How Many Amps Does a Furnace Blower Use

The furnace blower motor’s amperage affects electrical sizing, energy use, and troubleshooting. This article explains typical amperage ranges, how to calculate running and startup currents, differences between motor types, measuring tips, and safe breaker and wire sizing. It helps homeowners and technicians make informed decisions about replacement, diagnosis, and efficiency.

Blower Type	Typical Running Amps (120V)	Typical Running Amps (240V)	Notes
Small PSC Motor	2–6 A	1–3 A	Common in older furnaces
Medium PSC Motor	6–12 A	3–6 A	Typical residential
Large PSC Motor / 1 HP	9–15 A	4–8 A	Higher airflow models
ECM/Variable-Speed Motor	1–12 A	0.5–6 A	Lower average draw, variable by speed
Inducer/Blower Startup	2–3x Running	2–3x Running	Short-duration inrush current

Why Furnace Blower Amps Matter

The blower motor is the main electrical consumer in many gas furnaces and heat pumps. Knowing how many amps a furnace blower uses is vital for selecting proper breakers and wire sizes, assessing energy costs, and diagnosing electrical or mechanical issues. Undersized circuits cause tripped breakers; oversized circuits risk poor protection.

Common Furnace Blower Motor Types And Their Amperage

Blower motors come in two main categories: permanent split capacitor (PSC) single-speed or multi-speed motors and electronically commutated motors (ECMs) or variable-speed motors. Each has distinct amp profiles.

PSC (Permanent Split Capacitor) Motors

PSC motors are induction motors with fixed windings and speeds. Running amps are highest at full speed and lower at reduced speeds if multiple taps are used. Typical PSC running current ranges from about 2 to 15 amps at 120 volts depending on horsepower and blower design.

ECM (Electronically Commutated Motor) Motors

ECMs use brushless DC technology with electronic controls. They often draw lower average amperage because they run at optimized speeds and deliver consistent airflow with less wasted energy. Running amps typically range from under 1 amp at low speeds to around 12 amps at peak on 120V models.

Typical Amperage Ranges By Horsepower And Voltage

Amperage depends on motor horsepower (HP) and supply voltage. Below are approximate running-current ranges for common residential blower motor sizes. Actual values vary by efficiency and model.

Horsepower	120V Running Amps	240V Running Amps
1/4 HP	2–5 A	1–2.5 A
1/3 HP	4–7 A	2–3.5 A
1/2 HP	6–12 A	3–6 A
3/4 HP–1 HP	9–15 A	4–8 A

Running Current Vs. Inrush (Startup) Current

Blower motors draw a short-term inrush current at startup often 2–3 times the running amps for PSC motors. ECMs typically have controlled soft-starts and lower inrush. Circuit protection must handle both startup and running currents without nuisance tripping.

How To Measure Blower Amps Safely

Accurate measurement requires a clamp meter or multimeter capable of AC current measurement. A clamp meter on the blower’s hot conductor while the equipment runs gives the running amps. For startup inrush, use a meter with a fast capture or inrush mode.

• Turn off power and access the blower’s wiring compartment.
• Clamp around a single hot conductor, not the whole cable with both conductors.
• Record running current at several speeds and under different load conditions (filter clean vs dirty).
• For inrush, use meter in inrush mode or a meter with high sampling speed.

Calculating Amps From Horsepower And Efficiency

When nameplate amps are unknown, the approximate formula is useful: Amps = (Horsepower × 746) / (Voltage × Efficiency × Power Factor). This gives a theoretical electrical current based on mechanical output and motor efficiency.

Example: A 1/2 HP motor (373 W) at 120V and 80% efficiency: Amps ≈ 373 / (120 × 0.8) ≈ 3.9 A. Actual PSC motors often read higher due to lower efficiency and reactive power.

