⚠️ MAINS VOLTAGE HAZARD: Working inside a service panel or subpanel exposes you to lethal mains voltage (120V/240V AC). Always de-energize the upstream feeder, apply a lockout/tagout device, and verify the busbars are dead using a tested CAT III or CAT IV multimeter or non-contact voltage tester before touching any conductors. Local codes frequently require a licensed electrician for service entrance upgrades.

For a standard residential 100-amp service entrance, you need #4 AWG copper or #2 AWG aluminum. For a 100-amp feeder routing to a subpanel, you need #3 AWG copper or #1 AWG aluminum based on standard 75°C termination ratings. The correct cable size for a 100-amp service is the minimum American Wire Gauge (AWG) cross-section required to safely carry 100 amps of continuous electrical current without exceeding the temperature rating of the wire insulation or the connected equipment terminations.

Getting this sizing right changes three physical realities in your installation: it dictates your minimum conduit diameter (e.g., pulling three #1 AWG THHN wires requires a minimum 1-inch PVC conduit per NEC Chapter 9), it determines the exact inch-pound torque setting for your breaker lugs to prevent thermal arcing, and it establishes whether your voltage drop will stay under the 3% threshold at the end of a long run.

NEC Ampacity Tables: Service vs. Feeder Conductors

The most frequent point of failure in DIY electrical planning is confusing a service entrance conductor with a feeder conductor. The National Electrical Code (NEC) treats them differently because service conductors supply the main disconnect of a dwelling, while feeders supply downstream subpanels. You can verify these baseline rules in the NFPA 70 (National Electrical Code) guidelines.

Below is the definitive sizing matrix for 100-amp circuits. This table assumes standard residential/commercial equipment where the breaker and panel lugs are rated for 75°C. If you are using older equipment rated only for 60°C, you must use the 60°C column, which requires significantly thicker wire.

Application Conductor Material Min AWG Size Common Insulation Type NEC Article & Temp Column
Residential Service Entrance Copper #4 AWG THHN / XHHW-2 NEC 310.12(A) (Special Allowance)
Residential Service Entrance Aluminum #2 AWG XHHW-2 / SER Cable NEC 310.12(A) (Special Allowance)
Feeder to Subpanel (75°C Lugs) Copper #3 AWG THHN / THWN-2 NEC 310.16 (75°C Column)
Feeder to Subpanel (75°C Lugs) Aluminum #1 AWG XHHW-2 / THHN NEC 310.16 (75°C Column)
Legacy Feeder (60°C Lugs) Copper #1 AWG NM-B (Romex) NEC 310.16 (60°C Column)
Equipment Ground (EGC) Copper / Aluminum #8 Cu / #6 Al Bare or Insulated NEC 250.122
💡 The 80% Continuous Load Rule: If your 100-amp panel will supply a continuous load (defined by the NEC as a load expected to run for 3 hours or more, like an EV charger or baseboard heaters), the conductors and breaker must be sized at 125% of the continuous load. A true 100A continuous load requires conductors rated for 125A and a 125A breaker.

Worked Example: 100-Amp Subpanel Feeder with Voltage Drop

Ampacity tables only tell half the story. If your subpanel is located far from the main panel, resistance in the wire will cause voltage drop. The NEC recommends keeping voltage drop under 3% for branch circuits and feeders to ensure motors and appliances operate efficiently. Let's run the math on a real-world scenario using data aligned with standard Cerro Wire ampacity and resistance charts.

The Scenario: You are running a 100-amp subpanel to a detached garage 150 feet away. You plan to use aluminum XHHW-2 wire in underground PVC conduit. You expect a maximum continuous draw of 80 amps.

Option A: Using #2 AWG Aluminum (The Service Entrance Size)

  • Resistance: ~0.319 ohms per 1,000 feet for #2 Al.
  • Formula: Voltage Drop (VD) = (2 × Length × Resistance × Current) / 1,000
  • Calculation: (2 × 150 ft × 0.319 Ω × 80A) / 1,000 = 7.65 Volts
  • Percentage: 7.65V / 240V = 3.18%
  • Verdict: Fails. It exceeds the 3% recommended limit. Your 240V tools will only see ~232V under heavy load, causing motors to run hot and trip internal overloads.

