How Many Amps Does a Furnace Use? Typical Loads, Breakers, and Cost

Wondering how many amps a furnace uses and which breaker it needs? The answer depends on the fuel type, blower motor, and whether electric heat strips are installed. This guide explains real-world amp draws for gas, electric, and oil furnaces, how to size circuits, what nameplate terms like MCA and MOCP mean, and how to estimate operating cost from amps. It is written for U.S. homes and uses plain language with practical examples.

Quick Answer: Typical Furnace Amps At A Glance

Most gas furnaces draw 7–15 amps at 120V during normal operation, mainly for the blower and controls. Oil furnaces are similar, often around 8–14 amps. Electric furnaces draw far more current: 5–25 kW heat strips at 240V translate to roughly 21–104 amps and usually use one or more large breakers.

• Gas furnace (120V): 7–15 A running; usually on a dedicated 15A or 20A circuit.
• Oil furnace (120V): 8–14 A running; typically 15A or 20A circuit.
• Electric furnace (240V): 5 kW ≈ 21 A; 10 kW ≈ 42 A; 15 kW ≈ 63 A; 20 kW ≈ 83 A; 25 kW ≈ 104 A; multiple breakers common.
• Heat pump air handler with strips: Similar to electric furnace when auxiliary heat is on; blower alone ~3–10 A.

Always check the furnace nameplate for MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection) and size wiring and breakers accordingly.

How Furnace Amperage Works

“Amps” measure electrical current. A furnace’s amp draw depends on its components, voltage, and whether electric heating elements are energized. Gas and oil furnaces mainly power a blower motor, inducer fan, and controls. Electric furnaces additionally power large heating elements.

Most gas and oil furnaces use 120V circuits. Their largest load is the blower motor. Modern ECM blowers are more efficient than older PSC motors, so they often draw fewer amps for the same airflow.

Electric furnaces use 240V circuits and can require multiple breakers and feeders. Heat strips are resistive loads, and their current is calculated directly from power and voltage: amps = watts ÷ volts.

Motors have brief inrush current when starting, often 2–6 times the running amps. Breakers tolerate this short surge by design, so it does not require oversizing beyond code limits.

Typical Amp Draw By Furnace Type

These are typical ranges. The nameplate governs breaker/wire sizing for a specific model.

Gas And Oil Furnaces: What Drives Amp Draw

Gas and oil furnaces rely on a 120V branch circuit. Their current draw is determined by the blower motor horsepower, the type of motor, and small auxiliary loads like the inducer fan and igniter.

Blower Motor Types And Amps

Older PSC (permanent split capacitor) motors draw more current than modern ECM (electronically commutated) motors for the same airflow. ECM motors maintain airflow more efficiently under varying duct pressures.

Inducer fan motors often add 0.3–1.5 A. A hot-surface igniter may draw 1.7–3.3 A briefly. Control boards, gas valves, and sensors add a small amount.

Oil furnaces include a burner motor and ignition transformer, typically adding several amps, which is why oil models can be slightly higher than gas for the same blower size.

Why The Same Furnace Draws Different Amps

• Fan speed setting: Higher cooling speeds for AC require more blower current than heating speeds.
• Duct static pressure: Dirty filters, closed vents, or restrictive ducts force the blower to work harder.
• Staging and modulation: Low-fire stages often run lower blower speeds and amps.
• Altitude and air density: Can change airflow and motor load slightly.

Practical note: Many gas furnaces list MCA around 8–12 A and MOCP of 15 or 20 A. A dedicated 15A circuit is common; 20A is used where the nameplate requires it.

Electric Furnaces And Heat Strips: Calculating Amps And Breakers

Electric furnaces use resistive heating elements rated in kilowatts (kW). Current is straightforward to compute: Amps = Watts ÷ Volts. At 240V, every 5 kW adds about 21 A.

Fixed electric space heating is usually treated as a continuous load in the National Electrical Code (NEC). Branch-circuit conductors are typically sized at 125% of the heating element current, and overcurrent protection must not exceed the equipment’s MOCP. Most electric furnaces split heat strips across two or more breakers and feeders.

Some air handlers combine a 120V or 240V blower and controls with one or more 240V heat strip circuits. The nameplate will show separate MCA/MOCP values for each required circuit.

Heat Pump And Dual-Fuel Systems: Air Handler Amps

Heat pumps move heat using an outdoor unit on its own 240V circuit. The air handler indoors looks like an electric furnace but may only run the blower until auxiliary heat is needed.

• Blower-only mode: 3–10 A typical, depending on motor type and speed.
• Auxiliary/backup heat: Adds 5–25 kW of strips, increasing amps substantially, as shown in the table above.
• Defrost cycles: The system may energize strips briefly to temper supply air; current spikes accordingly.

In dual-fuel systems, a gas furnace provides backup heat; the air handler components are part of the furnace. The furnace circuit then resembles a standard gas furnace load.