Breaker And Wire Sizing Recommendations

Electrical code requires branch-circuit protection sized to the motor and its locked-rotor and running currents. For fixed motors, NEC rules and manufacturer instructions govern breaker sizing. General residential practice:

• Use a breaker sized to handle 125% of motor full-load amps for continuous loads.
• For PSC blowers on 120V drawing 8 A, select a 15 A breaker; for 12 A draw use a 20 A breaker.
• Use appropriate wire gauge: 14 AWG for up to 15 A, 12 AWG for up to 20 A, 10 AWG for up to 30 A.

Important: Always follow the motor nameplate and the HVAC manufacturer’s electrical data. Some motors require a dedicated circuit or have specific overcurrent protection requirements.

Energy Use And Cost Implications

Amps relate to watts and kilowatt-hours (kWh): Watts = Volts × Amps. A blower drawing 6 amps on 120V uses 720 watts. Running 24/7 would consume 17.3 kWh per day. Intermittent duty during heating season reduces actual energy use.

ECM motors typically reduce annual electricity consumption by 30–70% compared to PSC motors, depending on runtime and speed usage. Upgrading to an ECM can lower utility bills and improve comfort.

Diagnosing High Amperage Draw

High running amperage indicates issues such as bearing wear, a dirty blower wheel, collapsed ductwork, incorrect pulley or belt adjustments, or failing motor windings. Persistent high amps risk overheating and shortened motor life.

1. Check air filters and duct restrictions.
2. Inspect blower wheel for dust and damage.
3. Lubricate bearings if serviceable.
4. Measure voltage at the motor to ensure proper supply.
5. Compare measured amps to nameplate and manufacturer specifications.

Why Measured Amps May Differ From Nameplate

Nameplate amperage lists the motor’s rated full-load current under specific conditions. Real-world differences arise from supply voltage variations, mechanical load differences, duct static pressure, and motor wear. Measure under representative operating conditions for diagnosis.

Replacement And Upgrade Considerations

When selecting a replacement motor, confirm voltage, RPM, horsepower, frame and shaft size, wiring harness compatibility, and airflow (CFM) requirements. An ECM replacement may require compatibility with the furnace control board or additional wiring.

Replacing a PSC with an ECM generally yields lower amps and better control, but ensure the control system can command variable speeds and handle the motor’s electronic interface.

Practical Examples And Calculations

Example 1: A 1/3 HP PSC motor on 120V lists 6 A running on the nameplate. Running watts = 120 × 6 = 720 W. On a 15 A breaker, this motor should run without tripping, but startup current might approach 12–18 A briefly.

Example 2: An ECM lists 4 A running on 120V and has soft-start control. Running watts = 120 × 4 = 480 W, with lower inrush and smoother operation, often allowing smaller breakers while meeting code and manufacturer guidance.

Safety Tips And Best Practices

Always de-energize equipment before servicing and follow lockout/tagout procedures. Use appropriate PPE when accessing the blower compartment. If unsure about electrical work, hire a licensed electrician or HVAC technician.

• Read and follow the motor nameplate and furnace installation manual.
• Use a properly rated clamp meter and test leads.
• Check breaker and wire conditions for signs of overheating.
• Document measurements and compare across seasons for trends.

Frequently Asked Questions

Can A Blower Motor Trip A Breaker Repeatedly?

Yes. Repeated breaker trips often indicate an overloaded circuit, failing motor, or short. Inspect motor amps, wiring, and associated components to determine the cause.

Do Variable-Speed Motors Use Less Electricity?

Yes. Variable-speed ECMs reduce average amperage by matching motor speed to load, improving efficiency and comfort while lowering operating costs.

Is It Normal For Amps To Fluctuate With Furnace Demand?

Yes. Amperage varies with blower speed, heat exchanger load, and airflow restrictions. Higher fan speeds and more duct static pressure raise amperage.

Reference And Further Reading

Useful resources include motor nameplates, furnace installation manuals, NEC guidelines for motor branch-circuit sizing, and manufacturer technical bulletins. For precise electrical guidance, consult a licensed professional.