What Size Breaker for a Furnace: Guide to Circuit Requirements

Choosing the correct circuit breaker for a furnace is essential for safety, reliable operation, and code compliance. This guide explains how to determine the right breaker size for different furnace types, how electrical load calculations work, common breaker sizes, and practical tips for installation and troubleshooting.

Furnace Type	Typical Breaker Size	Notes
Single-Stage Gas Furnace (24V control)	15–20 Amp	Control circuits and intermittent blower motors
Two-Stage/Variable-Speed Gas Furnace	20–30 Amp	Larger blower motors and ECM motors
Electric Furnace (Residential)	30–60 Amp or more	Depends on kilowatt rating; often 240V with multiple breakers
Heat Pump / Furnace Combo	30–50 Amp	Depends on compressor and fan motor demands

How Furnace Electrical Requirements Are Determined

Furnace electrical requirements come from the unit’s nameplate, installation manual, and the National Electrical Code (NEC). The nameplate lists voltage, minimum circuit ampacity (MCA), and maximum overcurrent protection (MOCP). The MCA is the current the wiring must safely carry, and the MOCP defines the largest breaker allowed to protect the circuit.

Key Terms To Know

MCA (Minimum Circuit Ampacity) specifies the conductor size needed for continuous and non-continuous loads. MOCP (Maximum Overcurrent Protection) specifies the largest breaker that may be used to protect the circuit. Continuous Load is a load expected to run three hours or more; NEC requires wiring sized to 125% of continuous loads.

Common Furnace Breaker Sizes And Scenarios

Gas Furnace With Standard Blower Motor

Most residential gas furnaces with a standard PSC blower and 24V controls draw relatively little current on the 120V control circuit. Typical breakers for the furnace control and inducer/blower circuit are 15 or 20 amps on 120V.

Gas Furnace With ECM Or Variable-Speed Motor

Variable-speed and electronically commutated motors (ECMs) are more efficient but can draw higher inrush currents. Manufacturers often specify a 20 or 30 amp breaker for these units, especially if additional components such as humidifiers or advanced control boards are on the same circuit.

Electric Furnace (Entire Furnace Run On 240V)

Electric furnaces convert electrical energy to heat using heating elements and typically require much larger breakers and conductors. Residential electric furnaces commonly need 30 to 60 amps or higher depending on the kilowatt rating. Many electric furnaces will require a 240V dedicated circuit with appropriately sized double-pole breakers and multiple conductors.

Heat Pump Or Packaged Units

Heat pumps and packaged units have compressors and fan motors that require larger 240V breakers, often in the 30 to 50 amp range, but specifications vary widely. Always refer to the unit’s nameplate and installation manual for exact MCA and MOCP values.

Step-By-Step Method To Determine The Correct Breaker Size

1. Locate The Nameplate: Find the furnace nameplate or the installation manual which lists MCA and MOCP.
2. Identify Voltage: Confirm whether the unit uses 120V or 240V and whether it requires a single- or double-pole breaker.
3. Use MCA For Wire Sizing: Size conductors according to the MCA, applying NEC rules such as 125% for continuous loads.
4. Use MOCP For Breaker Sizing: Select a breaker that does not exceed the MOCP listed by the manufacturer; MOCP is the maximum allowed breaker size.
5. Account For Other Loads: If other devices (thermostat, humidifier, UV lights) are on the same circuit, include their current draw in the calculation.

NEC Rules And Safety Considerations

The NEC requires the circuit conductor to be rated at least equal to the furnace MCA. For continuous loads, wires must be sized to 125% of the continuous load. The breaker must not exceed the MOCP listed by the manufacturer. These rules protect wiring from overheating and ensure proper breaker coordination.

Examples With Calculations

Example 1: Gas Furnace With 0.8 A Control Circuit

If the nameplate shows a control circuit draw of 0.8 A at 120V and the motor startup current does not change MCA, the typical MCA could be 1–3 A. A 15A breaker on 120V is common and acceptable per manufacturer guidance.

