For a standard 60-amp subpanel, use 6 AWG copper THHN/THWN-2 wire protected by a 60A 240V double-pole breaker. If using aluminum, step up to 4 AWG XHHW-2. This assumes a run under 50 feet, copper conductors, 75°C terminations, and 30°C ambient temperature. Let's break down the exact code requirements and the physics behind these numbers.
- Conductor Material: Copper (unless aluminum is explicitly stated)
- Insulation Type: THHN/THWN-2 or XHHW-2
- Termination Rating: 75°C (Standard for modern Eaton/Square D/Siemens panels per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Conduit Setup: EMT or PVC with 3 current-carrying conductors (2 hots, 1 neutral; equipment ground does not count for derating)
The Baseline Sizing Rules for Sub Panel Wire Size
When pulling a feeder to a detached garage, workshop, or addition, you are governed by NEC Article 215 (Feeders) and Article 310 (Conductors for General Wiring). The most common DIY subpanel sizes are 60A and 100A. Below is the spec-sheet reference for standard residential feeder distances under 50 feet.
| Breaker Size | Copper AWG (75°C Col.) | Aluminum AWG (75°C Col.) | Max Distance (3% VD Limit) | Min. Ground Wire (Cu/Al) |
|---|---|---|---|---|
| 60A | 6 AWG | 4 AWG | 115 ft | 10 AWG / 8 AWG |
| 100A | 3 AWG | 1/0 AWG | 105 ft | 8 AWG / 6 AWG |
| 125A | 1/0 AWG | 3/0 AWG | 95 ft | 6 AWG / 4 AWG |
Note: Ground wire sizes are derived from NEC Table 250.122. Never size your equipment grounding conductor based on the 310.16 ampacity tables.
Why We Size It This Way (And Why You Can't Go Smaller)
A common bench question is: 'The 90°C column in NEC Table 310.16 says 8 AWG THHN is good for 55 amps. Why can't I use 8 AWG on a 60A breaker and use the next-size-up rule?'
The answer lies in NEC 110.14(C) - Temperature Limitations. While the insulation on your THHN wire can physically handle 90°C without melting, the brass or aluminum lugs inside your breaker and subpanel are only tested and rated for 75°C. If you push 55A+ through an 8 AWG wire, the wire won't melt, but the heat will transfer into the panel lug, potentially degrading the mechanical connection or damaging the breaker's internal thermal-magnetic trip mechanism.
Therefore, you must use the 75°C column for termination limits. In the 75°C column, 6 AWG copper is rated for 65A. Because 65A is greater than your 60A breaker, 6 AWG is the absolute minimum safe size. You cannot use the 90°C column for ampacity unless you are applying derating factors for bundling or high ambient temperatures—and even then, the final derated ampacity cannot exceed the 75°C termination limit.
What Changes the Answer: Distance, Bundling, and Material
The baseline chart above assumes ideal conditions. Real-world jobsites rarely cooperate. Here is how environmental factors force you to change your sub panel wire size.
1. Voltage Drop Over Distance
The NEC recommends a maximum 3% voltage drop for feeders (Informational Note to NEC 215.2). Let's run the math on a 60A, 240V feeder using 6 AWG copper pulled 150 feet to a detached garage.
- Formula: VD = (2 × K × I × L) / Circular Mils
- Variables: K = 12.9 (copper), I = 60A, L = 150 ft, CM = 26,240 (for 6 AWG)
- Calculation: (2 × 12.9 × 60 × 150) / 26240 = 8.84 Volts
- Percentage: 8.84V / 240V = 3.68%
At 150 feet, 6 AWG fails the 3% recommendation. You must bump up to 4 AWG copper (CM = 41,740), which drops the VD to 5.56V (2.3%). Always verify long runs using a reliable tool like the Southwire Voltage Drop Calculator.
2. Conduit Bundling and Derating
If you are pulling multiple circuits through the same conduit, the heat dissipates poorly. NEC Table 310.15(C)(1) requires derating when you have more than three current-carrying conductors (CCCs). If you pull two 240V circuits (4 hots) and share a neutral (1 CCC), you have 5 CCCs. This triggers an 80% derating factor.
