The correct wire size for tankless water heater installations depends on the specific model wattage and installation configuration. Small tankless units may require a single 60 amp circuit with 6 AWG copper wire, while larger whole-house models require multiple parallel circuits with 8 AWG or 6 AWG copper for each circuit. This article explains the NEC math behind tankless water heater wire sizing, common model categories and their circuit requirements, panel capacity considerations, wire type selection, and complete installation guidance for code-compliant electric tankless water heater circuits.

Across thousands of Southwire and Cerrowire orders shipped from IB Lighting nationwide, our electrician-led team advises DIY installers and licensed electricians on the correct wire size for residential tankless water heater installations. Founded in 2013 by a licensed Master Electrician with field experience across DTE Energy and Consumers Energy projects in Michigan, IB Lighting resells the full Southwire and Cerrowire electrical wire and cable lineup. This article is the walkthrough we provide to buyers researching wire size for their tankless installation, using specifications aligned with the current National Electrical Code (NFPA 70) and industry practice from the International Association of Electrical Inspectors.

The Fast Answer: Wire Size for Tankless Water Heater Circuits

The wire size for tankless water heater installations varies significantly by model. Small point-of-use tankless units may require only a single 30 or 40 amp circuit with 10 or 8 AWG copper wire. Mid-size units for warm climates typically require a single 60 amp circuit with 6 AWG copper wire or two 40 amp circuits with 8 AWG copper each. Whole-house units for cold climates typically require three or four 40 amp circuits with 8 AWG copper each, or multiple 60 amp circuits with 6 AWG copper each. Always verify the specific model requirements from the manufacturer installation manual.

Common electric tankless water heater categories

Electric tankless water heaters fall into several categories based on power output. Point-of-use models (3-11 kW) serve a single fixture or small application. Small whole-house models (13-18 kW) work for warm climates or single-bathroom homes. Standard whole-house models (18-27 kW) work for typical homes in moderate climates. Large whole-house models (27-36 kW) work for cold climates or larger homes with multiple simultaneous demand points.

240 volt operation without neutral

Unlike electric dryers and ranges which require a neutral conductor for 120 volt internal controls, most electric tankless water heaters operate entirely on 240 volts. This means the branch circuits typically use only two hot conductors plus grounding (like a tank water heater) rather than requiring a neutral. Standard configurations use 6/2 or 8/2 NM-B with ground for indoor installations or individual THHN in conduit for exposed runs.

Multiple parallel circuits for whole-house units

The most important concept for whole-house tankless water heater wire sizing is that manufacturers typically require multiple parallel dedicated circuits rather than a single high-amperage circuit. A 27 kW unit that could theoretically use a single 150 amp circuit is instead specified as three separate 40 amp circuits or two separate 60 amp circuits. This distributes the load across multiple panel positions and simplifies wire sizing for each individual circuit.

Why Tankless Water Heaters Are Fundamentally Different From Tank Units

Understanding why the wire size for tankless water heater installations differs so dramatically from tank water heater installations clarifies the entire sizing approach.

On-demand heating requires higher instantaneous power

Tank water heaters (like the 4,500 watt residential standard) heat water gradually over time by storing it in an insulated tank. The 4,500 watt heating element cycles on and off to maintain water temperature. Because heating is spread over hours, the peak power demand is relatively modest.

Electric tankless water heaters heat water on demand as it flows through the unit. To raise incoming cold water temperature by 60-80 degrees Fahrenheit within seconds requires 3 to 8 times more power than a tank unit. This translates directly to significantly larger circuits, larger wire sizes, and often multiple circuits for whole-house units.

Comparison of typical power draws

A standard 4,500 watt tank water heater draws 18.75 amps at 240 volts on a single 30 amp circuit with 10 AWG copper wire. A comparable output tankless water heater for the same household might require 24,000 watts (100 amps at 240 volts) distributed across multiple 40 amp circuits with 8 AWG copper each. This 5.3x power increase demonstrates why the wire size for tankless water heater installations is fundamentally different.

Continuous load classification differs

Tank water heaters are classified as continuous loads because they can operate for extended periods during heavy hot water use. Tankless water heaters are also classified as continuous loads but the operating pattern is different: tankless units run at full power for shorter durations (when hot water is flowing) rather than at partial power for longer durations. The NEC 125 percent continuous load factor applies to both, but the base current calculation is much higher for tankless units.

Installation location differences

Tank water heaters are typically installed in utility rooms, basements, or garages away from points of use. Tankless water heaters are often installed closer to major water use points to minimize hot water travel distance. This affects wire routing because the tankless location may be in a laundry room, kitchen, or bathroom rather than an isolated utility space.

