The correct 100 amp cable size is the minimum wire gauge required to safely carry 100 amperes of continuous or non-continuous current without exceeding the insulation's temperature rating or causing excessive voltage drop. For a standard residential or commercial installation, the absolute minimum 100 amp cable size is 3 AWG copper or 1 AWG aluminum, based on the 75°C column of the NEC ampacity tables. Getting this right changes the thermal ceiling of your termination points and ensures voltage stability at the load, preventing melted lugs and nuisance tripping.

The Baseline: Sizing Wire for 100 Amps

When you pull a 100-amp feeder, you are not just matching a number on a breaker handle; you are managing heat. The National Electrical Code (NEC) dictates wire sizing based on the lowest temperature rating of any connected component, which is almost always the breaker or panel lug. Most modern 100A breakers and panel lugs are rated for 75°C.

According to NEC Table 310.16, the ampacity of 3 AWG copper at 75°C is exactly 100 amps. If you prefer aluminum for cost savings on long runs, 1 AWG aluminum at 75°C also yields exactly 100 amps. While you might see 2 AWG copper used frequently on jobsites, 3 AWG is the code-minimum baseline for a 100A overcurrent protection device (OCPD).

Safety Warning: Always de-energize the main service panel, lock out the breaker, and verify the bus bars are dead with a properly rated CAT III or CAT IV multimeter before terminating any feeder cables. If you are upgrading a service entrance or working ahead of the main disconnect, local code likely requires a licensed electrician and a utility pull.

Where You Meet This In Practice

You will typically encounter the need for a 100 amp cable size in three specific scenarios:

  • Subpanel Feeders: Supplying a detached garage, workshop, or addition with a 100A main breaker panel.
  • Hardwired EV Chargers: Level 2 EVSE units that draw 80A continuously require a 125% multiplier (80A x 1.25 = 100A), mandating a 100A breaker and 100A-rated wire.
  • Large Appliances & Hot Tubs: Heavy commercial equipment or large spa setups that exceed 60A continuous loads.

What People Commonly Confuse It With

The most frequent bench and jobsite mistake is confusing the wire's insulation rating with the termination rating. Almost all THHN/THWN-2 wire you buy at the hardware store has a 90°C insulation rating. In the 90°C column of the NEC table, 4 AWG copper is rated for 95 amps, and 3 AWG is rated for 115 amps.

However, NEC 110.14(C) strictly limits your ampacity to the 75°C column because the breaker lugs are only tested and rated to 75°C. You cannot use the 90°C column to size your OCPD or terminate your wire, even if the wire jacket can physically survive the heat.

The Math: Ampacity, Derating, and Voltage Drop

Sizing for ampacity is only half the battle. If your run is longer than 50 feet, you must calculate voltage drop. Think of voltage drop like water pressure losing force over a long, narrow hose; if the hose is too thin for the distance, the sprinkler at the end won't pop up, even if the pump is running at full capacity.

Let us run a worked numeric example for a 100A, 240V subpanel feeder located 150 feet from the main panel.

Worked Numeric Example: Voltage Drop Calculation

The standard single-phase voltage drop formula is:

VD = (2 × K × I × D) / CM

  • K = 12.9 (constant for copper)
  • I = 100 Amps
  • D = 150 feet (one-way distance)
  • CM = 52,620 (circular mils for 3 AWG copper)

Calculation: (2 × 12.9 × 100 × 150) / 52,620 = 387,000 / 52,620 = 7.35 Volts.

On a 240V system, a 7.35V drop represents a 3.06% drop. The NEC recommends keeping feeder voltage drop under 3%. We are right on the ragged edge. If you add the branch circuit drop inside the subpanel, you will exceed the recommended 5% total system drop.

The Fix: Upsize to 2 AWG copper (CM = 66,360). Running the math again yields a 5.83V drop, or 2.4%, which is well within safe operational limits. You can verify these figures using tools like the Southwire Voltage Drop Calculator.

Scenario Walkthrough: The Melted EV Charger Lug

To understand why ignoring termination temperatures ruins installations, let us look at a real-world failure mode.

