Can I Use a 20 Amp Breaker Instead of 15? The Short Answer & Wire Rules
The direct answer is no, unless your branch circuit wiring is 12 AWG copper (or larger) AND every downstream device—including receptacles, smart switches, and electromechanical contactors—is explicitly rated for 20 amps. If your circuit uses 14 AWG wire, upsizing the breaker to 20A violates NEC Article 240.4(D) and creates a severe fire hazard, as the wire will melt before the breaker trips.
Even with 12 AWG wire, swapping a 15A breaker for a 20A breaker to stop nuisance tripping on a motor, well pump, or HVAC circuit often masks a dangerous mismatch. The breaker protects the wire, but the downstream electromechanical component (like a 15A-rated definite purpose contactor) relies on that breaker's trip curve to prevent its contacts from welding shut during a fault. If you push 19 amps through a 15A contactor, the contacts will overheat and fuse together long before a 20A breaker's thermal element trips.
Downstream Electromechanical Limits: Contactor Rating Tables
When evaluating if a 20A breaker is safe, you must look at the electromechanical switches controlling the load. Below is a specification table for standard Definite Purpose (DP) contactors commonly found in HVAC and pump applications. This data dictates whether your downstream hardware can survive the 20A breaker's let-through current.
| Contactor Model Class | Coil Voltage | Contact Rating (FLA) | Breaking Capacity (kAIC) | Governing Load Type |
|---|---|---|---|---|
| 15A DP Contactor (e.g., Eaton C25) | 120V AC | 15 Amps | 10 kA | Resistive / Small Motors |
| 20A DP Contactor | 120V AC | 20 Amps | 10 kA | Resistive / Fractional HP |
| 30A DP Contactor (HVAC Standard) | 240V AC | 30 Amps | 65 kA | Compressor (Inductive) |
| 40A DP Contactor (Heavy Duty) | 24V AC | 40 Amps | 65 kA | Compressor / Heat Strip |
Which Rating Column Governs This Load?
Beginners often look at the Resistive rating, but for motors and compressors, the FLA (Full Load Amps) column governs continuous operation, while the LRA (Locked Rotor Amps) dictates the inrush survival. If your compressor has an FLA of 18A, a 15A contactor will cook its internal contacts within a few cycles, even if the 15A breaker doesn't trip immediately due to thermal inertia. If you upgrade to a 20A breaker to accommodate the 18A load, you must also upgrade to a 30A-class contactor (the next standard size up) to provide a safety margin for contact degradation over time. For comprehensive industrial contactor specifications, refer to manufacturer catalogs like the Schneider Electric TeSys line.
Coil vs. Contact Wiring and Flyback Protection
Electromechanical contactors and heavy-duty relays split their wiring into two completely isolated circuits. Confusing these is a primary cause of burned-out control boards and tripped breakers.
- The Contact Side (Line/Load or L1/T1, L2/T2): This carries the high-current load (e.g., 240V AC to a compressor). The wire size here must match the breaker and the FLA of the load. If you are using a 20A breaker, this wiring must be a minimum of 12 AWG THHN or NM-B.
- The Coil Side (A1/A2): This is the low-current control circuit that energizes the electromagnet. It typically draws between 0.1A and 0.5A. This circuit is usually protected by a separate low-amp fuse or a 15A control circuit breaker, completely independent of the 20A main load breaker.
Selection Decision Path by Load Type & Breaker Curves
A common mistake is treating fuses and breakers as perfectly interchangeable without considering their trip curves. A standard 20A thermal-magnetic breaker will nuisance-trip on a 15A motor's startup inrush. A time-delay fuse (like an RK5) handles this inrush easily. To solve this with a breaker, you must use an HACR (Heating, Air Conditioning, and Refrigeration) rated breaker, which features a modified magnetic trip threshold designed specifically to tolerate motor inrush without compromising short-circuit protection.
| Load Type | Inrush Characteristic | Breaker / Fuse Selection Rule | Contactor Sizing Rule |
|---|---|---|---|
| Resistive (Heaters, Incandescent) |
Low (1.05x running current) | Standard thermal-magnetic breaker sized to 100% of load. | Match or exceed resistive amp rating. |
| Inductive (Transformers, Solenoids) |
Medium (5x to 8x running current) | Standard breaker, but size wire for 125% of continuous load. | Use contactor with high AC-3 / AC-4 utilization category. |
| Motor / Compressor (Pumps, HVAC) |
High (6x to 10x LRA) | HACR Breaker or Time-Delay Fuse. Size up to 250% of FLA per NEC 430.52. | Must be rated for Motor FLA and LRA; use DP contactors. |
Testing, Troubleshooting, and When to Replace
If your 15A breaker keeps tripping and you suspect a failing contactor or a degraded load, do not blindly swap to a 20A breaker. Test the electromechanical components first to find the root cause.
How to Test Dead (Power Off & Verified)
- Coil Resistance: Set your multimeter to Ohms (Ω). Place probes across A1 and A2. A healthy 120V AC coil typically reads between 10Ω and 50Ω. A 24V AC coil reads 1Ω to 5Ω. If it reads OL (open), the internal coil wire is broken. If it reads near 0Ω, the coil is shorted and will blow the control fuse instantly.
- Contact Continuity: With the coil de-energized, test across L1 and T1. It must read OL. Manually push the contactor plunger down with an insulated screwdriver; it should read < 0.5Ω. If it reads higher, the contacts are pitted or carbon-fouled.
How to Test Live (Mains Energized - Extreme Caution)
- Coil Voltage: Set meter to AC Volts. Measure across A1 and A2 while the thermostat or control board calls for operation. You should read nominal voltage (e.g., 114V-126V on a 120V system). A reading below 105V indicates a voltage drop in the control wiring, which will cause the contactor to chatter and arc.
- Contact Voltage Drop: With the contactor pulled in and the motor running, measure the voltage difference between L1 and T1. A healthy closed contact drops less than 0.2V. If you read 2V to 5V across a closed contact, the contact surface is degraded and generating immense heat (I²R losses).
When to Repair vs. Replace
Repair: In modern, sealed DP contactors and molded-case relays, repair is virtually impossible and highly discouraged. You cannot safely file down pitted contacts without altering the contact pressure and travel distance, which ruins the wiping action and guarantees future welding.
Replace: Replace the entire electromechanical component if you observe any of the following:
- Visible pitting, blackening, or melting on the plastic housing near the contacts.
- The contactor 'chatters' loudly (indicating a shorted shading coil on AC units or low control voltage).
- The contacts are welded shut (the load stays on even when the coil is de-energized).
- Resistance across closed contacts exceeds 0.5Ω.
Ultimately, if your circuit demands 18 amps, do not use a 20A breaker to save a 15A contactor. Upgrade the wire to 10 AWG, install a 30A HACR breaker, and fit a 30A definite purpose contactor. This ensures the breaker protects the wire, and the contactor can handle the mechanical and thermal stress of the load.
