Electric Furnace Breaker Size Guide: How to Choose the Right Breaker

The correct electric furnace breaker size is essential for safe, efficient operation and to meet electrical code requirements. This guide explains how to determine the proper breaker size, interprets manufacturer data, and covers common furnace amperages, wire sizing, and code considerations for U.S. installations.

Furnace Type	Typical Range (Amps)	Recommended Breaker	Typical Wire Gauge
Small Electric Furnace (10–20 kW)	42–83 A	50–100 A	6–3 AWG Copper
Medium Electric Furnace (20–30 kW)	83–125 A	100–150 A	3–2/0 AWG Copper
Large Electric Furnace (30–50 kW)	125–208 A	150–250 A	2/0–4/0 AWG Copper

How Electric Furnace Breaker Size Is Determined

Breaker sizing for an electric furnace depends on the furnace’s full-load current (FLC) or rated amperage, which the manufacturer lists on the nameplate or in the installation manual. The National Electrical Code (NEC) requires breakers to protect conductors and equipment without nuisance trips while allowing for safe continuous loads.

For electric furnaces, the NEC generally treats the heating elements as continuous loads. This means the circuit must be sized for at least 125% of the continuous load current and the overcurrent protection device must be rated no less than that adjusted conductor ampacity.

Key Terms To Know

• Full-Load Current (FLC): The current drawn at rated load; used for motor loads but also noted for heaters.
• Continuous Load: A load expected to run for three hours or more; heating elements typically qualify.
• NEC 125% Rule: Requires conductor ampacity to be at least 125% of continuous load current.
• OCPD: Overcurrent Protection Device such as a circuit breaker or fuse.

Step-By-Step: Calculating Breaker Size

Step 1: Locate the furnace nameplate or installation manual and note the rated amps and voltage. If only kilowatts are given, calculate current: amps = kW × 1000 ÷ volts ÷ power factor (for resistive loads PF≈1).

Step 2: Treat the heater as a continuous load and multiply the rated current by 125% to determine required conductor ampacity: Required Ampacity = Rated Amps × 1.25.

Step 3: Select a conductor size with ampacity equal to or greater than the required ampacity per NEC tables (Table 310.16 or manufacturer guidance).

Step 4: Choose an OCPD (breaker) that does not exceed conductor ampacity and follows the NEC rules for maximum breaker sizing for specific conductor types and installation conditions.

Example Calculations

Example 1: A 20 kW electric furnace on 240 V. Current = 20,000 ÷ 240 = 83.3 A. Required conductor ampacity = 83.3 × 1.25 = 104.1 A. A 125 A breaker with 3 AWG copper (allowed ampacity ~115–125 A based on insulation and conditions) might be appropriate, but the final selection depends on conductor ampacity tables and local code.

Example 2: A 10 kW furnace on 240 V. Current = 10,000 ÷ 240 = 41.7 A. Required ampacity = 41.7 × 1.25 = 52.1 A. A 60 A breaker with 6 AWG copper (ampacity ~55–65 A depending on insulation) is commonly used.

Common Electric Furnace Sizes And Typical Breakers

Furnace Rating	Approx. Current @240V	Typical Breaker Size	Common Wire Gauge (Copper)
5–7.5 kW	21–31 A	30–40 A	10–8 AWG
10 kW	42 A	60 A	6 AWG
15 kW	62.5 A	80 A	4 AWG
20 kW	83 A	100–125 A	3–2 AWG
30 kW	125 A	150 A	2/0 AWG

Wire Sizing And Conductor Ampacity

Wire gauge selection must satisfy conductor ampacity requirements after applying the 125% factor. The NEC ampacity tables vary by conductor insulation type (THHN, XHHW) and installation conditions. Copper conductors are common in residential settings, though aluminum or copper-clad aluminum may be used for larger feeders.

Examples of typical ampacities: 6 AWG copper THHN often rated for 65–75 A, 4 AWG for 85–95 A, 2 AWG for 115–130 A, and 2/0 AWG for 175–195 A depending on insulation and temperature rating. Always verify with the current NEC table and local code or an electrician.

