp feed, use 2 AWG aluminum. Always apply Noalox or similar anti-oxidant compound to the aluminum wire ends before torquing them into the panel lugs to prevent high-resistance oxidation over time.
Does the main breaker inside the subpanel need to exactly match the feed breaker?
No. If you are buying a "main breaker" panel to use as a subpanel, the rating of that main breaker simply acts as a local disconnect switch and an upper limit for the busbar. It is perfectly legal and common to install a 100-amp main breaker panel and feed it from a 60-amp breaker in the main house panel. The 60-amp breaker in the main panel will protect the 6 AWG feed wire, and the 100-amp breaker in the subpanel will just act as a switch. Just ensure the subpanel's busbar rating (e.g., 100A) is not exceeded by the sum of the loads connected to it.
What size ground wire do I need for a 100-amp subpanel feed?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) size for a 100-amp overcurrent device is 8 AWG copper or 6 AWG aluminum. However, if you had to upsize your hot wires for voltage drop (e.g., using 1 AWG copper instead of 3 AWG for a long 100A run), NEC 250.122(B) requires you to proportionally increase the ground wire size as well to maintain the same fault-current clearing capability.
