For a 60A subpanel, use 6 AWG copper wire with a 60A breaker. For a 100A subpanel, use 3 AWG copper with a 100A breaker. For a 200A subpanel, use 2/0 AWG copper with a 200A breaker. These sizes assume copper conductors, 75°C terminations, and 30°C ambient temperature.
- Material: Copper (Aluminum requires larger gauges; see chart below).
- Temperature Column: 75°C (per NEC Table 310.16, matching standard breaker lug ratings).
- Insulation Type: THHN/THWN-2 (rated 90°C, but base ampacity is capped by the 75°C termination rule).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Single raceway (conduit) or cable assembly, not bundled with other current-carrying conductors.
Note: All sizing follows NEC-style guidance (NFPA 70). Your local Authority Having Jurisdiction (AHJ) has final authority.
The Baseline: Subpanel Wire and Breaker Sizing Chart
The following spec-sheet-table provides the minimum allowable wire sizes for standard residential and light commercial subpanel feeders. These values are pulled directly from the 75°C column of NEC Table 310.16.
| Subpanel Rating | Breaker Size | Copper Wire (75°C) | Aluminum Wire (75°C) |
|---|---|---|---|
| 60 Amps | 60A | 6 AWG | 4 AWG |
| 100 Amps | 100A | 3 AWG | 1 AWG |
| 125 Amps | 125A | 2 AWG | 1/0 AWG |
| 200 Amps | 200A | 2/0 AWG | 4/0 AWG |
Important Grounding Note: In addition to the two hot wires and one neutral wire sized above, you must pull a separate Equipment Grounding Conductor (EGC). For a 100A subpanel, this requires an 8 AWG copper ground wire. The neutral and ground bars in a subpanel must remain strictly isolated.
Why This Size and Not One Smaller?
A common and dangerous mistake on DIY forums is recommending 4 AWG copper for a 100A subpanel. This happens because people confuse the 90°C ampacity of THHN wire with the actual termination limits of the equipment. Here is the physics and code reality:
The 75°C Termination Rule (NEC 110.14(C))
While THHN/THWN-2 wire has a 90°C insulation rating (where 4 AWG is good for 95A), the lugs inside standard residential breakers and subpanels are only tested and rated for 75°C. NEC 110.14(C) dictates that you must size your wire based on the lowest temperature rating of any connected component. In the 75°C column, 4 AWG copper is only rated for 85A. Therefore, to safely carry a full 100A continuous load without overheating the breaker lugs, you must step up to 3 AWG copper, which is rated exactly 100A at 75°C.
The 83% Residential Service Myth
Many builders use 4 AWG for 100A feeds by citing NEC Article 310.12, which allows sizing service conductors at 83% of the service rating. However, ECM Web and NEC experts consistently point out that Article 310.12 applies only to whole-house main service entrance conductors, not subpanel feeders. A subpanel is a feeder (Article 215), meaning you must size the conductors at 100% of the non-continuous load plus 125% of the continuous load. Do not apply the 83% rule to subpanels.
What Changes the Answer: Length, Bundling, and Material
The baseline chart assumes a short run in a cool environment. Real-world jobsite conditions frequently force you to upsize your wire. Here is the decision-tree-table for when to abandon the baseline chart:
1. Voltage Drop (The Distance Factor)
The NEC recommends a maximum 3% voltage drop for feeder conductors (NEC 310.15(B) Informational Note). If your subpanel is far from the main panel, the baseline wire size will result in dimming lights and tripping breakers under load.
Worked Example: 100A Subpanel at 150 feet vs. 200 feet
- Formula: VD = (2 × K × I × D) / CM. (K=12.9 for copper, I=80A practical continuous load, D=distance, CM=circular mils).
- At 150 feet using 3 AWG (52,620 CM): VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88V. This is a 2.45% drop on a 240V system. 3 AWG is acceptable.
