The correct wire size for a 100 amp breaker is 3 AWG copper or 1 AWG aluminum when terminating on modern 75°C rated lugs, per NEC Table 310.16. If your equipment terminals are rated only for 60°C (common in older panels or specific HVAC disconnects), you must step up to 2 AWG copper or 1/0 AWG aluminum. For feeder runs exceeding 50 feet, upsizing to 2 AWG copper is standard practice to mitigate voltage drop. However, sizing the wire is only half the battle when feeding heavy electromechanical loads; the breaker's trip curve and the downstream contactor's utilization rating must also align with the specific inrush characteristics of the load.

Wire Sizing and Breaker Trip Curves for 100A Loads

Before pulling wire, verify the terminal temperature rating of both the breaker and the downstream equipment. The NEC 110.14(C) termination rule dictates that you must use the lowest temperature rating in the circuit to determine ampacity. Below is the reference data for a standard 100A continuous or non-continuous feeder.

Table 1: Wire Sizing for 100A Breaker (Copper & Aluminum)
Wire Size (AWG/kcmil) Material Temp Column Used Ampacity (NEC 310.16) Max Run for <3% Drop (240V)
3 AWG Copper (THHN/THWN) 75°C 100A ~55 ft
2 AWG Copper (THHN/THWN) 75°C 115A ~85 ft
1 AWG Aluminum (XHHW) 75°C 100A ~45 ft
1/0 AWG Aluminum (XHHW) 75°C 120A ~75 ft
Safety & Code Caveat: Never treat fuses and breakers as interchangeable without analyzing the trip curve. A standard 100A thermal-magnetic breaker has a specific magnetic trip threshold (often 5x to 10x rated current). If you are protecting a high-inrush motor, a standard breaker might nuisance-trip on startup. In motor circuits, you need an HACR (Heating, Air Conditioning, Refrigeration) rated breaker or a Type D / high-magnetic setting breaker. A 100A Class RK5 fuse, by contrast, has a different let-through energy curve and time-delay profile. Always match the overcurrent protective device (OCPD) to the specific NEC Article 430 motor starting requirements.

Selecting the Downstream 100A Contactor

When a 100A breaker feeds a subpanel or a dedicated enclosure, the current often terminates on a heavy-duty electromechanical contactor (such as an Eaton C25 series or Siemens 3RT series) to switch large compressors, industrial chillers, or milling machine motors. Selecting the right contactor requires looking past the basic '100A' label and checking the utilization category.

Decision Path by Load Type

The governing rating column depends entirely on what the contactor is switching. Here is the decision matrix:

  • Resistive Loads (Heaters, Lighting Banks): Governed by the AC-1 rating. Inrush current is negligible (1x to 1.2x nominal). A 100A AC-1 contactor is sufficient.
  • Standard Motor Starting (HVAC Compressors, Pumps): Governed by the AC-3 rating. Squirrel-cage motors draw 6x to 8x full load current (FLC) during startup. A contactor rated '100A AC-3' is physically built with heavier silver-alloy contacts and stronger arc chutes than an AC-1 contactor of the same physical size.
  • Jogging, Plugging, or Reversing Motors: Governed by the AC-4 rating. This involves breaking locked-rotor current. You must severely derate the contactor; a 100A AC-3 contactor might only be rated for 60A under AC-4 conditions.
Table 2: 100A Contactor Rating & Breaking Capacity Matrix
Utilization Category Load Type Contact Rating (Nominal) Breaking Capacity Typical Application
AC-1 Non-inductive / Resistive 100A Low (1x In) Industrial space heaters, lighting
AC-3 Squirrel-Cage Motor 100A (approx 75 HP @ 460V) High (8x In breaking) Large HVAC compressors, conveyor motors
AC-4 Jogging / Plugging ~65A (Derated) Extreme (Locked rotor) Hoists, crane controls, rapid reversing
AC-8a Hermetic Refrigerant Compressor 100A (Definite Purpose) High (Specific to HVAC) Commercial rooftop AC units

For deeper specifications on IEC-rated contactors, refer to manufacturer selection guides like the ABB Contactor Catalog or Eaton's Definite Purpose series.