Option B: Upsizing to #1 AWG Aluminum (The Correct Feeder Size)

  • Resistance: ~0.254 ohms per 1,000 feet for #1 Al.
  • Calculation: (2 × 150 ft × 0.254 Ω × 80A) / 1,000 = 6.09 Volts
  • Percentage: 6.09V / 240V = 2.53%
  • Verdict: Passes. By spending slightly more on #1 AWG wire, you drop the loss to an acceptable 2.53%, ensuring safe, efficient operation at the garage.
Conduit Sizing Note: Pulling four #1 AWG THHN/XHHW-2 conductors (two hots, one neutral, one ground) requires a minimum of 1.25-inch Schedule 40 PVC conduit to meet the NEC 40% conduit fill rule. Do not attempt to jam these into 1-inch pipe.

Where You Meet This in Practice

You will typically encounter the 100-amp sizing requirement in three specific jobsite scenarios:

  1. Upgrading a Mid-Century Home: Homes built in the 1950s and 60s often have 60-amp service entrances. When upgrading the meter main and panel to 100 amps, the utility will require you to pull new #2 AWG aluminum SER (Service Entrance Round) cable from the weatherhead down to the new meter socket. You cannot reuse the old #6 AWG wire.
  2. Detached Workshops and Garages: As shown in the voltage drop example above, running a 100-amp feeder to an outbuilding almost always mandates #1 AWG aluminum or #3 AWG copper in conduit. You must also pull a separate Equipment Grounding Conductor (EGC) and install a grounding rod at the detached structure, bonding the neutral and ground only at the main panel, never at the subpanel.
  3. EV Charger Additions: A Level 2 EV charger drawing 48 amps continuous requires a 60-amp breaker. If you are adding this to an existing 100-amp main panel, you must perform an NEC Article 220 Load Calculation. If the home's calculated load exceeds the 100-amp main breaker limit, you will need to upgrade to a 200-amp service, which requires #2/0 AWG copper or #4/0 AWG aluminum service entrance conductors.

Common Sizing Mistakes and Confusions

Even experienced hobbyists get tripped up by the nuances of NEC wire sizing. Here is what people commonly confuse when sizing for 100 amps:

The 90°C THHN Trap

Most modern THHN wire in conduit is printed with a 90°C rating on the jacket. Looking at the 90°C column in NEC Table 310.16, #4 AWG copper is rated for 95 amps, and #3 AWG is rated for 115 amps. DIYers often assume they can use #4 AWG copper for a 100-amp feeder because 'the wire can handle it.' This is a code violation. While the wire insulation can handle 90°C, the brass or aluminum lugs inside your breaker and panel are almost universally rated for a maximum of 75°C. You must size the wire based on the weakest link in the chain, which is the 75°C termination limit. Therefore, #3 AWG copper is the minimum for a 100A feeder.

Service vs. Feeder Allowances

People frequently ask why a residential service entrance can use #4 AWG copper (via NEC 310.12), but a subpanel feeder requires #3 AWG copper (via NEC 310.16). NEC 310.12 is a special 'diversity' allowance granted only to the main service conductors of a single-family dwelling, acknowledging that a home rarely pulls its full calculated load simultaneously across all branch circuits. This allowance does not apply to feeders, subpanels, or commercial buildings. If you use #4 AWG on a 100-amp subpanel feeder, an inspector will fail the rough-in.

Neutral Conductor Sizing

For a standard 120/240V single-phase residential panel, the neutral conductor carries only the unbalanced load between the two hot legs. While the NEC allows the neutral to be sized smaller than the hot conductors in specific commercial feeder calculations, standard residential practice and most AHJs (Authorities Having Jurisdiction) require the neutral to be the exact same AWG as the ungrounded (hot) conductors. If you pull #1 AWG aluminum for your hots, pull #1 AWG for your neutral, and use #6 AWG aluminum for your ground.