The NEC Math Behind Tankless Water Heater Wire Sizing

Understanding the calculation that drives the wire size for tankless water heater specifications prevents common sizing mistakes.

Step 1: Determine the model wattage from the nameplate

Every electric tankless water heater has a nameplate showing the maximum wattage. Common residential models include 13 kW (13,000 watts), 18 kW, 24 kW, 27 kW, and 36 kW. This nameplate rating is the definitive electrical specification and drives all subsequent calculations. Do not use estimates or general product category assumptions.

Step 2: Calculate total current draw

Total current draw is calculated from wattage divided by 240 volts. For a 27 kW tankless water heater: 27,000 watts divided by 240 volts equals 112.5 amps total. This is the maximum instantaneous current draw when all heating elements are active. Some models never activate all elements simultaneously, but branch circuit sizing must accommodate the maximum possible demand.

Step 3: Apply the NEC 125 percent continuous load factor

Tankless water heaters are classified as continuous loads per NEC 422.13. The 125 percent factor applies: 112.5 amps multiplied by 1.25 equals 140.6 amps total required ampacity. This total ampacity is distributed across the multiple parallel circuits per the manufacturer specification.

Step 4: Determine the circuit configuration from manufacturer specification

The manufacturer installation manual specifies exactly how the total load distributes across parallel circuits. A 27 kW unit might be specified as three 40 amp circuits (each circuit provides 40 amps of the total 120 amp capacity) or two 60 amp circuits (each provides 60 amps). Never assume the circuit configuration; always use the manufacturer specification.

Step 5: Match wire ampacity to each circuit breaker per NEC 240.4(D)

Each individual circuit must have wire sized per NEC 240.4(D). For 40 amp circuits, use 8 AWG copper. For 50 amp circuits, use 6 AWG copper. For 60 amp circuits, use 6 AWG copper (permitted up to 65 amp ampacity). Each circuit needs its own dedicated wire from the panel to the tankless unit termination point.

Pro tip: Before ordering wire for a tankless water heater installation, download the specific model installation manual from the manufacturer website and confirm the exact electrical requirements. Manufacturers periodically update circuit specifications and some models have been reconfigured over time. The manual will specify the number of circuits, the amperage of each circuit, the wire gauge required, and any special installation requirements. This verification step prevents purchasing the wrong wire and finding out during installation.

Wire Size for Tankless Water Heater by Model Category

Different model categories have different typical wire requirements. Use this reference to understand the approximate wire specifications for your model category.

Point-of-use tankless (3-11 kW)

Small point-of-use tankless water heaters serving a single fixture or application typically require a single 30 or 40 amp circuit. Wire specifications typically include 10 AWG copper for 30 amp circuits or 8 AWG copper for 40 amp circuits. Use NM-B 10/2 with ground or NM-B 8/2 with ground for indoor installations, or individual THHN conductors in conduit for exposed runs. These units are often installed under a sink or near a specific fixture.

Small whole-house tankless (13-18 kW) for warm climates

Small whole-house tankless units designed for warm southern climates typically require a single 60 amp circuit with 6 AWG copper or two 40 amp circuits with 8 AWG copper each. Example models include Rheem RTEX-13 (13 kW single 60 amp circuit with 6 AWG) and comparable units from other manufacturers. These units work well in Florida, Texas, Arizona, and similar climates where incoming groundwater is warmer.

Standard whole-house tankless (18-27 kW)

Standard whole-house tankless water heaters for moderate climates typically require two to three parallel circuits. Common configurations include two 60 amp circuits with 6 AWG copper each, three 40 amp circuits with 8 AWG copper each, or two 50 amp circuits with 6 AWG copper each. Example models include EcoSmart ECO 27 (three 40 amp circuits with 8 AWG each). These units work well in moderate climates including California, the Southeast, and southern Midwest.

Large whole-house tankless (27-36 kW) for cold climates

Large whole-house tankless water heaters for cold northern climates typically require three or four parallel circuits. Common configurations include three 60 amp circuits with 6 AWG copper each or four 40 amp circuits with 8 AWG copper each. Example models include Rheem RTEX-36 (four 40 amp circuits) and Stiebel Eltron Tempra 36 (four 40 amp circuits). These units are necessary in northern climates where incoming groundwater temperature drops below 50 degrees Fahrenheit in winter.

Verifying your specific model requirements

The categories above provide general guidance, but individual manufacturer models within each category may have specific requirements that differ. Always download and review the specific model installation manual before finalizing wire and breaker purchases. Manufacturers occasionally revise electrical specifications between production runs, so verify against the current documentation for your specific unit.

