For a standard 60-amp subpanel, use 6 AWG copper THHN wire protected by a 60A double-pole breaker. If running aluminum, step up to 4 AWG. This assumes a standard residential 240V feeder under 100 feet. Let's break down the exact math, code requirements, and edge cases for your specific run.
The Baseline: 60A, 100A, and 125A Feeder Sizing
Before pulling any wire, confirm your run matches these baseline parameters. If it doesn't, you must adjust your sizing.
• Conductor Material: Copper (unless explicitly noted as Aluminum)
• Temperature Column: 75°C (Standard for most modern breakers and lugs)
• Ambient Temperature: 30°C (86°F)
• Conduit Type: PVC Schedule 80 or EMT, containing 3 or fewer current-carrying conductors
• Voltage: 240V split-phase residential
When planning your NFPA National Electrical Code (NEC) compliant feeder, you must reference Table 310.16. The most common mistake DIYers make is looking at the 90°C column because THHN wire is rated for 90°C. However, NEC 110.14(C) dictates that termination provisions are generally rated for 75°C. You must size the wire based on the 75°C column to protect the breaker lugs.
| Breaker Size | Copper AWG/Size | Aluminum AWG/Size | Max Continuous Load (80%) |
|---|---|---|---|
| 60 Amp | 6 AWG | 4 AWG | 48 Amps |
| 100 Amp | 3 AWG | 1 AWG | 80 Amps |
| 125 Amp | 2 AWG | 1/0 AWG | 100 Amps |
What Changes the Answer: Length, Bundling, and Material
The chart above gets you out of the gate, but three real-world variables will force you to upsize your wire: voltage drop, conductor bundling, and material choice.
1. Voltage Drop at Distance
The NEC recommends (via Informational Note to 210.19) that feeder voltage drop not exceed 3%. Let's run the math on a 60A copper feeder using 6 AWG THHN (26,240 circular mils) over a 100-foot one-way run.
Formula: VD = (2 × K × I × L) / CM
Math: (2 × 12.9 × 60 × 100) / 26,240 = 5.89 Volts
On a 240V circuit, 5.89V is a 2.45% drop. You are well within the 3% limit. However, if your detached garage is 150 feet away, the drop jumps to 8.84V (3.68%). At that distance, you must upsize to 4 AWG copper to maintain power quality and prevent motor burnout on heavy tools. Use the Southwire Technical Information calculators to verify long runs.
2. Conduit Bundling and Derating
If you are pulling feeders for two separate subpanels through the same 2-inch PVC conduit, you now have six current-carrying conductors (four hots, two neutrals; equipment grounds do not count). According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% derating factor. If you try to use 8 AWG THHN (rated 50A at 90°C for derating purposes), 50A × 0.80 = 40A. Your wire is now only good for 40A, meaning you must pull larger wire to compensate for the trapped heat inside the conduit.
3. Aluminum vs. Copper
Never treat aluminum and copper interchangeably. Aluminum is significantly cheaper and lighter, making it the standard for utility feeders and 200A+ residential services. However, it has a higher coefficient of thermal expansion and oxidizes rapidly. If you choose aluminum for a 100A subpanel (1 AWG), you must use an antioxidant compound (like Noalox) on the terminations and ensure your lugs are explicitly rated for aluminum (marked AL7CU or similar).
Why This Size and Not One Smaller?
A common question on the workbench is: 'My 60A subpanel will only ever run 40 amps of load, so why can't I use 8 AWG copper and save money?'
The answer lies in the thermal trip curve of the breaker and the melting point of wire insulation. A breaker protects the wire, not the load. If you install 8 AWG copper (ampacity 50A at 75°C) on a 60A breaker, a sustained 55A overload will not trip the 60A breaker immediately. The breaker will hold, but the 8 AWG wire will overheat, degrading the THHN insulation and creating a fire hazard inside the walls long before the breaker's thermal bimetallic strip bends enough to trip.
- Symptom: Breaker trips randomly, but wire feels cool. Fix: You likely have a short circuit or ground fault, not an overload. Check for pinched insulation.
- Symptom: Breaker trips after 10 minutes of heavy use, wire is warm to the touch. Fix: Continuous load exceeds 80% of breaker rating. Upsize wire and breaker, or shed load.
- Symptom: Breaker never trips, but wire terminations smell like burning plastic. Fix: IMMEDIATE SHUTDOWN. Wire is undersized for the breaker, or lugs were torqued incorrectly causing high resistance.
When an Engineer or the AHJ Must Confirm
While sizing a 60A or 100A feeder to a detached garage is well within the scope of standard NEC-style guidance, there are hard boundaries where you must defer to your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer:
- Service Entrance Upgrades: If adding the subpanel requires upgrading your main service from 100A to 200A, this involves the utility drop and meter base. Only licensed professionals and utility coordinators handle this.
- Continuous Industrial Loads: If your subpanel is feeding a CNC machine or commercial welder that runs for 3+ hours continuously, NEC Article 210.20 requires the breaker to be sized at 125% of the continuous load. An engineer must calculate the specific duty cycle.
- Local Amendments: Some municipalities have strict local amendments overriding the base NEC. For example, certain jurisdictions mandate a minimum 100A feeder for any detached habitable structure, regardless of your calculated load. Always pull a permit and consult your local inspector.
For deeper code interpretations, the EC&M National Electrical Code Resources provide excellent breakdowns of local amendment variations.
Frequently Asked Questions
Do I need a ground rod when wiring for sub panel in a detached garage?
Yes. For a detached structure, NEC 250.32 requires two separate grounding pathways. First, you must pull a 4-wire feeder (two hots, one neutral, one equipment grounding conductor) back to the main panel. Second, you must install a local Grounding Electrode System (GES) at the detached building—typically two 5/8-inch copper ground rods driven 6 feet apart and bonded to the subpanel's ground bar. Crucially, the neutral and ground bars in the subpanel must remain isolated (the green bonding screw must be removed).
Can I use 4-wire UF-B cable or do I need conduit for sub panel wiring?
You can use direct-bury UF-B (Underground Feeder) cable for short, simple runs, but it has limitations. UF-B is limited to the 60°C ampacity column, meaning a 60A run requires 4 AWG copper UF-B instead of 6 AWG THHN. Furthermore, UF-B must be buried at least 24 inches deep. If you use individual THWN-2 conductors inside rigid metal conduit (RMC) or PVC, the burial depth drops to 6 inches (for RMC) or 18 inches (for PVC), and you get to use the 75°C ampacity column. For most workshop runs, pulling THHN in PVC conduit is cheaper, easier, and allows for future upgrades.
Does the sub panel main breaker need to match the feeder breaker size?
No. The main breaker inside the subpanel acts purely as a local service disconnect switch; it does not protect the feeder wire (the breaker in the main panel does that). It is perfectly legal and common to feed a subpanel with a 60A breaker at the main panel, while the subpanel itself has a 100A main breaker. The only rule is that the subpanel's bus bar rating must be equal to or greater than the feeder breaker size.
