What Size Wire for Furnace: Choosing the Right Furnace Wire Gauge

Selecting the correct wire size for a furnace is essential for safe, efficient operation and code compliance. This article explains how to determine the right furnace wire gauge, breaker sizing, and installation considerations for typical residential systems in the United States.

Typical Furnace Full-Load Amps (FLA)	Recommended Copper Wire Gauge (THHN)	Typical Circuit Breaker
10–15 A	14 AWG	15 A
16–20 A	12 AWG	20 A
21–30 A	10 AWG	30 A
31–40 A	8 AWG	40 A
41–55 A	6 AWG	60 A

How Furnace Wiring Works

Furnaces require power for the blower motor, control board, gas valve or ignition system, and sometimes electric heat strips. Most modern gas furnaces use a 120V control circuit and a separate 240V or 120V circuit for blower and heating elements, depending on the model. Electric furnaces draw much higher currents and often need larger conductors and dedicated 240V circuits.

Determining Furnace Amps And Voltage

To choose the right wire size, the furnace’s full-load amps (FLA) or maximum current draw must be known. The furnace nameplate or installation manual gives this information as amps or kilowatts (kW) and voltage. If only kW is listed, current can be calculated: for single-phase systems, Amps = (kW × 1000) ÷ Voltage. Always use the highest listed value for safety.

Common Wire Sizes For Furnaces

Residential furnaces typically fall into a few amperage ranges. Standard recommendations for copper conductors in residential installations are based on the National Electrical Code (NEC) ampacity tables and common practice.

• 14 AWG Copper: Typically used for small 120V control circuits or thermostats and for loads up to 15 A. Not used for primary furnace power circuits except small add-on components.
• 12 AWG Copper: Common for 120V blower motors and small gas furnace power circuits up to 20 A.
• 10 AWG Copper: Used for medium loads such as larger blower motors or electric heat stages up to 30 A.
• 8 AWG Copper: Applied to heavy-duty blower motors or smaller electric furnaces up to 40 A.
• 6 AWG Copper: Used for electric furnaces or heat strips drawing up to 55–60 A, depending on installation and breaker sizing.

Why Breaker Size And Wire Gauge Must Match

The circuit breaker protects the wiring from overheating. NEC requires that the conductor ampacity be at least 125% of the continuous load for continuous heating equipment. This means a continuous load of 40 A requires conductors rated for at least 50 A and a breaker sized accordingly. Matching wire and breaker prevents nuisance tripping while maintaining safety.

Factors That Affect Wire Size Selection

Several factors influence the correct wire size beyond simple ampacity. These include conductor material (copper vs. aluminum), ambient temperature, conduit fill and derating, run length (voltage drop), and whether the load is continuous.

Conductor Material

Copper has higher conductivity than aluminum and requires a smaller gauge for the same current. If aluminum or copper-clad aluminum is used, select a larger gauge and ensure proper connectors rated for aluminum conductors are installed. Aluminum typically requires an increase of about two AWG sizes compared to copper for the same ampacity.

Ambient Temperature And Derating

NEC ampacity tables assume a standard temperature. Higher ambient temperatures or multiple current-carrying conductors in the same conduit require ampacity derating. When derating is required, a larger conductor must be used to prevent overheating.

Voltage Drop And Run Length

Long cable runs cause voltage drop, which can reduce equipment efficiency or prevent operation. For motors, NEC recommends keeping voltage drop to less than 3% on branch circuits. If the run is long, upsize the conductor to maintain acceptable voltage at the furnace.

Continuous Loads

Heating loads often run for extended periods and are considered continuous. NEC requires continuous loads be sized at 125% of the continuous current. Factor this into wire and breaker selection to ensure compliance and reliability.

Wiring Methods And Breaker Sizing

Typical wiring approaches vary with furnace type. For a gas furnace with a 120V control and 120V motor, a 15 A or 20 A dedicated circuit is common. For electric furnaces or heat strips, 240V circuits with appropriate amperage and double-pole breakers are used.

• Small Gas Furnaces: 120V, 15 A or 20 A circuit, 14 AWG or 12 AWG respectively for control and motor circuits.
• Gas Furnaces With Larger Motors: 120V or 240V motor circuits may need 12 AWG or 10 AWG and corresponding breakers.
• Electric Furnaces and Heat Strips: 240V circuits sized according to the heater kW rating; often 30 A to 60 A breakers and 10 AWG to 6 AWG copper conductors.

