Tank & Breaker
Reviewed

Written by the Tank & Breaker editorial teamReviewed by Eric Jackov, plumbing & heating professionalReviewed Oct 20, 2026Fact-checked against 5 sourcesUpdated

How-to

Electric Water Heater Installation

Short answer

An electric water heater installation comes down to four jobs: set the tank in a pan, connect the cold and hot lines and the relief valve piping, fill the tank completely, and only then connect and switch on the power. Get the order wrong and you'll burn out an element in seconds. Replacing an electric tank with the same size and voltage on an existing circuit is the friendliest water heater swap for DIY; new circuits and panel work belong to a licensed electrician.

Before you start

Check three things on the old tank's data plate and the new one's: voltage, element wattage and tank size. A typical residential electric tank like A.O. Smith's ProLine ENS-40 runs on 240 volts with 4,500-watt elements. If the new heater matches the old one, the existing circuit usually works. If it doesn't match, stop and get an electrician to check the breaker and wire size before you buy.

Then check the rules. A.O. Smith tells you to check with local and state authorities for codes that apply, and where there are none, follow NFPA 70 (the National Electrical Code) and the International Plumbing Code. Many areas want a permit for a water heater swap. Your local building department can tell you.

Electric tanks don't need venting, which is the main reason they're simpler to install than gas.

Tools and parts

  • Garden hose, bucket, towels
  • Pipe wrenches, tubing cutter, adjustable wrench
  • Non-contact voltage tester and a multimeter
  • Flexible water connectors or copper with dielectric unions, which A.O. Smith recommends to help prevent corrosion
  • T&P relief valve (if not factory installed) and copper or CPVC discharge pipe
  • Drain pan sized for the tank
  • Thermal expansion tank if your system needs one
  • A listed strain relief or conduit connector for the supply cable
  • Appliance dolly and a helper

How to install an electric water heater

  1. Switch the heater's breaker off. Open the junction box cover on top of the old tank and confirm with a tester that both hot wires are dead.

  2. Shut off the cold water to the heater and open a hot tap somewhere in the house.

  3. Drain the old tank with a garden hose. Sediment can clog the valve. Our how to drain a water heater page has tricks for a slow drain.

  4. Disconnect the supply wires and note which went where. Disconnect the water lines.

  5. Move the old tank out. Set the drain pan, then the new tank, level and with room to reach the element covers later. A.O. Smith says the pan must be at least two inches wider than the heater's diameter.

  6. Connect the cold supply to the inlet and the hot line to the outlet. Don't remove the heat traps, and don't solder while pipes are attached to the tank.

  7. Pipe the T&P relief valve. The discharge pipe needs at least a 3/4-inch inside diameter, sloped to drain, ending a maximum of six inches above a floor drain or outside the building. Never cap or plug it.

  8. Add the expansion tank on the cold line if required. A.O. Smith notes codes require a properly sized, properly pressurized one in almost all homes.

  9. Fill the tank. Open the cold supply and a hot tap and let the water run full stream for three full minutes to purge the air. Check every joint for drips.

  10. Connect the wiring. Bring the cable into the junction box through the strain relief, connect the two supply conductors to the heater leads, and the ground to the green ground screw. Replace the junction box cover.

  11. Turn the breaker on. Set the thermostats. A.O. Smith ships its heaters at about 120°F to reduce scalding risk. The first full heat-up takes a while, so give it time before you judge it.

Tap a number to tick off a step. 0 of 11 done.

Wiring, breaker and wire size

This is the part people ask about most. A.O. Smith's manual calls for 10 gauge solid copper wire and tells you to confirm the home's voltage, wire size and breaker rating against the data plate. The circuit is sized from the heater's wattage under the NEC and local code, and a dedicated circuit is the normal setup.

If you're adding a new circuit, upsizing to a bigger tank or moving the heater, that's electrician work. HomeGuide puts a new dedicated electrical circuit at $250 to $900, and a panel replacement at $850 to $1,700 if the panel can't handle it.

Mistakes to avoid

  • Powering a dry tank. A.O. Smith says dry firing, powering the tank before it's full, can burn out the upper heating element. Flip the breaker after the three-minute purge, not before.
  • Trusting the breaker label. Panels get mislabeled. Test the wires.
  • Running the relief discharge into the pan. Pipe it to a drain or outside, per the manual.
  • Leaving house pressure high. Most codes allow a maximum incoming pressure of 80 psi, and the manual recommends no higher than 50 to 60 psi.
  • Skipping dielectric unions on copper. Galvanic corrosion at the nipples is a slow leak waiting to happen.

Switching from gas to electric

It can be done, but it's more than a tank swap. You need a new dedicated circuit, a capped gas line and usually a sealed-off vent. HomeGuide lists a gas-to-electric conversion at $250 to $1,000 or more on top of the heater. Weigh a heat pump model while you're at it, since you're running new wiring anyway.

When it's a pro job

New circuits, panel upgrades, a different voltage, a tank in a tight attic, or any time the old wiring looks scorched. HomeGuide puts standard tank installation labor at $200 to $1,000, and Fixr's installed range for an electric tank is $500 to $3,500. For full numbers, see water heater installation cost.

FAQ

How do I install an electric hot water tank?

Set it in a pan, connect the water lines and the relief discharge, fill it completely, and only then connect power and switch on the breaker. A.O. Smith says to run a hot tap full stream for three minutes before powering it up.

How do I replace an electric water heater?

Turn off the breaker and water, drain the old tank, disconnect the wiring and pipes, and then install the new one using the steps above. Match the voltage and wattage to the old unit, or have an electrician check the circuit. Our water heater replacement guide covers the haul-away and permit side.

What wiring code applies to an electric water heater?

The NEC and your local electrical code, plus the heater's own data plate. Breaker and wire size depend on the wattage. If you're not sure, a licensed electrician should size the circuit.

Can I install an electric water heater myself?

Many homeowners do, especially a like-for-like swap. Check whether your area requires a permit, and follow the manual's order: water first, power last. If the circuit needs changes, hire an electrician.

Sources

Trade reviewReviewedReviewed by Eric Jackov, plumbing & heating professional with Plumbing Cor Inc, Bayside, NY, on Oct 20, 2026.
Updated

Eric works under NYC Master Plumber License #2128, held by John D'Auito (Tru-Flo Mechanical, Inc.), NYC Department of Buildings. Verify on the DOB list (opens in a new tab)

The numbers on this page come from the sources listed above. How we research and check pages