What Size Breaker for Furnace

The correct breaker size for a furnace depends on the furnace type, electrical load, and local code. This article explains how to determine the right amp breaker, common residential breaker sizes, wire gauge requirements, NEC guidelines, and practical steps to verify or upgrade a furnace circuit safely.

Furnace Type	Typical Breaker Size	Typical Wire Gauge
Gas Furnace (Control & Blower Only)	15A–20A	14–12 AWG
Electric Furnace (Whole-Home)	30A–60A Or Larger	10–6 AWG
HVAC Air Handler / Blower Motor	15A–30A	14–10 AWG
Two-Stage/Variable-Speed Systems	20A–40A	12–8 AWG

How Breaker Size Relates To Furnace Safety And Performance

A circuit breaker protects wiring from overheating due to overloads or short circuits. Selecting the right breaker for a furnace ensures the wiring capacity matches the furnace’s electrical demand. Too small a breaker causes nuisance trips; too large a breaker risks overheating the wire before the breaker trips.

Identify The Furnace Type And Electrical Components

First identify whether the furnace is a gas furnace with electric controls/blower or a fully electric furnace. A gas furnace typically has a small electrical load for the control board, inducer, and blower, while an electric furnace uses heating elements and draws much larger current.

Find The Manufacturer Nameplate And Electrical Ratings

The furnace nameplate or manual lists rated voltage, full-load amps (FLA), and minimum circuit ampacity (MCA) or the recommended branch-circuit overcurrent protection. Always follow the manufacturer’s stated MCA and recommended breaker size.

National Electrical Code (NEC) Basics For Furnace Circuits

The NEC requires the branch-circuit overcurrent protection to be sized to protect the conductors, not to match the equipment rating, but equipment rating and MCA determine conductor size. The NEC also requires continuous loads to be calculated at 125% of the load when sizing conductors and breakers.

Common Residential Breaker Sizes And Typical Applications

Understanding common breaker sizes helps identify typical installations.

• 15 Amp Breaker: Common for gas furnace control circuits and small blowers using 14 AWG wiring.
• 20 Amp Breaker: Used for larger blowers, two-speed motors, and control circuits with higher inrush currents; paired with 12 AWG wire.
• 30 Amp Breaker: Often used for smaller electric furnaces or heat strips; paired with 10 AWG wire.
• 40–60 Amp Breaker: Typical for larger electric furnaces or multi-element units; paired with 8–6 AWG wire.

Wire Gauge And Breaker Pairing

Matching wire gauge to breaker size is critical. Typical pairings: 14 AWG with 15A, 12 AWG with 20A, 10 AWG with 30A, 8 AWG with 40A, and 6 AWG with 60A. Using undersized wire on a larger breaker is a fire hazard.

Electric Furnace Load Calculation Example

For electric furnaces, calculate current using the furnace nameplate wattage: Amps = Watts ÷ Voltage. For example, a 9,600W furnace at 240V draws 40A. The branch-circuit conductor and breaker must account for continuous load: conductor sized for 125% × 40A = 50A, so choose a 60A breaker with 6 AWG copper conductors.

Gas Furnace With Blower — Typical Sizing Approach

Most residential gas furnaces list an MCA around 3–10A. A 15A or 20A breaker is common. If the furnace includes a high-efficiency ECM blower or optional electric heat strips, follow the nameplate values. Do not assume 15A is always correct; verify the label.

Multi-Stage, Variable-Speed Motors And Inrush Current

Variable-speed motors and ECMs can have higher inrush currents but lower steady-state draw. Use the manufacturer’s MCA and recommended breaker. Some motors may run on a 20A circuit despite a small steady draw due to startup characteristics.

When To Use Dedicated Circuits

Most furnaces require a dedicated circuit to avoid shared loads causing overloads. A dedicated circuit simplifies troubleshooting and reduces nuisance tripping.

Steps To Verify The Correct Breaker For A Furnace

1. Locate the furnace nameplate and record voltage, FLA, and MCA.
2. Calculate required conductor ampacity: For continuous loads, multiply by 1.25.
3. Choose a breaker that protects the conductor and aligns with the manufacturer’s recommended overcurrent protection.
4. Confirm wire gauge matches breaker ampacity per NEC tables.
5. If uncertain, consult a licensed electrician or the HVAC installer before making changes.

Measuring Furnace Current On-Site

A clamp meter measures operating current on the hot conductor. Measure steady-state current under typical operating conditions. Compare measured amps to breaker rating and nameplate MCA. If measured amperage exceeds safe conductor capacity, stop use and call a professional.

Common Mistakes And How To Avoid Them

Common mistakes include upsizing breakers without upgrading conductors, assuming identical systems use the same breaker, and ignoring nameplate MCA. Always prioritize label data and NEC conductor protection rules.

Signs The Breaker Or Circuit Is Incorrect

Signs of incorrect breaker sizing or circuit issues include frequent tripping, burning or melting at connections, discolored insulation, buzzing from the panel, or furnace not operating. These require immediate inspection by a licensed electrician.

Permits, Codes, And When To Hire A Pro

Electrical work often requires permits and inspection. Local codes may modify NEC requirements. For breaker or conductor changes, or when load calculations are unclear, hiring a licensed electrician ensures compliance and safety.

Upgrading An Older Furnace Circuit

Older homes may have circuits sized for past equipment. When installing a new furnace or adding electric heat strips, evaluate whether the existing breaker and wiring meet the new MCA and conductor requirements. Upgrade circuits and panels when necessary for safety and performance.

Special Cases: Dual-Fuel Systems And Heat Pumps

Dual-fuel systems and heat pumps may include outdoor compressors and indoor blowers with separate circuits. Each component must have properly sized overcurrent protection and dedicated wiring per nameplate ratings.

Quick Reference Checklist Before Changing Breaker Size

• Read the furnace nameplate for MCA and recommended breaker size.
• Perform load calculation and apply 125% for continuous loads.
• Verify wire gauge is appropriate for desired breaker.
• Confirm local code or AHJ requirements and get permits if needed.
• Hire a licensed electrician for breaker or conductor changes.

Frequently Asked Questions

Can A Furnace Circuit Be Shared With Other Devices?

Shared circuits are not recommended. A furnace requires reliable power; shared loads can cause overloads or nuisance trips. Use a dedicated circuit unless the manufacturer allows sharing.

Why Does The Breaker Trip When The Furnace Starts?

Startup inrush current or a weak/oversized breaker may cause trips. Verify breaker size, inspect for shorted motor windings, and compare startup current to breaker rating. A properly sized breaker should tolerate brief inrush if within limits.

Is It Okay To Replace A Tripping 15A Breaker With 20A?

Not without confirming conductor ampacity and furnace load. Replacing a 15A breaker with a 20A unit while the wiring is 14 AWG violates code and creates fire risk. Always ensure wire size matches the new breaker.

Who Is Responsible For Correct Breaker Size—HVAC Contractor Or Electrician?

HVAC contractors specify equipment requirements; electricians implement electrical work. Coordination is essential. The final breaker and wiring must meet both the equipment manufacturer’s specs and electrical code, so involve both trades when in doubt.

Resources And References

Useful resources include the furnace manufacturer’s installation manual, the National Electrical Code (NFPA 70), and local building department guidance. For accurate calculations and dangerous work, consult a licensed electrician and HVAC technician.

Key Takeaway: Match the breaker and conductor to the furnace’s nameplate MCA and the NEC rules. When uncertain, always verify with professionals to ensure safe, code-compliant installations.