Water Heater Wire Size: Match Wire and Breaker to Your Nameplate

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Water Heater Wire Size Chart: Quick Reference

Find your nameplate wattage in the table, then read across to the minimum ampacity, wire and breaker.

Common 4500 W and 5500 W storage-heater configurations point to 10 AWG copper. On page 8, the Rheem installation manual lists 12 AWG with 20 A overcurrent protection for 3500 W, 10 AWG with 25 A for 4500 W, 10 AWG with 30 A for 5500 W, and 8 AWG with 35 A for 6000 W at 240 V.

Vantecable 10/2 NM-B Wire With Ground

For compatible 4500–5500 W / 240 V storage-heater circuits specifying 10 AWG copper and no neutral.

10/2 AWG with ground · Solid copper · 600 V · 25 / 50 / 100 ft · ETL Listed (Intertek)

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Vantecable 12/2 NM-B Wire With Ground

For compatible 3500 W / 240 V storage-heater circuits specifying 12 AWG copper and no neutral.

12/2 AWG with ground · Solid copper · 600 V · 15 / 25 / 50 / 100 ft · ETL Listed (Intertek)

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Step 1: Read the Nameplate and Calculate Minimum Ampacity

Divide the nameplate watts by volts, then multiply by 1.25. NEC 422.13 treats fixed storage heaters of 120 gal or less as continuous loads, so the circuit must be rated for at least 125% of nameplate current. The nameplate is on the tank side or inside the access panel. Record the total input watts, the volts and any wiring notes.

On a dual-element heater, use the label’s total input and wiring configuration. Do not add the upper and lower element wattages unless the manufacturer states they operate at the same time.

For a 4500 W / 240 V heater, 18.75 A × 1.25 = 23.44 A. That requires 10 AWG copper with 30 A ampacity and a double-pole breaker of at least 25 A. A 25 A or 30 A breaker fits when permitted by the manufacturer’s instructions and does not exceed the heater’s marked maximum overcurrent protection.

Under this 125% calculation, a 30 A circuit supports up to 5760 W at 240 V:

Above that, such as 6000 W, move to a larger conductor size; the reference table shows 8 AWG copper.

Step 2: Choose the Cable: 10/2, 10/3 or Conduit

Use 10/2 NM-B when Step 1 calls for 10 AWG, the heater has no neutral terminal and the path allows NM-B.

Does the Heater Need a Neutral?

Follow the wiring diagram on the heater. Most conventional 240 V storage heaters need only two hots and a ground, because the heating elements connect across the two hot legs and do not require a neutral. If the circuit requires 10 AWG and the diagram shows a neutral connection, choose 10/3 NM-B: two hots, neutral and ground.

10/2 NM-B has a black conductor, a white conductor and a bare copper ground. When the white conductor serves as the second hot, permanently re-identify it at each termination and other visible, accessible locations. The identification should encircle the insulation; black marking tape is one suitable example.

The 600 V printed on the jacket is the insulation’s voltage rating, not the circuit’s load capacity.

Is NM-B Allowed on Your Path?

NM-B suits permitted dry indoor runs, including dry basement locations discussed in our NM-B in basements guide. Where the cable is exposed to physical damage, such as a garage wall near vehicles, protect it with conduit or another approved method.

A damp location rules NM-B out. If the design uses individual conductors in conduit, their rating must suit the location, including any wet conditions inside the raceway. Local code amendments may restrict the wiring method further.

Step 3: Check the Run Length

Check voltage drop using the actual cable route and heater current. The NEC informational note recommends keeping branch-circuit voltage drop at 3% or less. The table below estimates the drop at each one-way length with the heater’s nameplate current flowing.

Estimates assume a 240 V resistive load and solid, uncoated copper resistance at 75°C: 1.93 Ω per 1000 ft for 12 AWG and 1.21 Ω per 1000 ft for 10 AWG. They exclude upstream voltage drop and connection resistance. This resistance assumption does not change NM-B’s applicable ampacity limit.

The calculated 3% distance is approximately 159 ft for a 4500 W heater on 10 AWG and 130 ft for a 5500 W heater. These are estimates, not guaranteed maximum cable lengths.

If the estimated drop exceeds 3%, choose a larger conductor size and recalculate.

Step 4: Installation Overview

Confirm who may install the circuit before buying cable. Permit and licensing rules vary by locality, and some manufacturers require a qualified electrician: the Rheem manual says the branch circuit must be provided by one. Check your heater’s manual and your local building department.

Installation involves isolating the supply and verifying absence of voltage, routing and supporting the cable, completing the connections according to the manufacturer’s diagram, and checking the installation before energizing it. For typical NM-B installations, secure the cable within 12 in. of enclosure entries and at intervals not exceeding 4.5 ft, subject to the applicable box requirements and code exceptions.

FAQ: Water Heater Wire Size

Pick the Right Wire for Your Heater

Match the heater’s rated input and wiring diagram to the required conductor size, conductor count and breaker. Then confirm the route and purchase length before ordering.

For compatible dry indoor installations, Vantecable’s 10/2 and 12/2 NM-B cables provide solid copper conductors with ground and are ETL Listed (Intertek).

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