A 27 kW electric tankless water heater requires #8 AWG copper wire for each of its three 40-amp circuits, assuming a wire run under 50 feet. For runs over 50 feet, you must upsize to #6 AWG to compensate for voltage drop. Using undersized wire creates a fire hazard and violates the National Electrical Code.
Here is the complete wire sizing reference for every common electric tankless unit, including distance-based adjustments and conduit requirements.
Wire Sizing Fundamentals
Wire size is determined by two factors: the current it must carry safely (ampacity) and the voltage drop over distance.
Ampacity
The NEC (Table 310.16) specifies the maximum current each wire size can carry safely at various temperatures. For residential 240V circuits in NM-B (Romex) cable at 60C conductor rating:
Voltage Drop
Wire has resistance, which causes voltage to drop over distance. The NEC recommends a maximum 3% voltage drop for branch circuits and 5% total (feeder + branch). At 240V, a 3% drop is 7.2V — meaning the unit receives 232.8V instead of 240V.
Voltage drop formula:
VD = (2 x Length x Current x Resistance per foot) / 1,000
For #8 AWG copper at 37.5A over 50 feet: VD = (2 x 50 x 37.5 x 0.628) / 1,000 = 2.36V (0.98% — well within 3%).
For #8 AWG at 37.5A over 100 feet: VD = (2 x 100 x 37.5 x 0.628) / 1,000 = 4.71V (1.96% — still acceptable but approaching the limit).
At 75 feet with #8 AWG: VD = 3.53V (1.47%). This is fine, but upsizing to #6 AWG at this distance provides better margin and reduces energy loss.
Wire Size by Unit and Distance
This is the master reference table. Find your unit's kW rating and wire run distance:
Always check the manufacturer's installation manual. Some manufacturers specify wire sizes that are more conservative than the NEC minimum. The Stiebel Eltron Tempra 29 Plus, for example, specifies #6 AWG for each of its three 50A circuits, even for short runs where #8 might technically suffice by NEC ampacity alone.
Wire Types for Tankless Installation
NM-B (Romex)
The most common residential wire. NM-B is a non-metallic sheathed cable with 2 or 3 insulated conductors plus a bare ground. It is rated for dry locations inside walls, floors, and ceilings.
For tankless circuits: Use 2-conductor NM-B (two hots + ground) for 240V circuits. There is no neutral needed for a tankless water heater — it is a pure 240V load.
#8 AWG NM-B cost: $1.50–$2.50 per foot (2026 prices). #6 AWG NM-B cost: $2.50–$4.00 per foot.
THHN/THWN in Conduit
For exposed runs (basements, garages, exterior walls), individual THHN or THWN conductors run in EMT or PVC conduit. This allows higher ampacity ratings (Table 310.16 at 90C for THHN) but requires conduit installation labor.
#8 AWG THHN cost: $0.80–$1.50 per foot per conductor (need 3: two hots + ground). Conduit cost: $1.00–$3.00 per foot installed.
UF-B (Underground Feeder)
For outdoor or underground runs (such as feeding an outdoor tankless or a detached garage). UF-B has waterproof insulation and can be direct-buried.
Important: UF-B has lower ampacity than NM-B at the same gauge. Verify ratings before using UF-B for tankless circuits.
Conduit Sizing
If running conductors in conduit, the conduit must be sized for the number and size of wires:
Bundling derating: When more than 3 current-carrying conductors share a conduit, NEC Table 310.15(C)(1) requires derating ampacity. For 4–6 conductors, derate to 80%. For 7–9 conductors, derate to 70%. This may require upsizing wire. Running each circuit in its own conduit avoids derating entirely — and is the recommended approach.
Wire Cost Estimator
Wire cost can be a significant fraction of the installation budget, especially for long runs or upsized conductors. Keep the unit as close to the panel as possible to minimize wire costs and voltage drop.
Installation Best Practices
Route Planning
Plan the shortest possible path from the panel to the unit. Going through conditioned space (inside walls) is preferred over running through unconditioned attics or crawlspaces, where temperature extremes can affect wire ampacity ratings.
Securing and Protection
NM-B cable must be secured with staples or straps every 4.5 feet and within 12 inches of each junction box. Where cables pass through studs or joists within 1.25 inches of the edge, nail plates are required to protect against drywall screws.
Connection at the Unit
Most electric tankless units have a terminal block with labeled connections for each circuit. Each circuit connects to a pair of terminals (L1 and L2 for each element group). The ground wire connects to the grounding terminal. Use proper cable clamps at the unit's knockout openings.
Labeling
Label each circuit at the breaker panel clearly: "Tankless WH Circuit 1," "Tankless WH Circuit 2," etc. This helps future electricians and inspectors identify the circuits.
Real-World Examples
Example 1: Short Run — EcoSmart ECO 27
Setup: Panel in utility room, tankless on the opposite wall (15 feet away). Three circuits, #8 AWG NM-B.
Materials: 45 feet of #8/2 NM-B ($68), three 40A double-pole breakers ($36), cable staples and clamps ($15). Total materials: $119. Labor: 2 hours ($300). Total: $419.
Example 2: Medium Run — EcoSmart ECO 36
Setup: Panel in basement, tankless in 2nd-floor bathroom (55-foot run). Four circuits, upsized to #6 AWG due to distance.
Materials: 220 feet of #6/2 NM-B ($660), four 40A breakers ($48), nail plates and clamps ($30). Total materials: $738. Labor: 5 hours ($750). Total: $1,488.
Example 3: Detached Garage — Rheem RTEX-18
Setup: Panel in house, tankless in detached garage 80 feet away. Two circuits, #6 AWG UF-B (direct burial) in PVC conduit.
Materials: 160 feet of #6/2 UF-B ($640), 80 feet of 3/4-inch PVC conduit ($120), two 40A breakers ($24), trench supplies ($80). Total materials: $864. Labor (including trenching): 8 hours ($1,200). Total: $2,064.
Key Takeaways
- Most common wire: #8 AWG copper for 40A circuits under 50 feet
- Upsize to #6 AWG for runs over 50 feet to prevent voltage drop above 3%
- Each circuit needs its own dedicated wire run from the panel to the unit
- Use NM-B (Romex) inside walls, THHN in conduit for exposed runs, UF-B for underground
- A 27 kW unit needs about 75–150 feet of total wire (three runs); a 36 kW unit needs 100–200 feet (four runs)
- Wire cost ranges from $100–$1,200 depending on gauge, distance, and number of circuits
- Keep the unit close to the panel — every extra foot adds cost and voltage drop
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