The size of the electrical wire for a tankless water heater depends on the unit’s power rating, voltage, distance from the panel, and whether copper or aluminum conductors are used. Electric tankless models demand substantial electrical service and must be installed to code by a licensed electrician. Understanding key factors helps ensure a safe, efficient installation and compliance with local codes and the National Electrical Code (NEC).

Understanding Tankless Water Heaters Electrical Requirements

Electric tankless water heaters are rated by kilowatts (kW) and require a 240-volt supply in most residential installations. The higher the kW rating, the more amperage is drawn and the larger the circuit and wire size needed. Common residential setups range from small units around 5–9 kW to larger units 18–27 kW or more. The actual wire size must be determined from the manufacturer’s specifications and NEC ampacity tables, factoring in continuous operation and any safety margins.

Key points to consider include:

Unit power rating: Higher kW units draw more amperage and require larger conductors.
Voltage: Most homes use 240V, but some installations may use different configurations; always match the unit’s voltage with the available supply.
Distance from panel: Longer runs require larger conductors to minimize voltage drop and maintain performance.
Conductor type: Copper is common for its lower resistance and easier handling; aluminum is lighter and cheaper but requires different sizing.

Choosing Wire Size By Amperage And Distance

Electrical wire size is primarily determined by amperage (A) and the length of the run. Manufacturers publish the maximum continuous current for each model, and NEC tables translate that current into required conductor sizes and overcurrent protection. A typical approach looks like this:

Small to mid-range units (roughly 5–11 kW): Often require a dedicated 40–60 A circuit. Common practice uses 6 AWG copper conductors with a 40–60 A double-pole breaker, depending on the exact model and voltage drop considerations.
Mid-range to high-range units (around 12–27 kW): May require 100 A or larger service circuit. This often translates to 3 AWG to 4 AWG copper conductors or equivalent aluminum sizes, with corresponding multi-pole breakers as specified by the manufacturer.
Distance considerations: For runs over 50 feet, voltage drop becomes a factor. In such cases, a larger wire size than the minimum may be recommended to maintain performance.

Because exact wire sizing depends on the specific model, always verify with the unit’s installation manual and a licensed electrician who can apply NEC ampacity calculations and local amendments.

Copper Versus Aluminum Wire

Copper conductors are the preferred choice for tankless water heaters due to lower resistance and easier termination. They typically deliver the required amperage with smaller gauges than aluminum for the same length. Aluminum conductors are lighter and cheaper but have higher resistance, which can necessitate a larger size to carry the same current safely. If aluminum is used, ensure proper terminations rated for aluminum and confirm that the final breaker and conductor sizes satisfy NEC requirements for aluminum conductors in your jurisdiction.

Guidance by material:

Copper: Commonly 6 AWG for smaller units, 3–4 AWG for larger units, depending on distance and kW rating.
Aluminum: May require approximately 1–2 sizes larger than copper for equivalent current, with special lugs and anti-oxidant paste for connections.

Note that many local codes encourage or require copper for high-ampacity applications, so confirm local practice before selecting wire material.

Breaker Size And Code Considerations

Correct breaker sizing is essential for safety and equipment protection. The breaker must match the conductors’ ampacity and the manufacturer’s maximum input current. Common guidance includes:

Dedicated circuit: Tankless heaters typically require a dedicated circuit or circuits, not shared with other devices.
Overcurrent protection: Use a circuit breaker sized to protect the conductors, as specified by the manufacturer and NEC tables. This often means a 40–60 A breaker for smaller units and 100 A or more for larger units, depending on the exact kW rating and voltage drop considerations.
Permits and inspections: Most jurisdictions require permits, electrical work inspections, and adherence to NEC 2023/2026 editions or later as adopted locally.

Other important code considerations include correct wire routing, proper conduit or NM-B cable usage (where allowed), strain reliefs, and appropriate grounding/b bonding per NEC requirements.

Installation Tips And Common Mistakes

Effective installation supports reliability and safety. Consider these practical tips:

Follow the manual: Always install per the manufacturer’s wiring diagram and required wire gauge for the specific model.
Plan for future upgrades: If there is a possibility of upgrading to a higher kW unit, consider oversizing the panel or installing a subpanel to avoid rework.
Professional installation: Hire a licensed electrician to perform the wiring, breaker sizing, grounding, and permits.
Voltage drop assessment: For long runs, perform a voltage drop calculation to avoid under-voltage conditions that impair performance.

Safety And Compliance

Safe operation hinges on proper sizing and installation. Always ensure:

Adherence to NEC and local codes: Local amendments may affect permissible wire sizes, conduit types, and grounding methods.
Quality terminations: Use listed lugs and anti-oxidant compounds when working with aluminum conductors to prevent corrosion and loose connections.
Professional oversight: Electric tankless water heaters involve high current; incorrect sizing can lead to overheating, nuisance tripping, or electrical fires.

For most U.S. homes, accurate wire sizing for a tankless water heater requires manufacturer specifications, NEC ampacity charts, distance considerations, and a licensed electrician’s expertise to ensure a safe, compliant installation.