Why Whole-House Tankless Water Heaters Require Multiple Circuits

The multiple-circuit approach for whole-house tankless water heaters is a manufacturer design decision that affects wire sizing and installation planning.

Load distribution across the electrical panel

Distributing a 27 kW load across three 40 amp circuits rather than one 150 amp circuit spreads the load across three panel spaces instead of concentrating it. This improves electrical panel compatibility because residential panels rarely have single 150 amp breaker positions available, but they commonly have three 40 amp breaker positions available in the main panel.

Simpler wire sizing per circuit

Each individual 40 amp circuit uses standard 8 AWG copper wire, which is readily available and manageable for installation. A single 150 amp circuit would require 1/0 AWG copper wire, which is significantly bulkier, more expensive, and more difficult to route through walls. The multiple-circuit approach uses standard residential building wire sizes throughout.

Manufacturer element control simplification

Internal to the tankless water heater, multiple heating elements can be controlled independently across the parallel circuits. This allows the unit to activate only the elements needed for the current flow rate, improving efficiency and reducing unnecessary electrical demand. A single-circuit design would require more complex internal power distribution.

Safety and reliability considerations

Multiple parallel circuits provide redundancy that a single circuit does not. If one circuit trips or has an issue, the remaining circuits can continue providing some hot water capacity. This is particularly valuable for large families where complete loss of hot water capacity creates significant inconvenience.

Panel Capacity Requirements for Electric Tankless Installations

Electric tankless water heaters require significant electrical service capacity, and panel adequacy is often the limiting factor for installations in existing homes.

Typical panel capacity requirements

Whole-house electric tankless water heaters typically consume 60 to 100 amps of your electrical service capacity depending on the model. Point-of-use units consume 20 to 40 amps. Combined with other major loads (electric range, electric dryer, HVAC), a whole-house tankless installation can push total service demand above the capacity of older panels.

NEC 220 service load calculations

NEC 220 requires load calculations to verify that electrical service can accommodate all connected loads. For tankless water heater installations, this calculation includes the tankless load at 125 percent (continuous load factor) plus all other continuous and non-continuous loads. Existing panels near capacity may fail this calculation when tankless load is added.

When a panel upgrade is required

Panel upgrades are commonly required before whole-house tankless installations in homes with older 100 or 150 amp service. Upgrading to 200 amp service typically provides adequate capacity for tankless plus other typical loads. Some very large homes or homes with many electric appliances may require 400 amp service. Panel upgrade cost typically ranges $1,500 to $4,000 depending on region and complexity.

Alternative to panel upgrade

If panel upgrade is impractical or unaffordable, alternatives include installing a smaller point-of-use tankless unit that supplements an existing tank water heater rather than replacing it, installing a gas tankless water heater which does not require significant electrical capacity, or continuing with a traditional tank water heater. Consult a licensed electrician for panel capacity assessment before ordering an electric tankless unit.

Climate Impact on Tankless Water Heater Wire Sizing

Climate affects tankless water heater sizing because colder incoming groundwater requires more heating power to reach usable hot water temperature.

Groundwater temperature by region

Incoming groundwater temperature varies significantly by geographic region. Southern states typically see 65-75 degree Fahrenheit groundwater year-round. Moderate climates see 50-65 degree groundwater. Northern climates see 35-45 degree groundwater in winter. The wire size for tankless water heater installations must accommodate the coldest expected groundwater temperature for your region.

Required temperature rise calculation

Hot water at 105-120 degrees Fahrenheit is comfortable for typical household use. The temperature rise required is the difference between incoming groundwater temperature and target hot water temperature. Warm climates need 30-55 degree rise, moderate climates need 45-70 degree rise, cold climates need 65-85 degree rise. Larger required rises need larger tankless units and more circuits.

Tankless sizing by climate zone

The same household may need a 13 kW tankless in Florida but 27 kW in Michigan for equivalent hot water performance. This is not because the household uses more water in Michigan, but because each gallon requires more heating energy. Climate-appropriate sizing directly determines the wire size for tankless water heater installation.

Regional installation practices

Local plumbers and electricians in each region are familiar with the typical tankless sizing for local climate conditions. In southern states, single-circuit tankless installations dominate. In northern states, multiple-circuit installations are the norm. Consult local professionals for climate-appropriate model selection before finalizing wire sizing.

Wire Type Selection for Tankless Water Heater Circuits

Wire type selection for tankless water heater circuits depends on installation environment and local code requirements.