Finding The Right Breaker And Wire: Nameplate, MCA, And MOCP

The equipment data plate is the gold standard for sizing. It will list MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection) or a required breaker size. These values already account for motor rules and continuous loads.

• Size wire to at least the MCA, using appropriate conductor ampacity tables and temperature ratings.
• Do not exceed the MOCP for the breaker or fuse size.
• If MCA/MOCP are not provided, use applicable NEC articles (e.g., electric space heating and motor rules) or consult a licensed electrician.

Example (Gas Furnace): Nameplate MCA 9.8 A, MOCP 15 A. Use a 15A breaker and conductors meeting or exceeding the MCA (commonly 14 AWG copper on a 15A circuit, subject to local code and installation conditions).

Example (Electric Furnace 10 kW): 10,000 W ÷ 240 V ≈ 41.7 A. Conductor sizing typically at 125% ≈ 52 A, so a 60A breaker is common if the nameplate MOCP allows it. Many units split into two circuits (e.g., 7.5 kW and 2.5 kW).

Wire gauge reference (typical copper THHN/THWN-2): 30A ≈ 10 AWG; 60A ≈ 6 AWG; 90–100A ≈ 3–4 AWG. Always verify with the nameplate and NEC 310.16 tables for your installation.

Important: Local amendments and inspection practices vary. Follow the nameplate and local code, and when in doubt, hire a licensed electrician or HVAC contractor.

Measuring Furnace Amps Safely

A clamp meter can measure actual amp draw. If unfamiliar with electrical work, it is safest to hire a professional. If measuring:

• Use a true-RMS clamp meter rated for the expected current.
• Clamp around one conductor only (hot), not the entire cable.
• Measure blower motor amps on the motor lead during high speed for a worst-case reading.
• For electric heat, measure each heat strip leg; use caution with live 240V circuits.
• Compare readings to motor FLA and the equipment nameplate. Small variances are normal; large deviations suggest a problem.

Safety note: Turn off power before opening panels. Restoring power for live measurement should only be done by qualified persons using proper PPE.

Factors That Make Amps Go Up Or Down

Several conditions change how many amps a furnace uses. Understanding them helps explain energy bills and diagnose issues.

• Static pressure: Dirty filters, blocked returns, closed registers, or undersized ducts raise static pressure and blower current.
• Fan speed settings: Higher cooling speeds draw more amps than heating settings. Installer adjustments affect draw.
• ECM vs. PSC motors: ECMs can maintain airflow with lower amps, especially under higher static.
• Staging/modulation: Lower stages reduce airflow and amps; higher stages increase both.
• Supply voltage: Low voltage can raise motor current and heat, risking nuisance trips and motor stress.
• Mechanical issues: Bearing wear, wheel debris, or misalignment increase motor load.
• Heat strips enabled: In electric or heat pump systems, auxiliary heat dramatically increases amps.

Tip: Maintaining clean filters, open returns, and properly balanced airflow helps keep amp draw within design range.

Energy Use And Operating Cost From Amps

Electric cost depends on power (watts) and time. Convert amps to watts, then to kilowatt-hours (kWh):

• Watts = Volts × Amps
• kWh = (Watts ÷ 1,000) × Hours

Gas Furnace Example: 8 A × 120 V = 960 W (0.96 kW). If it runs 4 hours per day, that is 0.96 × 4 = 3.84 kWh/day. At $0.16/kWh, the blower and controls cost ≈ $0.61/day or ≈ $18/month. Gas fuel is billed separately and typically dominates heating cost.

Electric Furnace Example: 10 kW strip heat = 10 kW. If it runs 3 hours per day, 10 × 3 = 30 kWh/day. At $0.16/kWh, that is ≈ $4.80/day or ≈ $144/month just for electric heat runtime at that duty cycle.

Heat Pump With Aux Heat: The heat pump itself is efficient; the blower might draw 5 A at 240 V (≈ 1.2 kW) plus the outdoor unit. When auxiliary 10 kW strips engage, total power rises sharply to ~11–12 kW, increasing cost during very cold snaps.

Actual costs vary with thermostat settings, weather, home insulation, and utility rates. Smart thermostats and staged systems can reduce reliance on auxiliary heat.

Troubleshooting High Or Low Amp Draw

Abnormal amperage can signal issues. Professional evaluation is recommended when readings differ significantly from nameplate or historical values.

• High blower amps: Check filter, coil cleanliness, duct restrictions, blower wheel debris, bearing wear, or incorrect speed tap selection.
• Low blower amps with poor airflow: Possible ECM derate from high static, failed speed control, or low voltage.
• Nuisance breaker trips (gas/oil): Oversized blower, high static, or weak breaker. Verify circuit is dedicated and matches MOCP.
• Electric furnace tripping: Loose connections, partial element failures, improperly sized breakers/wires, or exceeding MOCP. Inspect contactors and sequencers.
• Heat pump auxiliary heat always on: Thermostat configuration, outdoor sensor issues, or failed heat pump forcing strip heat.