Coil vs. Contact Side Wiring and Protection

A common failure point in 100A circuits is confusing the high-current contact wiring with the low-current control wiring. The physical separation and termination requirements for these two sides are vastly different.

The Contact Side (Power Circuit)

This is where your 2 AWG or 3 AWG copper wire from the 100A breaker lands. Termination torque is critical. A loose 2 AWG lug on a 100A contactor will arc and melt the busbar within weeks under a 70A continuous load. Use a calibrated inch-pound torque screwdriver or a torque wrench. For a typical 100A lug, the manufacturer spec is usually between 45 and 50 in-lbs (or roughly 4 ft-lbs for larger hex-head lugs). Always use a wire brush and antioxidant paste (like Noalox) if terminating aluminum wire to copper lugs.

The Coil Side (Control Circuit)

The coil operates the electromagnet that pulls the heavy contacts closed. Coil wiring typically uses 14 AWG or 16 AWG stranded control wire, fed from a 120V AC control transformer or a 24V DC PLC output.

DC Coil Protection Warning: If your control circuit is DC (e.g., 24VDC from a solid-state PLC relay), you must install a flyback diode (such as a 1N4007) or an RC snubber module across the coil terminals. When the PLC turns off the DC coil, the collapsing magnetic field generates a massive inductive voltage spike (hundreds of volts) that will instantly destroy the PLC's internal transistor output. Wire the diode in reverse-bias (cathode to positive, anode to negative) so it only conducts during the spike.

Testing, Diagnostics, and Repair vs. Replace

When a 100A motor fails to start or the breaker trips immediately, you need a systematic diagnostic approach to determine if the contactor is at fault.

How to Test Dead (Power Off & Locked Out)

  1. Coil Continuity: Set your multimeter to Ohms (Ω). Measure across the coil terminals (A1 and A2). A healthy AC coil typically reads between 10 and 50 ohms. A reading of 'OL' (Open Loop) means the coil is burnt out. A reading near 0 ohms means an internal short.
  2. Contact Resistance: Manually press the contactor plunger in with an insulated tool. Measure resistance across Line to Load for each pole. It should read < 0.5 ohms. High resistance indicates pitted or carbon-scored contacts.
  3. Mechanical Check: The plunger should move smoothly without grinding. If it feels gritty, the armature is contaminated with rust or debris.

How to Test Live (Energized & Under Load)

Danger: Live testing a 100A circuit exposes you to fatal arc flash hazards. Wear appropriate PPE (Category 2 minimum), use a rated CAT III/IV meter, and keep your body clear of the enclosure dead-front.
  1. Coil Voltage: Measure AC voltage across A1 and A2 while the circuit is commanded 'ON'. It must be within ±10% of the coil rating. A 120V coil receiving only 95V will chatter, overheat, and burn out.
  2. Voltage Drop Across Poles: With the motor running under full load, measure the voltage from the Line side terminal to the Load side terminal of each pole. A healthy contactor will drop less than 0.05V per pole. If you read > 0.15V across a pole, the contacts are severely pitted and generating dangerous heat.

When to Repair vs. Replace

In the fractional-horsepower world, you might swap out just a coil. But at the 100A level, the decision matrix shifts heavily toward replacement.

  • Replace the Coil Only If: The coil is burnt (reads 'OL'), the mechanical armature and contacts are in pristine condition, and the exact replacement coil is readily available. This is common in massive, modular industrial contactors (e.g., 400A+ frames).
  • Replace the Entire Contactor If: The contacts are pitted, the arc chutes are melted, the armature is sticky, or the contactor has experienced a short-circuit fault. For standard 100A NEMA or IEC contactors, the labor cost to disassemble, file contacts, and reassemble safely exceeds the cost of a new unit (typically $150 to $350 for a quality 100A unit). Never file down modern silver-alloy contacts; you will remove the silver plating and expose the base metal, leading to rapid welding and failure.