NM-B for indoor dry locations

NM-B (Non-Metallic sheathed cable, Type B) is the standard wire type for tankless water heater circuits inside dry finished or unfinished spaces. Configuration depends on wire size: 6/2 with ground for 60 amp circuits, 8/2 with ground for 40 amp circuits, or 10/2 with ground for smaller 30 amp point-of-use circuits. Note that these are 2-conductor cables with ground because most tankless units do not require a neutral.

THHN individual conductors in conduit

THHN and THWN individual conductors in EMT or PVC conduit are appropriate for tankless installations in unfinished basements, garages, utility rooms, or where local code requires conduit. Pull three individual conductors per circuit: two hots (black and red or similar) and one green (or bare copper) for grounding. THHN provides more installation flexibility than NM-B especially for tankless units requiring multiple parallel circuits.

UF-B for uncommon damp locations

UF-B is rarely needed for tankless water heater circuits because tankless units are almost always installed indoors. UF-B may apply to specialty installations in outdoor equipment rooms or attached utility structures where the wire routing includes damp sections. For standard indoor tankless installations, NM-B is the more common and less expensive choice.

Multiple cables to the same unit

Whole-house tankless installations with multiple parallel circuits require multiple runs of cable from the electrical panel to the tankless unit location. Some installers pull all cables through a single conduit sleeve for organization, while others run cables as separate NM-B pulls. Local code and installation preference determines the approach. Either method is acceptable when properly executed.

Continuous Load Factor and Voltage Drop

Two additional considerations affect the wire size for tankless water heater installations: the continuous load factor and voltage drop for long wire runs.

The 125 percent continuous load factor

Tankless water heaters are classified as continuous loads per NEC 422.13. Branch circuit conductors must be sized at 125 percent of the actual load current. For an individual 40 amp circuit within a whole-house tankless installation, the 125 percent factor is already built into the 40 amp specification because 32 amp actual load times 1.25 equals 40 amps. Do not add another 125 percent factor on top of the manufacturer specification.

Voltage drop considerations for tankless

Voltage drop calculations for tankless water heater circuits should target the 3 percent maximum per NEC 210.19 informational note. For 40 amp circuits with 8 AWG copper, voltage drop stays within 3 percent for runs up to approximately 65 feet. For 60 amp circuits with 6 AWG copper, voltage drop stays within 3 percent for runs up to approximately 55 feet. Longer runs require wire size upgrades.

Multiple circuits and voltage drop

When multiple parallel circuits serve the same tankless unit, voltage drop must be calculated for each circuit independently. Each circuit’s actual draw is a fraction of the total tankless draw (for a three-circuit installation, each circuit carries approximately one-third of the total load). This makes voltage drop calculations somewhat more forgiving for multi-circuit installations than a single-circuit alternative would be.

Panel-to-tankless typical distance

Tankless water heaters are often installed near the electrical panel in the same utility room or basement space, so voltage drop is typically not a limiting factor. When the tankless is installed at some distance from the panel (kitchen installation with panel in basement, for example), voltage drop becomes relevant and may require upgrading from the minimum wire size specified.

Choosing the Right Wire Product for Your Tankless Installation

Understanding which specific wire products fit the wire size for tankless water heater specification helps buyers select the correct product for their installation.

Southwire 8/2 and 6/2 NM-B with ground

Southwire is one of the largest domestic manufacturers of building wire and cable. Their 8/2 NM-B with ground works for 40 amp tankless circuits, and their 6/2 NM-B with ground works for 60 amp tankless circuits. Southwire NM-B is available in standard reel lengths from 25 feet to 500 feet. For a typical tankless installation with multiple parallel circuits, calculate total wire needed and purchase reels that minimize waste.

Cerrowire 8/2 and 6/2 NM-B alternatives

Cerrowire manufactures a comparable line of 8/2 and 6/2 NM-B with ground. Cerrowire and Southwire products are electrically equivalent and meet the same UL 719 standard for NM-B cable. Both brands are widely accepted by electrical inspectors for tankless installations. Selection typically depends on regional availability and pricing.

THHN individual conductors for conduit installations

For tankless installations using conduit, purchase individual 8 AWG or 6 AWG THHN conductors as needed for each circuit: two hots (black and red or similar) plus one green or bare copper grounding per circuit. Whole-house installations may require pulling multiple sets of these conductors through the same conduit, so calculate conduit fill per NEC 314.16 to ensure adequate conduit size.

