The correct 100 amp wire size is the minimum conductor gauge capable of carrying 100 amps of current without exceeding the thermal limits of its insulation or termination points. Selecting the right gauge dictates your physical conduit diameter, bending radius, terminal lug compatibility, and overall material cost. DIYers frequently confuse the breaker's trip rating with the wire's baseline ampacity, or they mistakenly size wire using the 90°C insulation column instead of the 60°C or 75°C terminal temperature limit mandated by the National Electrical Code (NEC).
The Core Rule: Sizing Wire for a 100-Amp Load
To size a wire for a 100-amp circuit, you must consult NEC Article 310.16 (formerly 310.15(B)(16)), which outlines the allowable ampacities for insulated conductors. The most critical factor is the temperature rating of the terminations (the breaker lugs and panel bus bars), not just the wire insulation.
Under NEC 110.14(C), circuits rated 100 amps or higher can utilize the 75°C column for sizing, provided the equipment is rated for 75°C (which virtually all modern residential and commercial panels are). Even if you buy THHN wire with a 90°C insulation rating, you must use the 75°C column to determine your baseline ampacity because the breaker lugs will overheat if you push 90°C-rated current through them.
- Copper (75°C Column): 4 AWG is rated for 85A (too small). 3 AWG is rated for exactly 100A. 2 AWG is rated for 115A.
- Aluminum (75°C Column): 2 AWG is rated for 90A (too small). 1 AWG is rated for exactly 100A. 1/0 AWG is rated for 120A.
Where You Meet 100-Amp Feeds in Practice
You will rarely see a 100-amp circuit feeding a single standard appliance. Instead, this wire size is the backbone of heavy branch feeders and subpanels. Common real-world applications include:
- Detached Garage or Workshop Subpanels: A 100A feeder provides enough headroom for lighting, receptacles, and a 240V welder or compressor without tripping the main feeder breaker.
- Hardwired EV Level 2 Chargers: An 80-amp continuous EV charger requires a breaker sized at 125% of the continuous load (80A x 1.25 = 100A). This mandates a 100A breaker and correspondingly sized wire.
- Large HVAC or Commercial Equipment: Rooftop units, large electric furnaces, and commercial kitchen make-up air units frequently draw in the 80A–95A range, requiring a 100A disconnect and feeder.
- Hot Tubs and Spas: While many spas run on 50A or 60A GFCI breakers, large multi-pump luxury spas with inline electric heaters often require a dedicated 100A feed to an outdoor spa panel.
Worked Example: Voltage Drop on a 150-Foot Subpanel Run
Ampacity tables only tell you if the wire will melt. They do not tell you if the voltage at the other end will be sufficient to run your tools. NEC 310.15(B) recommends keeping voltage drop under 3% for feeders. Let us calculate the voltage drop for a 100A subpanel located 150 feet from the main panel, assuming a realistic continuous load of 80 amps.
Constants: K = 12.9 for Copper, 21.2 for Aluminum. I = 80A. L = 150 ft.
| Wire Choice | Circular Mils (CM) | Calculated Voltage Drop | Percentage (of 240V) | Result |
|---|---|---|---|---|
| 3 AWG Copper | 52,620 | 5.88V | 2.45% | Passes (Under 3%) |
| 1 AWG Aluminum | 83,690 | 6.07V | 2.53% | Passes (Under 3%) |
| 2 AWG Copper | 66,360 | 4.66V | 1.94% | Passes (Excellent headroom) |
While both the code-minimum 3 AWG Copper and 1 AWG Aluminum pass the 3% threshold at an 80A load, if you anticipate pulling a full 100A non-continuous load (like a large welder starting up), the voltage drop on 3 AWG Copper jumps to 3.06%, edging over the recommended limit. This is why many electricians automatically upsize to 2 AWG Copper or 1/0 AWG Aluminum for runs exceeding 100 feet. You can verify your specific run using the Southwire Voltage Drop Calculator.
