How to Replace an Electric Tank Water Heater
Why this matters
An electric tank swap looks easier than a gas one because there is no vent and no gas line. The two ways it goes wrong are both electrical: a tech who trusts a breaker label and puts hands on a live junction box, and a tech who closes the breaker before the tank is full and burns out the upper element in the first minute. The second one is a callback the same afternoon, on a brand-new heater, with a customer who watched you leave. This card is the swap done in the order that prevents both. The paperwork side is 'The Water Heater Permit and Inspection SOP'; where the heater sits and what it sits in is 'Water Heater Placement: Pans, Elevation and Access'.
Lock out before anything else
The heater is fed by a 240 V circuit that will kill you through wet hands on a wet floor. The first act on site, before the cold valve and before the drain hose, is the breaker.
- Find the two-pole breaker for the heater and turn it off. Do not trust the panel directory; old panels are mislabeled often enough that the label is a suggestion.
- Lock and tag it under your shop's written program. NFPA 70E Article 120 governs the lockout and the proving sequence (120.5 in the 2021 edition; check the current edition's numbering); the federal work practices are 29 CFR 1910.333(b)(2), and 29 CFR 1926.417 on construction work.
- Prove dead at the heater, not at the panel. Take the junction box cover off the top of the heater and measure L1 to L2, L1 to ground and L2 to ground with a meter you have just proved on a known live source, then prove the meter live again. Three zeros between two live checks is dead. A zero from a meter you never proved is a meter that might have a blown fuse.
- If anyone besides you could reach that panel, the lock is not optional. A breaker someone "helpfully" turns back on while your hands are in the box is the accident this whole sequence exists to prevent.
Where your shop's program does not qualify plumbers for this, the disconnect and the proving are an electrician's steps, and the swap waits for them.
Start state: what to confirm before you drain anything
The new heater's nameplate against the existing circuit. NEC 422.13 (2023 edition, as adopted) treats a storage water heater of 120 gallons or less as a continuous load, so the branch circuit has to be rated at least 125 percent of the nameplate current. Divide the nameplate watts by the nameplate volts, multiply by 1.25, and compare it to the breaker and the conductor. If the new heater's number fits what is there, you reconnect at the heater. If it does not, the circuit changes, and a circuit change goes to a licensed electrician wherever your state or locality licenses electrical work separately, which is most places. Plumbing licenses that cover the appliance usually stop at its terminals.
The supply voltage. A heater with 240 V elements on a 208 V apartment or light-commercial service makes about three quarters of its rated heat (see the worked job). Measure it before you order, not after the complaint.
The disconnect. NEC 422.31 wants a disconnecting means within sight of the heater or a breaker that can be locked open. Most houses rely on the lockable breaker. If the existing install has neither, note it on the ticket for the electrician rather than leaving it unsaid.
The rest of the job. A pan and its drain, a new T&P discharge if the old one is wrong, an expansion tank if the system is closed by a PRV or a check valve, and a permit where your jurisdiction requires one for a replacement. Sizing that tank is 'Expansion Tank Sizing for Closed-Loop Domestic Water Heaters'.
The swap, in order
1. Shut the cold and relieve the pressure. Close the cold valve at the heater, then open a hot tap somewhere in the house. Without a vent the tank drains at a dribble and pulls air through the drain valve in glugs. Skip it and you stand at the drain valve for an hour.
2. Drain to a safe place. Hose from the drain valve to a floor drain or outside, below the tank so it siphons. The water in a heater that was running this morning is at its set point, often 120 F or more, and a hose that pops off at the drain valve sprays it at your legs. Leave the breaker locked off for an hour first if the schedule allows, wear gloves, and clamp the hose. An old plastic drain valve clogged with scale may not flow at all; that is a reason to bring a spare valve, not a reason to force the old one.
3. Disconnect the wiring. With the circuit still locked and proven dead, open the junction box, note which conductor landed where, and take the circuit conductors and ground off the heater's leads. Remove the cable or flexible conduit from the box knockout and dress it out of the way, ends apart.
4. Disconnect the water. Unthread the connectors or cut the lines above the old fittings, and take off the T&P discharge pipe. Cap or plug nothing yet; you need to see whether the cold valve is holding.
5. Get the old one out. A drained tank still holds a few gallons below the drain valve plus the sediment, and a 50-gallon steel tank is a two-person lift on stairs. Use a dolly and a strap, not your back.
6. Set the new heater. In its pan, level, with the connections where the piping can reach them without strain and with the clearances the maker lists. Where seismic strapping is required, it goes on now.
7. Pipe the water. Cold to the port marked cold. That port carries the dip tube, which drops incoming cold water to the bottom of the tank. Swap the two and the cold water enters at the top, mixes with the hot right at the outlet, and the customer calls in a week to say the new heater runs out faster than the old one. Use dielectric separation where copper meets the steel tank, lead-free solder and flux, and fittings rated lead free. Pipe the T&P discharge full size to its termination; the rules are in 'The T&P Discharge Pipe and the Rules Other Trades Break'.
8. Fill completely, then purge the air, before any power. Close the drain valve, open the cold valve fully, and leave the hot tap you opened in step 1 running. It will spit air, then sputter, then run. Keep it open until it runs a steady stream with no spitting for a full minute, then run every other hot tap in the house until each one runs steady.
