The correct wire gauge for 100 amp circuits is #3 AWG copper or #1 AWG aluminum, paired with a 100A breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway.
The Baseline Assumptions (Why #3 Cu / #1 Al?)
Before pulling any wire, you must understand the baseline assumptions that make #3 AWG copper and #1 AWG aluminum the legal minimums. Wire sizing is not a guessing game; it is strictly governed by NFPA 70 (National Electrical Code) Table 310.16.
Baseline Sizing Assumptions
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2).
- Temperature Column: 75°C. (Most modern 100A breakers and panel lugs are rated for 75°C. If your equipment is older and only rated for 60°C, you must use the 60°C column, which bumps the requirement to #1 AWG Copper).
- Ambient Temperature: 30°C (86°F).
- Conduit Fill: Maximum of 3 current-carrying conductors in a single raceway.
- Load Type: Non-continuous (operating for less than 3 hours at a time).
Why this size and not one smaller?
A common mistake is assuming #4 AWG copper is sufficient because it is 'close enough.' According to the 75°C column of Table 310.16, #4 AWG copper has an ampacity of exactly 85A. NEC 240.4 requires that the overcurrent protective device (the breaker) must not exceed the ampacity of the wire. Because 100A is a standard breaker size (NEC 240.6), you cannot use the 'next size up' rule. If you put #4 copper on a 100A breaker, a sustained 95A load will pass through the wire indefinitely without tripping the breaker, slowly baking the THHN insulation until it fails and causes an arc fault. #3 AWG copper is rated for exactly 100A, making it the absolute code minimum.
Ampacity & Voltage Drop: The Real-World Sizing Matrix
While #3 AWG copper satisfies the NEC ampacity requirements for heat dissipation, the code also strongly recommends (and some local jurisdictions mandate) limiting voltage drop to 3% for branch circuits and 5% overall for feeders. If your 100A subpanel is located far from the main panel, the resistance of #3 wire will cause a significant voltage drop under heavy load, leading to dimming lights, tripping inverter chargers, and overheating motors.
Here is a real-world voltage drop check for a 240V, 100A load using copper conductors. The Copper Development Association (CDA) recommends using the formula: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is 100A, D is the one-way distance, and CM is the circular mil area of the wire.
| One-Way Distance | Nominal Voltage | Recommended Wire Size (Cu) | Calculated Voltage Drop | Drop % | Verdict |
|---|---|---|---|---|---|
| 50 feet | 240V | #3 AWG | 2.45V | 1.02% | Pass (Code Minimum) |
| 100 feet | 240V | #3 AWG | 4.90V | 2.04% | Pass (Under 3% limit) |
| 150 feet | 240V | #2 AWG | 5.88V | 2.45% | Pass (Upgrade Required) |
| 200 feet | 240V | #1 AWG | 6.24V | 2.60% | Pass (Upgrade Required) |
Pro-Tip: If you are running a 100A feeder to a detached garage or workshop over 100 feet away, skip #3 AWG and pull #2 AWG copper or #1/0 AWG aluminum. The marginal increase in wire cost is vastly cheaper than replacing a burnt-out well pump or air compressor motor down the line due to low-voltage brownouts.
What Changes the Answer? (Derating & Material Swaps)
The baseline assumptions rarely survive contact with the actual jobsite. When you alter the installation environment, the wire's ability to shed heat decreases, forcing you to increase the wire gauge. Use this decision tree to determine if your specific installation requires upsizing.
| Installation Condition | Effect on Ampacity | Required Action / New Wire Size |
|---|---|---|
| Ambient Temp > 86°F (30°C) (e.g., conduit on a hot roof or in an attic) |
Insulation thermal limit is reached faster; ampacity must be multiplied by a correction factor (NEC 310.15(B)). | At 104°F (40°C), multiply ampacity by 0.88. #3 Cu (100A × 0.88 = 88A) is now too small. Upsize to #2 AWG Copper. |
| 4 to 6 Current-Carrying Conductors (e.g., two multi-wire branch circuits in one conduit) |
Conductors heat each other; ampacity must be adjusted downward (NEC 310.15(C)(1)). | Multiply ampacity by 0.80. #3 Cu (100A × 0.80 = 80A) fails. Upsize to #1 AWG Copper. |
| Continuous Load (e.g., EV charger, server rack, running 3+ hours) |
NEC 210.20(A) requires the circuit to be sized at 125% of the continuous load. | 100A × 1.25 = 125A required capacity. Upsize to #1 AWG Copper or #1/0 AWG Aluminum. |
| Using Aluminum Instead of Copper | Aluminum has higher resistance and expands/contracts more under heat. | Use #1 AWG Aluminum minimum. Apply Noalox antioxidant paste to terminations and torque lugs to manufacturer specs to prevent arcing. |
Never present aluminum and copper as interchangeable without verifying the termination ratings. Modern 100A breakers and panel lugs are typically marked 'AL/CU', meaning they accept both. However, if you are landing aluminum wire on a lug that is only rated for copper, you will create a high-resistance connection that will eventually melt the lug and start a fire.
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline framework, your local Authority Having Jurisdiction (AHJ) has the final say. You must pull a permit and have an inspector confirm your wire gauge and breaker sizing in the following scenarios:
- Service Entrance Conductors: If this 100A feed is coming directly from the utility meter to your main service panel (rather than a subpanel feeder), it falls under NEC Article 230. Utilities often have specific requirements for service drop wire sizes and types (e.g., triplex/quadruplex overhead cables).
- Mobile Home Feeders: NEC Article 550 has highly specific, sometimes more restrictive rules for mobile home service equipment and feeders. Many jurisdictions mandate a minimum of #2 AWG copper or #1/0 AWG aluminum for all 100A mobile home feeds regardless of distance.
- Local Amendments: Some municipalities amend the NEC to eliminate the 3% voltage drop 'recommendation' and make it a strict 'requirement.' In these areas, an inspector will fail a 150-foot run of #3 copper on a 100A breaker, even though it meets the thermal ampacity rules.
Frequently Asked Questions
Can I use 2/0 aluminum wire for a 100 amp subpanel?
Yes, absolutely. 2/0 AWG aluminum is rated for 150A at 75°C. Using it on a 100A breaker is perfectly safe and code-compliant; the breaker will simply protect the oversized wire. In fact, because aluminum pricing fluctuates, 2/0 aluminum (often sold as 'Mobile Home Feeder' cable) is frequently cheaper and easier to source than #1 aluminum or #3 copper. The only drawback is that 2/0 wire is physically thicker and stiffer, making it harder to bend into tight panel gutters. Ensure your panel lugs are rated to accept the larger wire diameter.
What size ground wire do I need for a 100 amp feeder?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100A overcurrent device is #8 AWG copper or #6 AWG aluminum. However, there is a critical exception: if you upsized your current-carrying conductors to mitigate voltage drop (for example, using #1 AWG copper instead of #3 AWG), NEC 250.122(B) requires you to increase the ground wire proportionately. If you bumped your hot wires up two sizes, you must bump your ground wire up two sizes as well (e.g., to #4 AWG copper).
Will #4 copper wire work for a 100 amp breaker if the run is very short?
No. This is a dangerous misconception. Wire ampacity is determined by the insulation's ability to shed heat into the surrounding environment, not by the length of the run. A 5-foot run of #4 AWG copper will get just as hot as a 50-foot run if you pull 95 amps through it continuously. Because #4 copper is only rated for 85A at 75°C, a 100A breaker will not trip in time to prevent the insulation from degrading, melting, and potentially shorting out against the panel enclosure. Always respect the thermal limits in Table 310.16, regardless of distance.
