Understanding how many amps a furnace uses helps homeowners choose the right breaker, size wiring, estimate energy costs, and troubleshoot electrical issues. This guide explains typical amp draws for gas and electric furnaces, distinguishes startup versus running currents, and provides measurement, sizing, and safety advice for U.S. installations.
| Furnace Type | Typical Running Amps | Typical Breaker Size | Notes |
|---|---|---|---|
| Gas Furnace (Blower Only, ECM) | 1–6 A (120V) | 15–20 A | Modern variable-speed ECM motors draw less current |
| Gas Furnace (Blower PSC Motor) | 3–10 A (120V) | 15–20 A | Older motors draw more and have higher inrush |
| Inducer Motor / Ignition / Controls | 1–5 A (120V) | Shared with blower circuit | Small continuous loads |
| Electric Furnace (Residential) | 30–100+ A (240V) | 30–200 A | Depends on kilowatt rating (e.g., 10 kW ≈ 41 A at 240V) |
| Heat Pump Air Handler | 1–15 A (120/240V) | 15–30 A | Includes compressor circuit separate from air handler |
Why Furnace Amps Matter
Knowing a furnace’s amp draw is crucial for breaker sizing, wire gauge selection, energy cost estimates, and preventing overloads. Incorrect sizing can trip breakers, overheat wiring, or cause equipment failure. Electric furnaces have the highest electrical demands, while gas furnaces primarily use electricity for blowers, controls, and ignition.
Typical Amp Draws By Furnace Type
Furnaces fall into broad categories: gas (natural gas or propane) that use electricity only for controls and fans, and all-electric furnaces that produce heat entirely by electric heating elements. Typical running current varies widely across these types.
Gas Furnaces
Gas furnaces generally need a small 120V branch circuit for furnace controls, the blower motor, inducer motor, and ignition system. Most gas furnaces run on a 15–20 amp circuit. Modern ECM (electronically commutated motor) blowers often draw 1–6 amps while older PSC (permanent split capacitor) motors can draw 3–10 amps or more when running.
Electric Furnaces
Electric furnaces can draw substantial current because heating elements convert electricity directly to heat. Typical residential electric furnaces range from 7.5 kW to 30 kW or higher. At 240 volts, a 10 kW furnace uses ~41.7 amps, a 15 kW furnace ~62.5 amps, and a 20 kW furnace ~83.3 amps. Large systems may require 100–200 amp service and dedicated circuits sized per NEC requirements.
Breakdown Of Common Furnace Components And Their Amp Use
Understanding individual component loads clarifies the total demand. The table below lists typical running amps for common parts of a gas furnace on a 120V circuit.
| Component | Typical Running Amps (120V) | Notes |
|---|---|---|
| Blower Motor (ECM) | 1–6 A | Efficient, variable speed |
| Blower Motor (PSC) | 3–10 A | Higher inrush and running amps |
| Inducer Motor | 1–4 A | Runs briefly during ignition and shutdown |
| Hot Surface Igniter or Spark Module | 0.5–3 A | Short-duration load |
| Control Board & Thermostat | 0.1–1 A | Minimal continuous draw |
| Humidifier/Accessory | 0.5–5 A | Depends on accessory type |
Startup (Inrush) Versus Running Current
Motors have higher inrush currents at startup—often several times the running amps. A PSC blower might surge 6–10 times its running current briefly when starting, while ECM motors have lower and softer starting currents. Breakers are designed to tolerate short inrush if the sustained current stays below rating. Distinguish between peak inrush and continuous running amps when sizing breakers.
How To Measure Furnace Amps Safely
Accurate measurement requires proper tools and safety precautions. A clamp meter is the preferred tool for measuring current without disconnecting wiring.
- Turn off power and access the furnace electrical compartment.
- Identify the correct conductor (hot lead) to clamp. Do not clamp around both hot and neutral together.
- Restore power and observe running amps under normal operation and during startup.
- Record peak and steady-state readings. Consider repeating for blower speeds and heat stages.
If the user is not experienced with electrical systems, a licensed electrician or HVAC technician should perform measurements. Never clamp across a multiconductor bundle or measure current on a live wire without training.
Breaker And Wire Sizing Basics
Breaker and conductor size depend on the continuous load rating and NEC rules. For continuous loads (three hours or more), NEC requires sizing at 125% of the continuous current. For example, a 41.7 A load (10 kW at 240V) multiplied by 125% gives 52.1 A, so a 60 A breaker is typical.
Common wire size guidelines for copper conductors at typical ampacities: 14 AWG up to 15 A, 12 AWG up to 20 A, 10 AWG up to 30 A, 8 AWG up to 40–50 A depending on insulation, 6 AWG up to 55–65 A, 4 AWG up to 70–85 A. Always follow NEC tables and local code, and use an electrician for final sizing.
Practical Examples And Calculations
Example 1: A 15 kW electric furnace at 240V draws 62.5 A. Multiply by 125% for continuous load: 62.5 × 1.25 = 78.1 A. A 90 A or 100 A breaker with appropriately sized conductor is used depending on local code.
Example 2: A gas furnace with a PSC blower that runs at 8 A and an inducer at 2 A results in ~10 A continuous. A 15 or 20 A breaker is typical, but check startup surge if the blower motor is aged or oversized.
Energy Use And Cost Estimates From Amps
To estimate electrical power, multiply amps by volts to get watts. For a 10 kW electric furnace, the current at 240V is 41.7 A and power consumption is 10,000 W. Running for one hour equals 10 kWh. Multiply kWh by the local electricity rate to estimate cost. For gas furnaces, electrical draw is modest and costs are typically dominated by the fuel bill rather than electrical consumption.
Troubleshooting Electrical Symptoms Related To Amps
Common electrical issues and what amp-related measurements reveal:
- Frequent breaker trips: Check whether running amps exceed breaker rating or if a high inrush is mis-sized.
- Blower not starting or slow speeds: Measure starting current; low inrush may indicate failing motor or capacitor.
- Burning smell or overheating: Inspect wiring and measure current; overloads can overheat conductors.
- Intermittent operation: Check control board, relays, and current draw during cycles.
Code, Safety, And Best Practices
National Electrical Code (NEC) and local amendments guide circuit protection and wire sizing. Key best practices: use dedicated circuits for furnaces, size breakers for continuous loads at 125%, follow manufacturer’s data plate for electrical ratings, ensure proper disconnects are installed, and obtain permits for major changes. Always hire a licensed electrician or HVAC technician for panel upgrades, wiring, or when the required breaker exceeds typical household branch circuits.
When To Call A Professional
Schedule professional assistance when the furnace requires a new dedicated circuit, the home needs a service panel upgrade for electric furnace installation, when inrush currents trip breakers frequently, or when measurements reveal surprisingly high currents. Licensed HVAC electricians will check compatibility, calculate loads, and install correct breakers and wiring to code.
Key Takeaways And Quick Reference
Gas furnaces: Typically use a 15–20 A 120V circuit for blower, controls, and inducer. Electric furnaces: Range widely from ~30 A to 200 A at 240V depending on kW rating. Start-up inrush can be several times running current for PSC motors. Always use correct breaker sizing rules (125% for continuous loads) and consult professionals for measurement, wiring, and code compliance.
For personalized calculations or installations, the safest approach is to consult the furnace nameplate for rated amperage and contact a licensed electrician or HVAC technician to verify circuit and panel capacity.