Wire and cable pricing considerations for multi-circuit installations

Wire and cable costs for whole-house tankless installations are significantly higher than for tank water heater installations because of the multiple parallel circuits. Total wire cost for a three-circuit installation typically runs 200 to 500 dollars depending on wire type, gauge, and total length. This is in addition to the multiple breakers, junction boxes, and other components required.

Point-of-Use vs Whole-House Tankless Wire Requirements

Understanding the significant difference between point-of-use and whole-house tankless installations clarifies the wire sizing approach.

Point-of-use installations

Point-of-use tankless water heaters serve a single fixture or small application such as an under-sink installation for a kitchen or bathroom, a garage utility sink, or an outbuilding shower. These small units (3-11 kW) typically require a single 30 or 40 amp dedicated circuit with 10 or 8 AWG copper wire. Panel impact is modest and installation is straightforward.

Whole-house installations

Whole-house tankless water heaters replace the existing tank water heater and serve all hot water fixtures in the home. These larger units (13-36 kW) require single high-amperage circuits or multiple parallel circuits with heavier wire sizes. Panel impact is significant and installation requires more planning including potential panel upgrade.

Hybrid installations

Some installations combine a smaller whole-house tankless unit with point-of-use tankless units at distant fixtures. This approach reduces the size of the main unit needed while providing quick hot water at remote fixtures without long pipe runs. Each unit requires its own dedicated circuit or circuits sized per the specific model.

Cost implications

Point-of-use installations are typically much less expensive than whole-house installations because of simpler electrical requirements, smaller unit cost, and simpler installation. Whole-house tankless installations often exceed $3,000 to $6,000 total cost including the unit, electrical work, plumbing modifications, and potential panel upgrade. Consider this significant cost difference when evaluating whether tankless is the right choice for your situation.

5 Common Mistakes When Wiring a Tankless Water Heater

The mistakes below cause the most frequent buyer questions about wire size for tankless water heater installations.

Mistake 1: Assuming tank water heater wire specifications apply to tankless

The most common mistake is assuming that the 10 AWG on 30 amp specification used for tank water heaters also applies to tankless units. Electric tankless water heaters draw 3 to 8 times more power than tank units and require significantly larger circuits (often multiple parallel circuits). Never use tank water heater wire specifications for tankless installations without verifying manufacturer requirements.

Mistake 2: Skipping the panel capacity assessment

Whole-house tankless water heaters can consume 60 to 100 amps of electrical service capacity. Installing tankless without verifying panel adequacy can result in tripped main breakers, incomplete installations, or expensive panel upgrades after the fact. Always perform a NEC 220 service load calculation before purchasing a whole-house tankless unit.

Mistake 3: Not verifying manufacturer-specific requirements

Tankless water heater wire requirements vary significantly between manufacturers and even between models from the same manufacturer. Downloading and reviewing the specific model installation manual is essential before ordering wire. General category assumptions can result in undersized or overspecified wire that requires expensive rework.

Mistake 4: Using a single circuit when multiple are required

Some installers attempt to use a single high-amperage circuit (100 or 150 amp) when the manufacturer specifies multiple smaller circuits (three 40 amp or two 60 amp). This violates the manufacturer specification and may create warranty issues. Always follow the specified circuit configuration exactly.

Mistake 5: Using indoor NM-B for outdoor equipment room installations

Rare but expensive: some installers use NM-B for tankless circuits when the installation is in an outdoor equipment room or attached utility structure with damp conditions. NM-B is not rated for damp locations. Use UF-B for damp installations or THHN in conduit for outdoor exposed runs.

Conclusion

The correct wire size for tankless water heater installations depends heavily on the specific model wattage and can range from a single 60 amp circuit with 6 AWG copper for small units to four parallel 40 amp circuits with 8 AWG copper each for larger whole-house models. Electric tankless water heaters require 3 to 8 times more power than tank water heaters, which drives fundamentally different wire and circuit requirements.

Whole-house tankless installations typically require multiple parallel circuits, significant electrical service capacity, and potentially a panel upgrade in older homes with 100 or 150 amp service. Always download and review the specific manufacturer installation manual before ordering wire because tankless requirements vary significantly between models. Most tankless units operate on 240 volts without a neutral, using 2-conductor cables with ground for each circuit.

This article was last reviewed and updated in June 2026 to reflect current National Electrical Code requirements and residential electric tankless water heater installation best practices. Wire sizing recommendations are based on the current NEC (NFPA 70) and common manufacturer specifications and should be verified against your specific model installation manual and local code amendments. For complex installations or when in doubt, consult a licensed electrician. Improperly sized wire can cause fires and voids insurance coverage.