Do not upsize breakers to stop tripping unless the equipment nameplate allows it and conductors are appropriately sized.

Common Breaker And Circuit Questions

These short answers address frequent wiring and protection questions about furnace amperage.

• Does a furnace need a dedicated circuit? Yes, most gas, oil, and electric furnaces require a dedicated branch circuit per manufacturer instructions and code.
• Is 15A enough for a gas furnace? Often yes. Many gas furnaces list MOCP 15A. Some require 20A; check the nameplate.
• Can a furnace share a circuit with lights or outlets? Generally no. A dedicated circuit is standard practice and often required by the manufacturer.
• GFCI/AFCI requirements? Requirements vary by NEC edition and local adoption. Some locations require GFCI for receptacles in basements/garages, and AFCI in many habitable spaces. Many furnaces are hardwired. Check local code and inspector guidance.
• Why do some electric furnaces have two breakers? Large heat strips are split across multiple circuits for ampacity and sequencing. Follow the wiring diagram and nameplate MCA/MOCP for each circuit.

How To Read The Nameplate For Amps

The equipment nameplate or installation manual lists several values that govern current and protection.

• Voltage/phase: Most residences use 120/240V single-phase.
• MCA (Minimum Circuit Ampacity): The minimum ampacity of the supply conductors. Does not include 80% derating; already calculated.
• MOCP (Maximum Overcurrent Protection): The largest permissible breaker or fuse size.
• Motor FLA: Full load amps of blower or inducer motors for diagnostics.
• kW of heat strips: For electric furnaces and heat pump auxiliary heat; use to compute amps and energy.

Follow these values exactly. They supersede generic rules because they incorporate UL/CSA listing and internal wiring considerations.

Real-World Scenarios And Examples

These examples illustrate how many amps a furnace uses in common U.S. situations and how circuits are typically configured.

• 90,000 BTU gas furnace with PSC blower: Nameplate MCA 10.5 A, MOCP 15 A. On high-speed cooling, measured blower current is 9.2 A; heating speed is 7.8 A. Circuit: 15A breaker, 14 AWG copper.
• 80,000 BTU gas furnace with ECM: MCA 7.6 A, MOCP 15 A. Blower averages 3–5 A depending on speed. Lower amps, quieter operation, better efficiency.
• 10 kW electric air handler: Nameplate shows two circuits: 60A (strips + blower) and 15A (blower/control only) or a combined 60A depending on model. Running current ~42–46 A when strips are active; 3–6 A when only blower runs.
• Heat pump with 15 kW aux heat: Air handler MCA 70 A, MOCP 80 A; outdoor condenser on its own 30–40A breaker. In mild weather, the air handler draws ~5 A; in freezing temps, it can draw ~65 A when strips energize.

Key takeaway: A furnace’s “amps” is not one number—it changes with mode, stage, and components in use.

Upgrades That Can Reduce Amp Draw Or Improve Performance

Upgrades often lower current and energy use while improving comfort.

• ECM blower retrofit: Replaces PSC with an efficient variable-speed motor; typically lowers amps at equal airflow.
• Duct improvements: Enlarging returns, sealing leaks, and balancing airflow reduce static pressure and motor load.
• Proper filter selection: Use filters with appropriate MERV; avoid overly restrictive options that spike static and amps.
• Smart thermostat strategies: Staging and outdoor lockouts reduce unnecessary electric strip usage in heat pumps.

Note: Always ensure compatibility and follow manufacturer guidelines when changing motor types or controls.

When To Call A Professional

DIY amperage checks and calculations are helpful, but certain cases call for an HVAC or electrical pro.

• Breaker trips or overheating conductors despite correct breaker size.
• Measurements exceed nameplate limits or vary widely from normal operation.
• Electric furnaces and multi-circuit air handlers, which have complex wiring and sequencing.
• Code compliance questions about GFCI/AFCI, conductor sizing, or panel capacity.

Professional commissioning includes measuring motor amps, static pressure, temperature rise, and verifying MCA/MOCP compliance—valuable for safety and efficiency.

SEO-Friendly Summary And Key Phrases

If searching “how many amps does a furnace use,” the short answer is: gas furnaces use about 7–15 amps at 120V, while electric furnaces with heat strips draw 21–104 amps at 240V depending on kW. Check MCA and MOCP on the nameplate, use the correct breaker and wire, and remember that ECM blowers usually draw fewer amps than PSC motors.

• Related terms: furnace amperage, furnace breaker size, gas furnace amps, electric furnace amps, blower motor amps, MCA, MOCP, heat strip amps.
• Best practice: Follow nameplate ratings and local code; avoid upsizing breakers without verifying conductor size and manufacturer limits.