To wire a 100 amp sub panel, use 3 AWG copper or 1 AWG aluminum wire for the hot and neutral conductors, protected by a 100A double-pole breaker. You must also pull a 6 AWG copper or 4 AWG aluminum equipment grounding conductor. This assumes standard 75°C terminations.
- Conductor Material: Copper (unless aluminum is explicitly stated)
- Temperature Rating: 75°C column (standard for residential panel lugs and breakers)
- Ambient Temperature: 30°C (86°F) or lower
- Installation: Single raceway/conduit, no more than 3 current-carrying conductors (no derating applied)
- System: 120/240V single-phase split-phase
The Core Sizing Rules for a 100A Feeder
When figuring out how to wire a 100 amp sub panel, the National Electrical Code (NEC) dictates that your wire ampacity must meet or exceed the breaker rating. According to NFPA 70 (NEC) Table 310.16, we look at the 75°C column because standard residential panel lugs and breaker terminals are rated for 75°C, even if the wire insulation itself is rated for 90°C (like THHN).
| Conductor Size | Material | Insulation Type | 75°C Ampacity | 90°C Ampacity |
|---|---|---|---|---|
| 3 AWG | Copper | THHN/THWN-2 | 100A | 115A |
| 1 AWG | Aluminum | XHHW-2 | 100A | 115A |
| 6 AWG | Copper | THHN/THWN-2 | 65A (Ground) | N/A |
Why this size and not one smaller?
A common mistake is attempting to use 4 AWG copper wire. In the 75°C column, 4 AWG copper is rated for only 85A. You cannot safely land an 85A-rated wire on a 100A breaker. While NEC 240.4(B) allows the "next size up" rule for overcurrent protection, that rule applies when the exact calculated load does not match a standard breaker size. For a 100A subpanel feeder, the breaker is sized to protect the wire and the panel bus. Using 4 AWG creates a fire hazard if the panel load actually approaches 100A, as the wire will overheat before the breaker trips.
Voltage Drop and Environmental Derating
The baseline answer of 3 AWG copper assumes a relatively short run and standard conditions. Several real-world factors will force you to upsize your wire.
Voltage Drop Check at 100 Feet
The NEC recommends a maximum 3% voltage drop on feeders. Let us calculate the voltage drop for a 100-foot run of 3 AWG copper carrying an 80A continuous load (the realistic maximum for a 100A breaker).
- Formula: VD = (2 × K × I × L) / CM
- K (Copper): 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 = 3.92V.
On a 240V system, 3.92V is a 1.63% drop. This is well within the 3% limit. However, if your run is 200 feet, the drop doubles to 7.84V (3.2%), at which point you must upsize to 2 AWG copper to maintain code compliance and tool performance.
What changes the answer: Bundling and Aluminum
If you pull two separate subpanel feeds through a single conduit, you now have four current-carrying conductors. NEC Chapter 9, Note 4 to Table 310.15(B)(3)(a) requires an 80% derating factor. You must use the 90°C column for derating math: 115A × 0.80 = 92A. Your 3 AWG wire is now only good for 92A and fails the 100A requirement. You must upsize to 2 AWG or 1 AWG copper. Furthermore, if you switch to aluminum to save money, you must use 1 AWG, apply anti-oxidant paste (like Noalox) to the terminations, and use a torque screwdriver to hit the exact lb-in spec printed on the panel label to prevent high-resistance arcing.
| Scenario | Hot/Neutral Wire Size (Copper) | Hot/Neutral Wire Size (Aluminum) | Ground Wire (Copper) |
|---|---|---|---|
| Standard run (< 100 ft), single conduit | 3 AWG | 1 AWG | 6 AWG |
| Long run (100 ft - 150 ft) at 80A load | 2 AWG | 1/0 AWG | 6 AWG |
| Bundled (4+ current-carrying conductors) | 2 AWG | 1/0 AWG | 6 AWG |
When an Engineer or AHJ Must Confirm
While sizing wire and breakers for a standard subpanel is straightforward, certain conditions require professional engineering or explicit Authority Having Jurisdiction (AHJ) approval:
- High Available Fault Current: If your utility company recently upgraded the neighborhood transformer, the available fault current at your main panel might exceed 10,000 Amps (10kA). If you buy a standard 100A breaker rated for 10kAIC (Ampere Interrupting Capacity) and the system can deliver 22kA, the breaker can violently fail during a short circuit. You must verify the main panel's fault rating and buy a 100A breaker with a matching or higher AIC rating (e.g., 22kAIC or 42kAIC).
- Separate Structures: If the subpanel feeds a detached garage or shed, NEC Article 225 requires a separate grounding electrode system (ground rods) at the second building, and the neutral and ground bars in the subpanel must be bonded differently than in an attached structure.
- Service Entrance Upgrades: If this 100A feed is actually acting as a service entrance disconnect rather than a subpanel, entirely different NEC Article 230 rules apply regarding physical protection and disconnect grouping.
Frequently Asked Questions
Can I use 2 AWG wire for a 100 amp sub panel?
Yes. 2 AWG copper is rated for 115A in the 75°C column. Using a wire larger than the minimum requirement is perfectly legal and often recommended for long runs to mitigate voltage drop. The only physical constraint is ensuring the 2 AWG wire will physically fit into the lugs of your 100A breaker and the subpanel's main lugs. Always check the manufacturer's datasheet for the maximum wire termination size before purchasing.
What size ground wire do I need for a 100 amp sub panel?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100A overcurrent device is 6 AWG copper or 4 AWG aluminum. This ground wire runs alongside your hot and neutral conductors and connects to the ground bar in the subpanel and the ground bar or bus in the main panel. Never use the neutral wire as a ground, and ensure the neutral bar in the subpanel is isolated from the metal enclosure (the bonding screw or strap must be removed).
Do I need a main breaker in a 100 amp sub panel?
Not necessarily. If the subpanel is located in the same building as the main service panel, the 100A double-pole breaker back in the main panel serves as the required disconnecting means. You can use a "Main Lug" subpanel, which has no main breaker, just lugs to land your feeder wires. However, if the subpanel is in a detached structure, NEC 225.31 and 225.32 require a local disconnecting means. In that case, buying a "Main Breaker" 100A panel is the easiest way to satisfy the code requirement for a local disconnect.
How many circuits can I put on a 100 amp sub panel?
There is no physical limit to the number of breaker spaces you can have. You can install a 12-space, 24-space, or even 42-space 100A subpanel. The limit is based on the calculated load, not the number of breakers. You must perform an NEC Article 220 load calculation to ensure the sum of the continuous and non-continuous loads drawing from the subpanel does not exceed 80A continuous or 100A peak. Adding twenty 15A lighting and receptacle circuits is perfectly fine, provided you are not plugging in twenty 1500W space heaters simultaneously.