The Setup: A DIY enthusiast is wiring a hardwired 80A continuous Level 2 EV charger in a detached garage. Following the 125% continuous load rule, they correctly select a 100A breaker. To save money, they pull 4 AWG THHN copper wire. Looking at the wire packaging, they see it is rated for 95 amps at 90°C. They figure 95A is 'close enough' to 100A and terminate it directly into the 100A breaker lugs.

The Numbers: The EV charger pulls a steady 78A during charging cycles. The 100A breaker never trips because 78A is well below the 100A magnetic and thermal trip thresholds.

The Outcome: After three weeks of daily charging, the plastic housing around the breaker lug warps. The wire insulation turns brittle, brown, and cracks near the termination point. The connection begins to arc, eventually tripping an upstream AFCI breaker.

What Went Wrong: The installer ignored NEC 110.14(C). The 4 AWG wire was operating at 78A. At the 75°C termination limit, 4 AWG copper is only rated for 85A. The wire was operating just below its 75°C ampacity, but the heat generated at the high-resistance lug connection had nowhere to dissipate. The lug cooked the wire insulation, despite the wire's 90°C jacket rating. The correct 100 amp cable size here was 3 AWG copper minimum, which would have kept the termination well within its thermal envelope.

Step-by-Step: Terminating and Verifying Your 100A Feeder

Once you have pulled the correct 3 AWG copper or 1 AWG aluminum wire, proper termination is critical. Loose lugs cause high resistance, which generates heat and starts fires.

  1. Strip to the Exact Length: Use a wire stripping jig or measure against the breaker's wire gauge marker. Typically, you need 5/8' to 3/4' of bare conductor. Do not nick the copper strands; a nicked strand reduces the effective CM and creates a hot spot.
  2. Prepare Aluminum (If Used): If using 1 AWG aluminum, wire-brush the strands and immediately coat them with a Noalox or similar anti-oxidant compound to prevent galvanic corrosion and high-resistance oxidation.
  3. Seat the Wire Fully: Push the wire into the lug until the insulation jacket just touches the plastic barrier. Ensure no bare copper is exposed outside the lug, and no insulation is jammed inside the clamping mechanism.
  4. Torque to Spec: Do not guess. Read the torque specification printed on the breaker label (usually between 45 and 75 inch-pounds for 100A residential breakers). Use a calibrated inch-pound torque screwdriver, not a foot-pound automotive torque wrench.
  5. The Tug Test: Give the wire a firm, sharp tug. It should not move a millimeter.
  6. Verify Voltage: After energizing, measure line-to-line and line-to-neutral at the subpanel lugs. You should read 240V (±5%) and 120V (±5%) respectively.

FAQ: Common 100 Amp Wire Questions

Do I need to run a 100A-rated ground wire?

No. The equipment grounding conductor (EGC) does not carry continuous load current; it only carries fault current long enough to trip the breaker. Per NEC 250.122, a 100A breaker requires a minimum 8 AWG copper or 6 AWG aluminum ground wire.

Can I use 2 AWG copper instead of 3 AWG?

Yes, and you often should. While 3 AWG is the code minimum for ampacity, 2 AWG copper is highly recommended for runs over 50 feet to mitigate voltage drop. Furthermore, 2 AWG is physically more robust and often easier to source in standard NM-B (Romex) or MH-F cable formats than 3 AWG.

What conduit size do I need for three 3 AWG THHN wires and a ground?

For three 3 AWG THHN current-carrying conductors and one 8 AWG ground, NEC Chapter 9 Table 1 dictates a maximum 40% fill for three or more wires. A 1-inch PVC or EMT conduit will comfortably accommodate this bundle while leaving room for pulling.

Does the 100 amp cable size change if the wire is in a hot attic?

Yes. If your conduit runs through an attic where the ambient temperature exceeds 86°F (30°C), you must apply temperature correction factors from the bottom of NEC Table 310.16. If the attic hits 110°F, you must multiply the wire's ampacity by 0.82. In that scenario, 3 AWG copper (100A × 0.82 = 82A) is no longer sufficient, and you must upsize to 1 AWG copper.