For an 18 kW tankless water heater, you typically need a 60-amp double-pole circuit breaker to ensure safe and efficient operation.
Installing an 18 kW tankless water heater requires careful electrical planning. The right breaker size ensures safety and optimal performance. This guide explains everything you need to know about breaker requirements for 18 kW units.
Breaker Size for 18 kW Tankless Water Heaters
Most 18 kW tankless water heaters require:
- Two 40-amp double-pole breakers (common requirement)
- 75 amp total circuit (some models specify this)
- #8 AWG copper wire minimum (or larger depending on distance)
The exact breaker size depends on the manufacturer’s specifications. Always check your unit’s manual before installation.
Why Two Breakers?
18 kW tankless heaters often have multiple heating elements. These elements typically divide the electrical load across two separate circuits. This design:
- Reduces strain on any single circuit
- Prevents excessive voltage drop
- Allows for staged heating
Electrical Calculations
Here’s how to calculate the amperage for an 18 kW unit at 240V:
| Calculation | Formula | Result |
|---|---|---|
| Total Amps | 18,000W ÷ 240V | 75A |
| Per Circuit (2 circuits) | 75A ÷ 2 | 37.5A |
Manufacturers typically recommend 40A breakers for each circuit to provide a safety margin. This follows the NEC ampacity guidelines.
Service Panel Requirements
Your electrical panel must have:
- Minimum 150-amp main service
- Two available slots for double-pole breakers
- Sufficient capacity for additional household loads
Consider a professional evaluation of your electrical system before installation.
What If Your Panel Is Too Small?
Options for insufficient panel capacity:
- Upgrade to a larger service panel (200A or more)
- Install a smaller tankless unit with lower power requirements
- Use a load management device to prioritize water heater operation
Installation Considerations
Wire Size
Recommended wire gauges:
- #8 AWG copper for 40A circuits (most common)
- #6 AWG copper for longer runs (over 50 feet)
- #4 AWG aluminum as a cost-saving alternative (where permitted)
Voltage Drop
Long wire runs can cause voltage drop. For runs over 50 feet:
- Increase wire gauge by one size
- Consider installing the unit closer to the panel
- Verify voltage at the unit during operation
Manufacturer Specifications
Different brands have slightly different requirements:
| Brand | Model | Breaker Requirement |
|---|---|---|
| Rheem | RETEX-18 | 2 x 40A |
| A.O. Smith | R4LA-180E | 2 x 40A (75A total) |
| EcoSmart | ECO 18 | 2 x 40A |
Always verify with your specific model’s documentation before installation.
Safety Tips
- Hire a licensed electrician for installation
- Use properly rated breakers and wire
- Install a disconnect switch near the unit
- Follow all local electrical codes
- Consider whole-house surge protection
Proper breaker sizing protects your investment and ensures safe operation of your tankless water heater for years to come.
