For a standard 100-amp subpanel, use 3 AWG copper THHN wire with a 100-amp double-pole breaker. If running aluminum, step up to 1 AWG. This assumes a standard residential feeder under 50 feet, copper conductors, 75°C terminations, and 30°C ambient temperature. Let's break down exactly how to calculate this for your specific run.
- Material: Copper (unless aluminum is explicitly stated)
- Insulation: THHN/THWN-2 in conduit, or XHHW-2
- Termination Temperature Rating: 75°C column (standard for modern breakers and lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Type: PVC Schedule 80 or EMT, single feeder run (no bundling derating applied yet)
The Baseline Assumptions for Subpanel Wire Sizing
Wire sizing is not a guessing game; it is a strict mathematical application of the National Electrical Code (NEC). The most common mistake DIYers make is looking at a wire gauge chart without checking the temperature column or insulation type. A 3 AWG copper wire is rated for 115A in the 90°C column, but you cannot use that number for a 100A breaker. NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected termination. Since standard residential breakers and subpanel lugs are rated for 75°C, you must use the 75°C column in NEC Table 310.16.
Ampacity and Voltage Drop: The Two-Step Sizing Check
Sizing a subpanel feeder requires passing two distinct tests: ampacity (will the wire melt?) and voltage drop (will the equipment starve?).
Step 1: The Ampacity Check
For a 100-amp feeder, the wire must have an allowable ampacity of at least 100A in the 75°C column. Looking at the spec sheet below, 3 AWG copper hits exactly 100A. If you are using aluminum, you must jump to 1 AWG to achieve 100A.
| AWG / kcmil | Copper Ampacity (75°C) | Aluminum Ampacity (75°C) | Common Subpanel Application |
|---|---|---|---|
| 6 AWG | 65A | 50A | 60A Subpanel (Copper only) |
| 4 AWG | 85A | 65A | None (falls between standard breaker sizes) |
| 3 AWG | 100A | 75A | 100A Subpanel (Copper) |
| 2 AWG | 115A | 90A | Long-run 100A / Short 125A |
| 1 AWG | 130A | 100A | 100A Subpanel (Aluminum) |
| 2/0 AWG | 175A | 135A | 200A Subpanel (Copper) |
| 4/0 AWG | 230A | 180A | 200A Subpanel (Aluminum) |
Step 2: The Voltage Drop Check
Ampacity ensures safety; voltage drop ensures performance. The NEC recommends a maximum 3% voltage drop on feeders. Let's run the math for a 100A subpanel located 100 feet from the main panel, assuming an 80A continuous load (the realistic maximum for a 100A panel under the 80% continuous load rule).
Using the standard single-phase voltage drop formula: VD = (2 × K × I × L) / CM
- K (Copper constant) = 12.9
- I (Current) = 80A
- L (Length) = 100 ft
- CM (Circular Mils for 3 AWG) = 52,620
VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92 Volts
3.92V on a 240V system is a 1.63% drop. This easily passes the 3% threshold. However, if that same subpanel is 200 feet away, the drop doubles to 7.84V (3.26%). At 200 feet, 3 AWG fails the voltage drop test, and you must upsize to 2 AWG copper to keep the drop under 3%. Always verify long runs using a dedicated voltage drop calculator.
What Changes the Answer: Length, Bundling, and Material
Why use 3 AWG and not one size smaller? A 4 AWG copper wire is rated for 85A at 75°C. Even if your actual connected load only draws 70A, you cannot use 4 AWG wire on a 100A breaker. The breaker's job is to protect the wire, not the load. If a fault or unexpected load spike pulls 95A, the 100A breaker will not trip, but the 85A-rated 4 AWG wire will overheat, melt its insulation, and start a fire inside the conduit.
The Bundling Derating Trap
NEC 310.15(C)(1) requires you to derate wire ampacity if you bundle multiple current-carrying conductors (CCCs) in a single raceway. However, here is a detail many get wrong: in a standard 120/240V single-phase subpanel feeder (2 hots, 1 neutral, 1 ground), only the 2 hot wires count as CCCs. Per NEC 310.15(E), the neutral in a single-phase system carrying only unbalanced current is not counted, and the equipment grounding conductor (EGC) is never counted. Because you only have 2 CCCs, and derating does not begin until you have 4 CCCs, standard residential subpanel feeders do not require bundling derating unless you are pulling multiple separate circuits through the same conduit.