You apply this factor to the 90°C column to find your derated ampacity, but you still cannot exceed the 75°C termination limit. If bundling forces a derate that drops your wire below the breaker size, you must increase the wire gauge or pull a second conduit.
3. Copper vs. Aluminum Decision Matrix
Aluminum (specifically XHHW-2) is significantly cheaper and perfectly safe if installed correctly, but it requires different handling. Never treat them interchangeably.
| Criteria | Copper (THHN/THWN-2) | Aluminum (XHHW-2) |
|---|---|---|
| Material Cost (approx/ft) | $1.10 - $1.40 | $0.55 - $0.75 |
| Termination Prep | Strip, insert, torque to spec | Strip, wire-brush oxide, apply Noalox, torque |
| Conduit Space (OD) | Smaller (easier pulls) | Larger (may require upsizing conduit) |
| Creep / Loosening Risk | Very Low | Moderate (requires precise torque screwdriver) |
Per NEC 110.14(D), you must use a calibrated torque screwdriver to tighten breaker and lug connections to the manufacturer's specified inch-pounds. 'Hand tight' or 'giving it an extra quarter turn' causes arcing fires, especially with aluminum feeders. Check the sticker inside your Square D or Eaton panel for exact torque values.
When to Call an Engineer or the AHJ
While the National Electrical Code (NFPA 70) provides the baseline, local Authorities Having Jurisdiction (AHJ) and specific load profiles can override standard sizing charts.
- Continuous Loads (NEC 210.20): If your subpanel will feed an EV charger or a dust collection system that runs for 3 hours or more, that is a continuous load. You must multiply the continuous load by 125% to size the breaker and wire. A 48A continuous EV charger requires a 60A breaker (48 × 1.25 = 60), but leaves zero headroom. Most inspectors will require a 70A or 80A feeder (requiring 4 AWG or 3 AWG copper) to prevent thermal nuisance tripping.
- Local Amendments: Some municipalities have amended the NEC to require a minimum 100A feeder for any newly constructed detached garage, regardless of the calculated load, to future-proof for EV charging.
- Service Entrance vs. Feeder: If you are upgrading your main service entrance (the wires from the meter to the main panel), different rules apply (NEC Article 230), and utility company specifications will dictate your sizing. Always defer service entrance work to a licensed contractor.
Sub Panel Wire Size FAQ
Do I need 4 wires for a sub panel wire size setup?
Yes. Modern NEC (since the 2008 cycle) strictly requires a 4-wire feeder to any subpanel: two ungrounded conductors (hots), one grounded conductor (neutral), and one equipment grounding conductor (ground). At the subpanel, the neutral bar and ground bar must remain physically and electrically isolated from each other. The neutral carries return current; the ground only carries current during a fault. If you bond them at the subpanel, normal neutral current will flow on your grounding paths, creating a shock hazard.
Can I use underground direct burial cable for my sub panel wire size?
You can use direct burial cable like UF-B or USE-2, but you must check the ampacity column carefully. Many standard UF-B cables are limited to the 60°C column per NEC 339.3. If you use 6 AWG UF-B copper, the 60°C column limits it to 55A, meaning you cannot legally use it on a 60A breaker. If you want to direct bury without conduit, look specifically for USE-2 or XHHW-2 rated for direct burial and 75°C/90°C wet locations, or pull individual THWN-2 conductors inside a continuous run of Schedule 40/80 PVC conduit.
Does the ground wire need to be the same size as the hots?
No. The equipment grounding conductor (EGC) is sized based on the rating of the overcurrent device (breaker), not the ampacity of the hot wires. According to NEC Table 250.122, a 60A breaker requires a minimum 10 AWG copper or 8 AWG aluminum ground wire. A 100A breaker requires an 8 AWG copper or 6 AWG aluminum ground. The only exception is if you had to upsize your hot wires significantly for voltage drop; in that specific case, NEC 250.122(B) requires you to proportionally increase the ground wire size as well.
