Electric Heater Circuit-Sizing Tables
Use these tables to estimate the heater’s current draw, minimum copper wire size, and breaker or fuse size. Before using the tables, confirm the heater’s wattage, voltage, and phase on the product label or electrical schedule, and make sure they match the available electrical supply.
Not sure what heater wattage you need? Start with our Heater Sizing Calculator.
How to Use the Tables:
- Check Heater Information.
Find heater’s wattage, voltage, and phase on the product label. If multiple heaters share a circuit, add wattage of all heaters that may run at same time. - Choose Correct Table.
Use single-phase table for most residential applications. Use three-phase table only when the building has three-phase electrical service. - Find Heater Wattage.
Locate the heater’s wattage in the first column. If the exact wattage is not listed, use the next higher value – never a lower one. - Read Across the Row.
Use row that matches the wattage and voltage to find the heater current (amps), minimum wire size, and recommended breaker or fuse rating. - Confirm Before Installation.
Always verify the table values against the heater nameplate and installation instructions. Product requirements and local codes take priority.
*Wire gauges are sized based on 75C copper amperage limits to comply with typical 75C terminal ratings of electronic circuit breakers. Any 90C rated wire (or higher) must be sized for the terminal rating of the circuit breaker.
*Wire gauges are sized based on 75C copper amperage limits to comply with typical 75C terminal ratings of electronic circuit breakers. Any 90C rated wire (or higher) must be sized for the terminal rating of the circuit breaker.
Important Circuit-Sizing Information
1. This Is a Planning Guide
These tables provide general estimates. The final wire and breaker sizes depend on the heater nameplate, installation instructions, wire type, terminal temperature ratings, installation conditions, local electrical codes, and electrical-inspector requirements.
2. Why the Circuit Is Larger Than the Heater’s Running Current?
Fixed electric heaters are normally treated as continuous electrical loads. The wire and circuit protection aregenerally sized for at least 125% of the total heater load.
Another way to think about this is that the heater load should generally not exceed 80% of the branch circuit rating. This extra capacity helps prevent the circuit from operating continuously at its maximum rating.
Always verify circuit sizing against the heater nameplate, installation instructions, and applicable local electrical codes.
3. Connecting More Than One Heater
When multiple heaters are connected to one circuit, add the wattage or amperage of all heaters that may run at the same time. When the combined load is too large for the circuit, a larger circuit or an additional circuit may be required.
4. Confirm Voltage and Phase
Verify the heater’s rated voltage and phase from the product nameplate before installation. In a typical residential 120/240V panel, 120V circuits commonly use a single-pole breaker and 240V circuits commonly use a two-pole breaker. Do not rely on breaker appearance alone to determine circuit voltage.
5. Use the Correct Heater Voltage
Heaters are not interchangeable between voltages. Connecting a 240V heater to 120V will produce approximately 25% of its rated wattage. Connecting a 120V heater to 240V can produce approximately four times its rated wattage, damaging the heater and creating a serious fire hazard. For heaters with the same wattage, operating cost is generally the same regardless of voltage, but higher-voltage circuits draw less current.
Typical Residential Circuit Examples
Figure 1: Typical 120-Volt Heater Circuit
A 120-volt heater commonly connects to a single-pole circuit breaker.
Figure 2: Typical 240 V Residential Circuit Example
A 240-volt heater commonly connects to a two-pole circuit breaker.
These illustrations are examples only. Always confirm the required voltage using the heater nameplate, installation instructions, and electrical-panel schedule.