How to Wire an Electric Water Heater Circuit

Why this matters

A tank water heater is the simplest 240 V appliance in the house, and the circuit still gets four things wrong on a regular basis: sized at the element's current with no allowance for it running for hours, no disconnect the next tech can lock, a flexible whip relied on as the ground path when it does not qualify, and the first power-up done on a tank the plumber has not finished filling, which burns out the element in seconds. This card is the circuit for a residential electric tank: sizing, the disconnect, the whip, the bond and the amp test on each element. The plumbing, including the relief valve, the expansion tank and the water connections, belongs to the plumber, and the handoff between the two trades is part of the job.

Safety first

Power. Install with the circuit de-energized: breaker off, locked and tagged, proved dead with live-dead-live on a CAT III or better meter, proved on a known live source before and after (NFPA 70E Article 120, 120.5 in the 2021 edition and renumbered in 2024; work practices at 29 CFR 1910.333(b)(2), and 29 CFR 1926.417 on construction work). On a replacement, the old heater's circuit may be fed through a junction box shared with another circuit; prove every conductor in it.

Water. The tank is full before the breaker closes, and full means air purged: a hot tap in the house run until water flows without sputtering. An element energized in air glows and fails, often within a minute.

The relief valve. If the temperature and pressure relief valve is discharging when you arrive or during your test, turn the heat off first, at the breaker, and only then shut the cold supply. Shutting off the cold first leaves the element heating a closed tank. Then the call goes to the plumber; a discharging relief valve is usually a pressure, expansion or valve problem. The electrical exception is a stuck thermostat or a failed high limit overheating the tank, and that gets checked dead.

Sizing: 4500 W at 240 V

The worked job: a house converting from a gas water heater to a 50 gallon electric tank. The plumber has set and piped the tank. Its data plate reads 240 V, 4500 W upper element and 4500 W lower element, non-simultaneous, total connected load 4500 W. Non-simultaneous means the two thermostats are interlocked so only one element runs at a time, so the circuit sees 4500 W at most, not 9000 W.

Why it is a continuous load. NEC 422.13 (2023 edition, as adopted) treats a fixed storage water heater of 120 gallons or less as a continuous load and requires the branch circuit rating to be at least 125 percent of the nameplate. The 125 percent arithmetic is the continuous-load card's; see related: 'Continuous Load and the 125 Percent Rule'.

Why 30 A rather than 25 A. NEC 422.11(E) as adopted limits the overcurrent device for a single non-motor appliance to the rating marked on it, and where none is marked and the appliance is rated over 13.3 A, to 150 percent of its rated current, with the next standard size up permitted. 150 percent of 18.75 A is 28.1 A, so 30 A is permitted, and it is what most tank instructions call for. Read this tank's instructions: if they name a maximum breaker, that governs.

The conductor. 10 AWG copper, the conductor 240.4(D) allows on a 30 A device and 30 A in the 60 C column for NM (334.80). The circuit is 240 V only, so 10/2 NM with ground: the white is re-identified as an ungrounded conductor at each end and anywhere else it is visible and accessible (see related: 'Reidentifying a White Conductor in Cable').

A 3800 W tank at 240 V is 15.8 A, x 1.25 = 19.8 A, so a 20 A circuit on 12 AWG; on a replacement, read the plate before assuming the existing circuit is right or wrong.

The disconnect

NEC 422.31(B) as adopted lets the branch-circuit breaker be the disconnect for an appliance rated over 300 VA if it is within sight of the appliance, or if it is capable of being locked in the open position with the locking provision staying in place whether or not the lock is installed (110.25).

On the worked job the panel is in the garage, 35 ft away and through a wall: not within sight. The options are a lockable handle accessory installed permanently on the 30 A breaker, or a disconnect switch next to the tank. The shop installed the breaker accessory: it costs a fraction of the time, it satisfies 110.25, and the next tech can lock out the heater at the panel. A disconnect at the tank is the better choice where the panel is far away or locked in another unit; see related: 'Within-Sight Disconnect Rules for Equipment'.

The run and the whip

What each leg of the run carries, panel to tank:

 panel: 30 A two-pole breaker, lock accessory on it
    |
    | 10/2 NM: black, white re-identified as a hot,
    | bare ground
    v
 junction box on the wall, above the tank's top cover:
 bare and green spliced, and to the box if metal
    |
    | FMC whip, about 4 ft: black, white, green 10 AWG
    v
 tank junction compartment: both hots to the supply
 leads or block, green to the ground screw; no neutral

1. NM to a junction box beside the tank. Run the 10/2 NM from the panel and land it in a junction box on the wall or a stud next to the tank, at a height above the tank's top cover. Support and protect it per 334.30 (see related: 'NM Cable Support, Protection, and Clearance Rules'). A box on the wall rather than NM straight into the tank means the tank can be replaced without touching the house wiring.

