The Direct Answer: Sizing Wire for a Sub Panel
For a 60-amp subpanel, use 6 AWG copper THHN wire on a 60A double-pole breaker. For a 100-amp subpanel, use 3 AWG copper or 1 AWG aluminum. These sizes assume standard residential terminations, 30°C ambient temperature, and runs under 100 feet where voltage drop remains below 3%.
- Material: Copper (unless explicitly noted as Aluminum)
- Temperature Column: 75°C (per NEC 110.14(C) termination limits)
- Ambient Temperature: 30°C (86°F)
- Conduit Type: Standard Schedule 40 PVC, maximum 3 current-carrying conductors in the raceway
Ampacity, Breaker Sizing, and the 75°C Termination Rule
When sizing feeders, the most common mistake DIYers make is looking at the 90°C column for THHN/THWN-2 wire. While the wire insulation can handle 90°C, the lugs inside your main breaker panel and subpanel are almost universally rated for 75°C. Per NEC 110.14(C), you must size the conductor based on the lowest temperature rating of any connected component. Therefore, we use the 75°C column of NEC Table 310.16.
Why use 6 AWG for 60A and not 8 AWG? 8 AWG copper at 75°C is only rated for 50A. While 6 AWG is rated for 65A at 75°C, the next standard breaker size down is 60A. You cannot use a smaller wire (like 8 AWG) and rely on the "next standard size up" rule (NEC 240.4(B)) because the wire's actual ampacity (50A) does not correspond to a standard overcurrent device that protects it without exceeding its thermal limit.
| Breaker Size | Copper AWG | Aluminum AWG | 75°C Ampacity | Max Distance (3% VD at 240V) |
|---|---|---|---|---|
| 60 Amp | 6 AWG | 4 AWG | 65A (Cu) / 65A (Al) | ~115 feet |
| 100 Amp | 3 AWG | 1 AWG | 100A (Cu) / 100A (Al) | ~125 feet |
| 125 Amp | 1 AWG | 2/0 AWG | 130A (Cu) / 135A (Al) | ~110 feet |
| 200 Amp | 2/0 AWG | 4/0 AWG | 175A (Cu) / 180A (Al)* | ~100 feet |
*Note: For residential 200A service feeders, NEC 310.12 allows specific residential derating exceptions, but for standard subpanel feeders, standard ampacity rules apply. Always verify with your local AHJ.
What Changes the Answer: Voltage Drop and Derating
The chart above assumes a relatively short run. If your subpanel is in a detached garage or a distant workshop, voltage drop becomes the governing factor, not thermal ampacity. The NEC recommends a maximum 3% voltage drop for feeders.
Voltage Drop Check at 150 Feet (100A Subpanel):
Let's test 3 AWG Copper for a 100A load at 150 feet. Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper constant) = 12.9
- I (Current) = 100A
- D (Distance) = 150 ft
- CM (Circular Mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 100 × 150) / 52,620 = 387,000 / 52,620 = 7.35V
Percentage = 7.35V / 240V = 3.06%
Because 3.06% exceeds the 3% recommendation, 3 AWG is technically undersized for a 150-foot run at full load. You must upsize to 2 AWG Copper (CM = 66,360) to bring the drop down to 2.42%. You can verify these calculations using tools like the Cerrowire Voltage Drop Calculator.
| Installation Condition | NEC Rule | Required Action |
|---|---|---|
| Run exceeds 100-125 feet | Informational Note (VD) | Calculate VD; upsize 1-2 AWG sizes if >3%. |
| 4 to 6 current-carrying conductors in one conduit | 310.15(C)(1) | Multiply 90°C ampacity by 80%. Upsize if result < breaker rating. |
| Ambient temp exceeds 86°F (30°C) | Table 310.15(B)(1) | Apply temperature correction factor (e.g., 0.88 for 104°F). |
Copper vs. Aluminum: Material Selection and Torque
Copper and aluminum are not interchangeable. Aluminum is significantly cheaper and lighter, making it the default choice for 100A+ feeders and service entrances. However, it has a higher coefficient of thermal expansion and is prone to galvanic corrosion if improperly terminated.
If you choose aluminum (specifically AA-8000 series alloy, as mandated by the NEC), you must follow three strict rules:
- Use Anti-Oxidant Paste: Apply a conductive anti-oxidant compound (like Noalox or Penetrox) to the stripped aluminum conductor before inserting it into the lug. This prevents aluminum oxide from forming, which is highly resistive and causes heat buildup.
- Check Lug Ratings: Ensure the breaker and subpanel lugs are explicitly marked "AL" or "AL/CU". Never terminate aluminum in a copper-only lug.
- Torque to Spec: This is where most failures happen. Use a calibrated torque screwdriver or torque wrench. A 1 AWG aluminum wire in a standard panel lug typically requires between 150 and 250 inch-pounds of torque, depending on the manufacturer. Under-torquing leads to arcing; over-torquing shears the softer aluminum strands.
When an Engineer or the AHJ Must Confirm
Local Authority Having Jurisdiction (AHJ) always has final say. You must pull a permit and have an inspector verify your work for any subpanel installation. Furthermore, you must consult a licensed Professional Engineer (PE) if your project involves:
- Parallel Conductors: Running multiple sets of wires per phase (only permitted for 1/0 AWG and larger per NEC 310.10(H)).
- Complex Thermal Environments: Routing feeders through insulated walls packed with thermal barriers, or near high-heat sources like steam lines.
- Separate Buildings: Supplying a detached structure requires a local disconnecting means and a separate grounding electrode system (ground rods) per NEC 250.32.
Frequently Asked Questions
Can I wire a sub panel with 3 wires instead of 4?
For any new installation, no. Since the 2008 NEC update, NEC 250.32 strictly requires a 4-wire feeder for subpanels: two ungrounded "hot" conductors, one grounded "neutral" conductor, and one separate equipment grounding conductor (EGC). The only exception is for existing, pre-2008 installations that are not being modified, or specific mobile home setups. Never bootleg a ground or bond the neutral to the ground bar in a subpanel.
What size ground wire do I need for a 100-amp sub panel?
The equipment grounding conductor (EGC) is sized based on the breaker rating, not the current-carrying conductors. According to NEC Table 250.122, a 100-amp overcurrent device requires a minimum 8 AWG copper or 6 AWG aluminum ground wire. If you upsized your hot wires for voltage drop (e.g., using 1 AWG copper instead of 3 AWG for a long run), NEC 250.122(B) requires you to proportionally increase the size of the ground wire as well.
Does the sub panel feeder wire need its own disconnect?
If the subpanel is located in the same building as the main panel, the breaker in the main panel serves as the disconnect. However, if you are wiring a sub panel in a detached building (like a garage or shed), NEC 225.31 and 225.32 require a local disconnecting means at the building. This can be achieved by using a main breaker panel as your subpanel, or by installing a separate disconnect switch ahead of a main-lug-only subpanel.
How do I separate grounds and neutrals in a sub panel?
In a main service panel, the neutral and ground bars are bonded together. In a subpanel, they must be strictly isolated. When you install the subpanel, locate the green bonding screw or bonding strap that connects the neutral bar to the metal enclosure and remove it entirely. Route all bare/green ground wires to the ground bar (which remains bonded to the metal box), and route all white neutral wires to the isolated neutral bar. If they touch or share a bar, return current will travel on the ground wire, creating a shock hazard and tripping GFCI/AFCI devices upstream.