Example 2: Furnace With 7 A Blower Motor (120V)

For a blower drawing 7 A continuous, NEC requires conductor sizing to 7 A × 125% = 8.75 A. A 15 A breaker with #14 or #12 AWG conductors typically suffices; always check the furnace MOCP for maximum breaker size.

Example 3: 10.5 kW Electric Furnace (240V)

A 10.5 kW electric furnace at 240V draws 10,500 W / 240 V = 43.75 A. NEC requires 125% for continuous load: 43.75 × 1.25 = 54.69 A. The next standard breaker size is 60 A; therefore, a 60 A double-pole breaker with appropriately sized conductors (often #6 AWG copper) is used.

Wiring And Breaker Types

Furnace circuits commonly use single-pole breakers for 120V control circuits and double-pole breakers for 240V heating circuits. Use copper conductors unless the equipment listing allows aluminum; aluminum requires larger sizes and special terminations. Ensure the breaker type matches panel requirements and equipment listing (standard thermal-magnetic breakers or HACR-type where specified).

Common Mistakes And How To Avoid Them

• Assuming All Furnaces Use The Same Breaker: Breaker size varies by model, motor type, and heating method.
• Ignoring Nameplate Values: Manufacturer MCA and MOCP supersede generic rules.
• Undersizing Conductors: Leads to overheating and code violations; always size conductors to the MCA and continuous load rules.
• Using The Wrong Breaker Type Or Location: Use the correct single- or double-pole breaker and ensure disconnecting means are accessible as required by code.

When To Call A Licensed Electrician Or HVAC Technician

A licensed electrician or HVAC professional is necessary when changes to breaker size, panel upgrades, running new circuits, or when wire terminations must be inspected. Permits and inspections may be required for major electrical changes. Professionals ensure code compliance and safe operation.

Troubleshooting Breaker Trips And Electrical Problems

If the breaker trips frequently, potential causes include an overloaded circuit, short circuit, ground fault, or failing motor. Start by checking whether additional loads share the same circuit. If the breaker trips on startup, the motor may have high inrush current or mechanical binding. Persistent issues warrant professional diagnosis.

Upgrading An Older Furnace Or Electrical Panel

Older homes may have undersized panels, 60 A service, or no dedicated furnace circuit. Modern furnaces may require larger breakers and dedicated circuits. When upgrading, verify service capacity, panel bus rating, and clearances. Upgrades should follow local codes and often require permits.

Documentation And Code References

Always keep the furnace installation manual and nameplate information accessible. Reference the National Electrical Code (NEC) articles related to HVAC equipment, motors, and branch-circuit sizing. Local codes may adopt NEC with amendments; verify with the local authority having jurisdiction.

Practical Checklist For Determining Breaker Size

1. Locate Furnace Nameplate And Manual.
2. Note MCA, MOCP, Voltage, And Phase.
3. Calculate Conductor Size Using MCA And 125% For Continuous Loads.
4. Select Breaker Size That Does Not Exceed MOCP And Protects The Conductor.
5. Confirm Breaker Type (Single Or Double Pole) And Panel Compatibility.
6. Hire A Licensed Professional For Any Panel Work Or Circuit Changes.

Additional Tips And Best Practices

Always use a dedicated circuit for the furnace unless manufacturer instructions allow shared loads. Label the furnace circuit in the service panel. Replace breakers and wiring with components rated for the furnace MCA and MOCP. Test safety controls and ensure the furnace disconnect is functional and accessible.

Key Takeaway: The correct breaker for a furnace depends on the unit’s MCA and MOCP values, the voltage, and whether the load is continuous. Always use the furnace nameplate and installation manual as the authoritative source and consult a licensed electrician for installation or upgrades.

For further guidance, refer to the furnace manufacturer’s installation instructions and the latest NEC code editions adopted locally. Professional inspection and installation ensure safe, code-compliant operation.