Copper vs. Aluminum: Never Interchange Blindly
Aluminum is significantly cheaper and lighter than copper, making it the standard for utility service entrances and long feeder runs. However, you cannot simply swap 3 AWG copper for 3 AWG aluminum. Aluminum has higher resistance and expands/contracts more under thermal cycling.
| Condition / Variable | Impact on Sizing | Required Action |
|---|---|---|
| Switching from Copper to Aluminum | Lower ampacity per AWG | Upsize by 2 AWG steps (e.g., 3 AWG Cu becomes 1 AWG Al for 100A) |
| Ambient Temp > 30°C (86°F) | Wire cannot dissipate heat | Apply NEC Table 310.15(B)(1) temperature correction factors; likely upsize |
| Run exceeds 100 feet | Voltage drop exceeds 3% | Upsize wire by 1 or 2 AWG steps regardless of ampacity |
| Terminating Aluminum Lugs | Oxidation and thermal creep | MUST use anti-oxidant paste (Noalox) and a calibrated torque screwdriver |
When an Engineer or AHJ Must Confirm
While the rules above cover 95% of residential garage, workshop, and detached building subpanels, certain scenarios require professional engineering or explicit AHJ approval:
- Continuous Industrial Loads: If the subpanel feeds equipment that runs at maximum capacity for 3 hours or more (NEC Article 100 Continuous Load definition), the feeder and breaker must be sized at 125% of the continuous load.
- Extreme Ambient Temperatures: Running conduit through an unventilated attic in a southern climate where ambient temperatures regularly exceed 110°F (43°C) requires aggressive derating that may push a 100A feeder up to 1 AWG copper.
- Service Entrance vs. Feeder: If you are upgrading the main service to the house (the wires from the utility meter to the main panel), different NEC articles apply (Article 230), and utility companies often mandate specific aluminum gauges (like 2/0 or 4/0) regardless of standard feeder tables.
For a comprehensive understanding of local code adoptions and safety standards, always consult the NFPA National Electrical Code resources and your local building department.
Frequently Asked Questions
What size wire for a 60 amp subpanel?
For a 60-amp subpanel, use 6 AWG copper or 4 AWG aluminum wire, protected by a 60-amp double-pole breaker. This assumes a standard run under 50 feet. If the run exceeds 75 feet, upsize to 4 AWG copper to mitigate voltage drop. You will also need an 8 AWG or 10 AWG copper equipment grounding conductor, depending on your local AHJ's interpretation of NEC 250.122.
Can I use 4 AWG wire for a 100 amp subpanel if my actual load is only 70 amps?
No. You must use 3 AWG copper (or 1 AWG aluminum). The breaker protects the wire's insulation from melting, not the connected appliances. A 4 AWG copper wire is rated for 85A. If you install a 100A breaker on 4 AWG wire, the breaker will allow up to 99A to flow continuously without tripping, which will overheat the 85A-rated wire and create a severe fire hazard inside the walls or conduit.
Do I need to pull a ground wire to a subpanel in a detached garage?
Yes. Under current NEC rules (NEC 250.32), you must run a dedicated Equipment Grounding Conductor (EGC) with your feeder wires to a detached structure. You cannot rely on a ground rod alone. For a 100A feeder, this ground wire is typically 8 AWG copper. Crucially, in the detached subpanel, the ground bar and neutral bar must remain isolated (do not install the green bonding screw). The neutral and ground only bond at the main service disconnect.
What size wire for a 200 amp subpanel?
For a 200-amp subpanel, use 2/0 AWG copper or 4/0 AWG aluminum wire, protected by a 200-amp double-pole breaker. Because 200A feeders are often run to large detached workshops or secondary dwelling units (ADUs), voltage drop is a major factor. If the run is longer than 100 feet, you will likely need to upsize to 3/0 AWG copper or 250 kcmil aluminum to keep the voltage drop under the recommended 3% threshold.