- At 200 feet using 3 AWG: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V. This is a 3.26% drop. 3 AWG fails the 3% guideline. You must upsize to 2 AWG copper (66,360 CM), which drops the loss to 2.5%.
2. Conduit Bundling (Derating)
If you pull multiple circuits through the same conduit, the wires heat each other up. Per NEC Table 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a single raceway, you must derate the wire's ampacity to 80%. If your 3 AWG wire (100A base) is bundled with two other hot/neutral pairs, its effective ampacity drops to 80A. You would need to upsize to 1 AWG copper to maintain a 100A feeder capacity.
3. Aluminum vs. Copper
Aluminum (like SER cable) is significantly cheaper and lighter than copper, but it has higher resistance and expands/contracts more under load. You can never use the same AWG for aluminum as you do for copper. As shown in the baseline chart, a 100A aluminum feeder requires 1 AWG, and a 200A feeder requires 4/0 AWG. Always use anti-oxidant paste (like Noalox) on aluminum terminations and torque lugs to the manufacturer's exact inch-pound specification to prevent high-resistance arcing over time.
When an Engineer or AHJ Must Confirm
While the guidelines above cover 95% of standard residential garage, workshop, and detached building subpanels, you must pull a permit and have an electrical engineer or your local AHJ review the plans under the following conditions:
- High Ambient Temperatures: If the feeder conduit runs across a rooftop, through an unventilated attic in a hot climate, or near a boiler, you must apply the temperature correction factors in NEC Table 310.15(B)(1). A 30°C baseline assumption is invalid in a 50°C attic.
- Solar Backfeed (NEC 705.12): If you are adding a subpanel to integrate a solar inverter or battery backup system, the 120% busbar rule applies. An engineer must verify that your main panel's busbar can handle the combined utility and solar current without overheating.
- Complex Article 220 Load Calculations: If the subpanel will supply heavy continuous loads (e.g., a commercial woodshop with 5HP dust collectors, welders, and HVAC), a formal Article 220 load calculation is required to prove the feeder size matches the actual calculated demand, rather than just the breaker rating.
FAQ: Subpanel Wire Sizing Questions
What size wire for a 50 amp sub panel?
For a 50A subpanel, use 6 AWG copper or 4 AWG aluminum with a 50A double-pole breaker. While 8 AWG copper is rated for 50A at 75°C, using 6 AWG is highly recommended in practice to account for voltage drop over distance and to provide physical rigidity when terminating on the lugs. Don't forget an 10 AWG copper equipment grounding conductor.
Can I use 100 amp wire for a 60 amp sub panel?
Yes, you can always use wire that is larger (thicker) than the minimum required by code. Using 3 AWG copper (rated 100A) on a 60A breaker is perfectly safe and will result in near-zero voltage drop. The only challenge is physical: 3 AWG wire is stiff and may be difficult to bend into the tight spaces of a smaller 60A breaker lug. If the wire physically fits and torques down securely without fraying strands, it is code-compliant.
Does a sub panel need a ground rod?
A subpanel inside the same building as the main panel does not need a ground rod; it only requires the Equipment Grounding Conductor (EGC) pulled from the main panel. However, if the subpanel is in a detached building (like a detached garage or shed), NEC 250.32 requires both an EGC pulled with the feeder wires and a local grounding electrode system (typically two ground rods driven 6 feet apart) bonded to the subpanel's ground bar. The neutral and ground must still remain isolated in the detached subpanel.
What size wire for a 200 amp sub panel 200 feet away?
For a 200A subpanel located 200 feet from the main panel, the baseline 2/0 AWG copper will suffer from excessive voltage drop. To keep the drop under the recommended 3% at a practical 160A continuous load, you must upsize to 4/0 AWG copper or 350 kcmil aluminum. At this size and cost, many electricians opt to trench and lay direct-burial aluminum URD cable rather than pulling massive copper THHN through 3-inch PVC conduit.