The Decision Path: Copper vs. Aluminum for 100 Amps
Choosing between copper and aluminum is a trade-off between material cost, conduit space, and pulling effort. Use this decision tree to select your exact wire.
| If your installation condition is... | Then choose this wire... | Why? |
|---|---|---|
| Run is under 50 feet, and conduit is tight (e.g., 3/4-inch PVC with existing pulls) | 3 AWG THHN Copper | Copper is physically smaller, making it easier to pull through crowded conduits. Cost difference is negligible at short distances. |
| Run is over 50 feet, budget is a primary concern, and you have 1-inch or larger conduit | 1/0 AWG XHHW-2 Aluminum | Aluminum is roughly 60% cheaper than copper by the foot. Upsizing to 1/0 AWG (120A rating) eliminates voltage drop concerns and provides future-proofing. |
| Terminating directly into a main breaker with tight lug spacing and no room for large bending radii | 2 AWG THHN Copper | Aluminum requires a larger bending radius and larger lugs. Copper's flexibility and smaller diameter save space inside the panel gutter. |
| Buried direct underground (no conduit) | 1/0 AWG URD (Underground Residential Distribution) Aluminum | URD cable is specifically jacketed for direct burial and uses aluminum to keep the cost of long trench runs manageable. |
Common Installation Mistakes and Terminal Ratings
Sizing the wire correctly is only half the battle. Improper termination is where 100-amp circuits fail, overheat, or cause fires.
1. The 90°C Ampacity Trap
Wire manufacturers print the 90°C rating on the jacket (e.g., THHN/THWN-2). A common mistake is looking at the 90°C column in NEC 310.16, seeing that 4 AWG Copper is rated for 95A, and assuming it is close enough to 100A, or using the 90°C column to derate for conduit fill. While you can use the 90°C column for derating adjustments (like bundling multiple wires in a conduit), your final adjusted ampacity must still be equal to or greater than the required size in the 75°C column. You cannot terminate a 100A breaker on a wire sized solely by the 90°C column.
2. Ignoring Anti-Oxidant Paste on Aluminum
Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates intense heat under load. If you are using 1 AWG or 1/0 AWG aluminum wire, you must apply a UL-listed anti-oxidant compound (like Noalox or Ideal Noalox) to the stripped conductor before inserting it into the lug. This paste displaces oxygen and prevents the oxide layer from forming. Never use aluminum in standard plug-on bus bar stabs unless the breaker is explicitly rated for aluminum (most are, but always check the breaker label).
3. Guessing the Torque
A 100-amp lug requires significant mechanical pressure to maintain a low-resistance connection. Under-torquing leads to arcing and melted lugs; over-torquing strips the threads or deforms the wire. According to NEC 110.14(D), you must use a calibrated torque screwdriver or wrench set to the exact inch-pound value printed on the breaker or panel label (typically between 40 and 50 in-lbs for 100A lugs, but this varies by manufacturer like Eaton or Square D).
FAQ: 100 Amp Wire Sizing Edge Cases
What size equipment grounding conductor (EGC) do I need for a 100 amp feeder?
According to NEC 250.122, the minimum equipment grounding conductor for a 100-amp breaker is 8 AWG Copper or 6 AWG Aluminum. If you upsized your ungrounded (hot) conductors for voltage drop (e.g., using 1/0 AWG Aluminum instead of 1 AWG), NEC 250.122(B) requires you to proportionally increase the ground wire size as well. In practice, most electricians just pull a 6 AWG Copper ground for any 100A feeder to avoid the math and ensure a robust fault-current path.
Can I use 2 AWG copper wire on a 100 amp breaker?
Yes. The NEC specifies the minimum wire size. Using a larger wire (lower AWG number) like 2 AWG or 1 AWG copper is always legal and often recommended for long runs to mitigate voltage drop. The only limitation is physical: ensure the wire is stripped to the correct length and that the breaker's lug is rated to accept the larger gauge. Some 100A breakers max out at 1 AWG or 1/0 AWG.
What if my 100-amp load is continuous (running for 3 hours or more)?
If the load is continuous, NEC 210.20(A) requires the breaker to be sized at 125% of the load. Therefore, a 100A continuous load requires a 125-amp breaker. Consequently, your wire must be sized for 125 amps, meaning you must step up to 1/0 AWG Copper or 3/0 AWG Aluminum based on the 75°C column. Never put a 100A continuous load on a 100A breaker.