2. Flexible conduit from the box to the tank's junction compartment. A short whip of flexible metal conduit, about 4 ft on this job, from the box to the knockout on the tank's top cover, with listed connectors at both ends. It allows the tank to be moved slightly when the plumber works on it.

3. Pull an EGC through the whip. This is the step most often skipped. NEC 250.118 as adopted allows listed flexible metal conduit to serve as the equipment grounding conductor only in short lengths (6 ft or less) on circuits protected at 20 A or less, among other conditions. This circuit is 30 A, so the FMC does not qualify, and a green 10 AWG conductor (the 250.122 size for a 30 A circuit, and the same size as the circuit conductors) goes through the whip with the two 10 AWG circuit conductors.

4. Terminate at the tank. The two circuit conductors to the tank's supply leads or terminal block, the green to the tank's ground screw. Every connection is made with connectors sized for 10 AWG; the tank's own leads may be smaller and are listed with the tank. Torque the breaker terminals and any terminal block to the values marked on them with a calibrated tool (NEC 110.14(D)).

5. Bonding. The tank's jacket is bonded through the EGC. The water piping is bonded at the service under 250.104(A); see 'How to Bond the Water Pipe at the Service'. Some jurisdictions also want a jumper between the hot and cold lines at the tank, where dielectric fittings break continuity; check the local amendment.

The finish test: one element, then the other

Dead first. With the breaker locked off and the element access covers removed, lift one lead from each element and ohm across the element's two terminals; with both leads on, the meter can read back through the thermostat and the other element. Expected, 240 x 240 / 4500 = 12.8 ohms. On this job: upper 12.9 ohms, lower 12.8 ohms. Then each element terminal to the tank: open. A reading to the tank means a grounded element, which will trip a GFCI if there is one and leak current if there is not. Reland the lifted leads and torque them.

Fill confirmed. The plumber confirms the tank is full and purged. On this job, the hot tap at the kitchen ran clear and steady.

Restore, then measure the upper element. Thermostat guards, insulation and element covers go back on before power: nothing on the tank is opened live. Close the breaker standing to the side of the panel. With the tank full of cold water, the upper thermostat calls first, so the first current is the upper element. Clamp one circuit conductor at the junction box (cover off, energized diagnostic work: the PPE your shop's 70E assessment sets, the clamp jaw around a single insulated conductor, no tools on terminals). Measure the supply voltage there too.

• Supply: 242 V. Expected current: 242 / 12.8 = 18.9 A. Measured: 18.8 A, upper element.

Wait for the hand-off, then measure the lower element. When the top of the tank reaches the upper thermostat's setting, it transfers power to the lower element. On this job that came about 25 minutes after power-up. Clamp again.

• Measured: 18.9 A, lower element.

Both within a few percent of 18.9 A. A reading of about half that means the element is seeing 120 V or the circuit is miswired; zero on the upper with the tank cold means the upper thermostat's high-limit has tripped or the element is open, and both get checked dead, not by poking a live thermostat.

Close up. Breaker off and locked to put the junction box cover on, then restore and confirm the tank keeps heating. Write on the ticket: plate data, breaker and conductor size, element ohms, both currents, the supply voltage, and the lock accessory installed. Panel directory: "Water heater, 30 A".

What changes the answer

• A tankless electric heater. It draws far more than a tank, through its own set of circuits, each sized by the maker, often several large circuits on one unit. Size every circuit from its installation instructions, do a load calculation before promising the customer the service can carry it (see related: 'Electrical Load Calculation Reference'), and leave the plumbing, flow and venting questions to the plumber.
• Simultaneous elements. A tank wired or plated for both elements at once is 9000 W at 240 V, 37.5 A, x 1.25 = 46.9 A: a 50 A circuit, and a different conductor. It is rare in a house and common in some commercial tanks; read the plate.

References

• NFPA 70, National Electrical Code, 2023 edition (2020 where numbering moved) as adopted, local amendments checked: 110.14(D), 110.25, 240.4(D), 250.104(A), 250.118, 250.122, 334.30, 334.80, 422.11(E), 422.13, 422.31(B)
• NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2); 29 CFR 1926.417 for construction work
• See related: Continuous Load and the 125 Percent Rule
• See related: Within-Sight Disconnect Rules for Equipment
• Water heater manufacturer's data plate and installation instructions