Installation Tips And Safety

Proper installation protects equipment and occupants. Ensure the following practices are followed: have a licensed electrician perform furnace power connections, use copper conductors unless the equipment and local code allow aluminum, install a dedicated circuit and proper grounding, and follow manufacturer instructions and NEC requirements.

Grounding And Equipment Bonding

The furnace chassis must be grounded with the equipment grounding conductor sized per NEC Table 250.122. The grounding conductor size often corresponds to the overcurrent device protecting the circuit. Proper bonding ensures safe fault clearing and reduces shock risk.

Disconnect Switches And Accessibility

The NEC requires a disconnecting means for HVAC equipment near the unit. For outdoor units or air handlers, install a readily accessible switch or fused disconnect within sight of the equipment to allow safe servicing.

Troubleshooting Wire And Circuit Issues

Common signs of incorrect wire sizing include frequent breaker trips, warm wire or terminal connections, voltage fluctuations, intermittent blower operation, and blown fuses. If any of these occur, shut down the furnace and have an electrician inspect the circuit and wiring.

Measuring Current And Voltage

Use a clamp meter to measure motor running current and a multimeter to check supply voltage at the furnace. Compare these readings to the nameplate amps and voltage. Significant deviations indicate wiring, breaker, or supply issues that require correction.

Identifying Overheated Conductors

Overheated wires or connections often have discolored insulation or a burnt odor. Loose connections at the breaker or furnace terminals can create heat and should be tightened or replaced by a professional to prevent fire risk.

Code And Manufacturer Considerations

Local codes and the NEC govern wire sizing and installation. Manufacturers may specify minimum circuit sizes, wire gauges, and breaker ratings in installation manuals. Always follow the more restrictive requirement between local code and manufacturer instructions.

NEC Continuous Load Rule

Because heating is usually continuous, NEC requires conductors and overcurrent protection to be sized at 125% of the continuous load. This affects the choice of breaker and wire: a 40 A continuous load typically needs a 50 A-rated circuit and conductors rated for that current.

Examples And Calculations

Example 1: A gas furnace blower draws 8 A at 120V. For a non-continuous 8 A load, 14 AWG copper on a 15 A breaker is acceptable, but if considered continuous, use 12 AWG on a 20 A breaker to meet the 125% rule.

Example 2: An electric furnace rated 10 kW at 240V draws 41.7 A. Applying the continuous load factor, required circuit current = 41.7 A × 1.25 = 52.1 A. This would typically require a 60 A breaker and 6 AWG copper conductors.

Frequently Asked Questions

Is 14 Gauge Wire Ok For A Furnace?

14 AWG is acceptable only for small 120V control circuits or non-continuous loads up to 15 A. It is not suitable for primary furnace power when the blower or heater draws more current. Check the furnace nameplate and local code.

Can Aluminum Wire Be Used For Furnace Circuits?

Aluminum can be used if permitted by code and manufacturers, but a larger gauge and correct connectors are required. Many professionals prefer copper for durability and lower resistance.

How Important Is Voltage Drop?

Voltage drop affects motor starting and heating performance. For long runs, increasing conductor size to limit voltage drop to 3% or less is recommended to maintain reliable operation.

Who Should Size And Install Furnace Wiring?

A licensed electrician should size and install furnace wiring to ensure compliance with the NEC, local codes, and manufacturer requirements, and to provide safe, reliable operation.

Key Takeaways

• Identify The Furnace Full-Load Amps From The Nameplate Or Manual Before Choosing Wire Size.
• Apply The NEC 125% Rule For Continuous Heating Loads When Selecting Wire And Breaker Sizes.
• Consider Run Length, Ambient Temperature, And Conductor Material When Upsizing For Voltage Drop Or Derating.
• Use A Licensed Electrician For Final Sizing And Installation To Ensure Safety And Code Compliance.

For a specific furnace model, always refer to the manufacturer’s installation instructions and consult local electrical codes. Proper wire sizing extends equipment life, reduces fire risk, and ensures efficient heating performance.