100 amp wire is a conductor sized to safely carry 100 amperes of electrical current without exceeding its thermal limits, which under standard NEC 75°C termination rules requires 3 AWG copper or 1 AWG aluminum.
When you step up to a 100A circuit, you are no longer dealing with standard branch-circuit wiring. This gauge of wire changes the physical footprint of your installation: it dictates the minimum conduit diameter (typically 1.25-inch or 1.5-inch PVC depending on the number of conductors), requires specific lug torque values, and forces you to calculate voltage drop on runs longer than 50 feet. The most common mistake DIYers and junior electricians make is confusing the 90°C ampacity column with the 75°C termination limit. You might see that 4 AWG copper can handle 100A at 90°C, but because standard breakers and panel lugs are rated for 75°C, the NEC mandates you size the wire based on the 75°C column.
The Baseline: Sizing 100 Amp Wire for Standard Installations
To select the correct wire, we look at NEC Table 310.16 published by the National Fire Protection Association (NFPA). The table provides ampacities based on insulation type and temperature ratings, but NEC Article 110.14(C) acts as the ultimate bottleneck for terminations.
| Conductor Material | AWG Size | Ampacity (75°C Column) | Ampacity (90°C Column) | Allowed for 100A Breaker? |
|---|---|---|---|---|
| Copper (THHN/THWN-2) | 4 AWG | 85A | 95A | No (Undersized for lugs) |
| Copper (THHN/THWN-2) | 3 AWG | 100A | 115A | Yes (Minimum Size) |
| Aluminum (XHHW-2) | 2 AWG | 90A | 100A | No (Undersized for lugs) |
| Aluminum (XHHW-2) | 1 AWG | 100A | 115A | Yes (Minimum Size) |
Worked Example: 100A Subpanel Feeder at 150 Feet
Ampacity is only half the battle. If you are running a 100A feeder to a detached garage subpanel 150 feet away, a wire that can handle the heat might still suffer from unacceptable voltage drop. The NEC recommends a maximum 3% voltage drop for feeders (7.2V on a 240V system).
Let us calculate the voltage drop using the standard formula: VD = (2 × K × I × D) / CM
- K (Constant) = 12.9 for Copper, 21.2 for Aluminum
- I (Current) = 100A
- D (Distance) = 150 feet
- CM (Circular Mils) = Cross-sectional area of the wire
Scenario A: Using minimum 3 AWG Copper (CM = 52,620)
VD = (2 × 12.9 × 100 × 150) / 52,620 = 387,000 / 52,620 = 7.35V (3.06%).
This slightly exceeds the 3% recommendation. While technically legal under strict NEC ampacity rules, it is poor practice for a 150-foot run.
Scenario B: Upsizing to 2 AWG Copper (CM = 66,360)
VD = 387,000 / 66,360 = 5.83V (2.43%).
This is well within the 3% threshold and represents the correct professional choice for this distance.
Scenario C: Using 1/0 AWG Aluminum (CM = 105,600)
VD = (2 × 21.2 × 100 × 150) / 105,600 = 636,000 / 105,600 = 6.02V (2.5%).
Because aluminum is significantly cheaper per foot than copper, 1/0 AWG Aluminum (often sold as 1/0-1/0-1/0-2 SER cable) is the most common jobsite choice for long 100A subpanel feeders. Consult Southwire's technical resources for exact CM values and derating factors if bundling multiple cables.
Where You Meet 100 Amp Wire in Practice
You will rarely pull 100A wire for standard wall receptacles. In residential and light commercial environments, this wire gauge shows up in specific, high-demand scenarios:
- Detached Structure Subpanels: Feeding a 100A subpanel in a garage, workshop, or barn. This allows for simultaneous use of a welder, air compressor, and lighting without tripping the main feeder breaker.
- Heavy-Duty EVSE (EV Chargers): While most Level 2 home chargers run on 50A or 60A circuits, hardwired commercial-style chargers or future-proofed residential installations often pull a 100A circuit to support 80A continuous charging rates (yielding roughly 19.2 kW of power delivery).
- Large Spa and Hot Tub Subpanels: A large spa with multiple heating elements, high-flow circulation pumps, and landscape lighting often requires a dedicated 100A GFCI-protected subpanel, fed by 100A wire.
- Tankless Electric Water Heaters: Whole-home electric tankless units can draw massive current. While they usually require multiple 40A breakers, some consolidated commercial setups utilize a single 100A fused disconnect fed by 3 AWG copper.
Frequently Asked Questions
What size wire do I need for a 100 amp sub panel 100 feet away?
For a 100-foot run, voltage drop is less severe than at 150 feet. Using the voltage drop formula, 3 AWG Copper yields a 2.04% drop (4.9V), and 1 AWG Aluminum yields a 2.11% drop (5.08V). Both are well under the 3% recommended limit, meaning you can safely use the minimum NEC ampacity sizes (3 AWG Cu or 1 AWG Al) without upsizing for distance.
Can I use aluminum wire for a 100 amp breaker?
Yes, absolutely. Aluminum is the industry standard for feeders and service entrances due to its cost-effectiveness. For a 100A breaker, you must use 1 AWG Aluminum (assuming 75°C rated terminations). The critical rule when using aluminum is to apply an anti-oxidant compound (like Noalox) to the stripped conductor ends before torquing them into the breaker or panel lugs, and to strictly follow the manufacturer's inch-pound torque specifications to prevent thermal expansion failures over time.
Does 100 amp wire need to be in conduit?
It depends on the cable type and installation location. If you are using individual THHN/THWN-2 conductors, they must be pulled through a raceway (like 1.5-inch PVC or EMT conduit) for physical protection. However, if you are using a bundled cable assembly like SER (Service Entrance Cable) or MH-Feeder, it can be stapled directly to framing or run through bored joist holes inside finished walls without conduit. If you transition from cable to individual wires (e.g., entering a concrete slab or exiting a wall to surface-mount a panel), you must use conduit for the exposed transition.