Breaker Type And Characteristics

Electric furnaces generally use standard thermal-magnetic circuit breakers sized for continuous loads. For larger furnaces, two-pole breakers on 240 V circuits are common. Some installations use fused disconnects or molded-case circuit breakers (MCCB) when higher amperages are present.

Important: The breaker must coordinate with the conductor ampacity and interrupting rating to protect the circuit and handle short-circuit currents. Use breakers listed for the specific application and environment.

NEC Requirements And Code Considerations

The NEC includes specific rules that impact furnace breaker sizing: treat heating elements as continuous loads, apply the 125% rule to conductor sizing, and ensure overcurrent protection is appropriate. NEC also requires proper disconnecting means, labeling, and clear access to service equipment.

Local jurisdictions may adopt NEC editions with amendments. An electrician should confirm the local code version and any additional requirements such as panelboard limitations, service capacity, or utility restrictions.

Practical Installation Considerations

• Service Capacity: Verify the home’s main service can support the furnace load in addition to existing electrical demands. A service upgrade may be required for large electric furnaces.
• Voltage Drop: For long runs, consider voltage drop and increase conductor size if necessary to maintain efficiency and prevent nuisance tripping.
• Disconnect Location: NEC requires a local means of disconnect within sight of the furnace or at the equipment. Use lockable disconnects when required.
• Breaker Accessibility: Breaker space in the panel must be available; tandem breakers are not acceptable for high-amperage furnace feeds.

When To Use Dual Breakers Or Subpanels

Large electric furnaces with high current draw often require a dedicated two-pole breaker or a subpanel fed from the main service. Subpanels can house branch circuits for the furnace, controls, and other loads while keeping the main panel organized.

In multi-unit or commercial settings, three-phase power may be used. Breaker sizing follows the same principles but uses three-phase current calculations and NEC three-phase rules.

Common Mistakes And How To Avoid Them

• Using Nameplate Amps Directly: Failing to apply the 125% continuous load rule leads to undersized conductors.
• Ignoring Voltage Drop: Long cable runs without voltage drop calculations can cause underperformance and overheating.
• Overlooking Service Limits: Installing a large furnace without checking main service capacity can overload the service.
• DIY Without Permit: Working without permits or inspection risks code violations and safety hazards.

Safety And Maintenance Tips

Ensure breakers are properly labeled with the furnace circuit identification. Inspect connections for signs of heat, corrosion, or loose terminals. Periodic professional inspection can catch issues early and maintain safe operation.

Safety Reminder: Electrical work can be dangerous. A licensed electrician should perform sizing calculations, select the proper breaker and conductors, and complete installation according to code.

How To Read The Furnace Nameplate

The nameplate typically lists rated voltage, kilowatt rating, and full-load amps or maximum overcurrent device rating. If kilowatts are shown without amps, use the formula amps = kW × 1000 ÷ volts. Always use the nameplate data as the authoritative source when available.

When To Call A Professional

Call a licensed electrician when the calculation involves service upgrades, three-phase power, potential code issues, or when the required breaker size exceeds standard residential panel capacities. Permit and inspection are often required for new furnace installations or service changes.

Resources And References

• National Electrical Code (NEC) — See sections on continuous loads and conductor ampacity.
• Manufacturer Installation Manual — Always follow the furnace manufacturer’s instructions and nameplate data.
• Local Building Department — Verify local amendments and permit requirements.

Quick Reference Checklist For Installing A Furnace Breaker

1. Obtain furnace nameplate data: voltage, kW, amps.
2. Calculate rated current if necessary: amps = kW × 1000 ÷ volts.
3. Apply 125% for continuous load conductor sizing.
4. Select conductor with ampacity ≥ required ampacity per NEC.
5. Choose breaker that protects conductor and meets NEC rules.
6. Verify service capacity and obtain permits.
7. Install disconnect and label circuit; schedule inspection.

This article gives the core information needed to determine and select the proper electric furnace breaker size and conductors. For final approval, calculations, and installation, a licensed electrician and local code officials provide the definitive guidance required for safe, compliant installation.