The reason is physical, not ceremonial. Inside the tank, the dip tube carries the cold to the bottom, and the upper element sits just under the space where the last air collects:
cold in junction box hot out
| [ ] |
+-----|---------------------------|-----+
| | last air collects up here |
| | =====| upper element
| | dip tube |
| | |
| | =====| lower element
| v cold comes out at the bottom |-- drain valve
+---------------------------------------+
An element energized in air has nothing to carry its heat away; the sheath overheats and the element usually opens within the first minute. That is a dead element on a new heater, a second trip and a part you pay for. Filling the tank is not the same as purging it.
9. Wire it. Circuit conductors to the heater's two leads, equipment grounding conductor to the heater's ground screw, connectors sized for the conductors, the cable or conduit clamped at the knockout, cover back on. A loose wire nut on a 19 A load cooks itself and the cover above it.
10. Set both thermostats. Remove the access covers with the circuit still locked, set the upper and lower thermostats to the same value (120 F is the common residential setting), and put the insulation and covers back. Scald protection at the shower is a separate device: the IRC or IPC, as adopted, limits shower and tub outlets to 120 F with a listed valve (ASSE 1016 type at the shower, ASSE 1070 type at the tub filler), not with this dial.
11. Restore and watch it start. Check every joint dry first. Then remove your lock, stand to the side of the panel rather than square in front of it, and close the breaker. A breaker that trips at once means a fault you did not find, often a damaged conductor at the knockout or an element shorted to the tank; turn it off, re-lock it, and go back to step 9 rather than resetting it.
Worked job: a 50-gallon swap on an existing 30 A circuit
The old heater is a 12-year-old 50-gallon unit leaking from the tank. It was fed by a 30 A two-pole breaker on 10 AWG copper with a ground. The new heater's nameplate reads 50 gallons, 240 V, 4,500 W upper and 4,500 W lower, non-simultaneous (only one element on at a time).
The circuit check. 4,500 W divided by 240 V is 18.75 A. Times 1.25 is 23.4 A. The 30 A breaker and 10 AWG conductor carry that with room, so this is a reconnect at the heater and no electrician is needed. Cross-check from the other side: 30 A divided by 1.25 is 24 A of continuous load allowed, and 18.75 A is under it.
The voltage. The meter read 243 V across L1 and L2 at the heater before the old one came out.
What the current should be. An element's resistance comes from its rating: 240 V squared is 57,600, divided by 4,500 W is 12.8 ohms. At the measured 243 V, the element draws 243 divided by 12.8, which is 19.0 A. The nameplate current (18.75 A) is at 240 V; 19.0 A at 243 V is the same element.
After restore. With the junction box cover off as energized diagnostic work (PPE per the shop's 70E assessment, one conductor alone in the clamp jaw, nothing loosened), L1 read 18.9 A with the upper element on. About half an hour later the upper thermostat satisfied and the same conductor read 18.9 A again, now feeding the lower element. That half hour is about right: heating the top 15 gallons or so from 55 F to 120 F is about 8,100 Btu, and 4,500 W is about 15,350 Btu per hour, so a little over 0.5 hours.
What the customer is told. Hot at the kitchen in about half an hour, a full tank in close to two hours. That figure checks: 50 gallons times 8.33 lb per gallon times 65 F of rise is about 27,100 Btu, divided by 15,350 Btu per hour is 1.76 hours. Customers who are not told that run the shower at the 40-minute mark and call it broken.
The finish test
- Hot at a tap. A thermometer in a stream at the nearest hot tap reads within a few degrees of the thermostat setting once the upper element has satisfied.
- Amperage within the nameplate. Each element's current, read at the measured voltage, is within a few percent of the figure the resistance predicts. A reading well under it on one element is a weak or wrong-rated element; zero with the thermostat calling is an open element or a thermostat that never closed, which is 'How to Replace Electric Water Heater Elements and Thermostats'.
- Both elements proved. Seeing the lower element draw current after the upper satisfies is the only proof the handoff between thermostats works. Skip it and a dead lower element shows up as "runs out of hot water" a week later.
- Covers on, dry, written down. Junction box and access covers on before you leave, no weep at any joint after the tank reached temperature, and the voltage, both currents and the breaker size on the ticket.
Where the job forks
- 208 V service. The same 12.8-ohm element on 208 V draws 208 divided by 12.8, which is 16.25 A, and makes 208 times 16.25, about 3,380 W, roughly three quarters of its rating. Recovery takes about a third longer. Order the heater with 208 V elements, or tell the customer in writing before you install.
- A bigger heater than the circuit. A 6,000 W, 240 V nameplate is 25 A, times 1.25 is 31.25 A, which is past a 30 A circuit. That is a new breaker and larger conductors, and it routes to an electrician before the heater is set.
- Simultaneous elements. Some heaters run both elements at once. Add the two element currents before multiplying by 1.25; reading only one element's watts undersizes the circuit by half.
References
- NFPA 70, National Electrical Code, 2023 edition as adopted: 422.13 and 422.31
- NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2) and, for construction, 29 CFR 1926.417
- IRC 2021 Chapters 25 to 33 or IPC 2021, as adopted, for the water heater, relief and discharge requirements; heater manufacturer installation instructions
- See related: Water Heater Changeout: Day of Install; The T&P Discharge Pipe and the Rules Other Trades Break
- See related: Expansion Tank Sizing for Closed-Loop Domestic Water Heaters