An electric water heater is one of the biggest loads in an American home. It usually runs on 240 volts and can draw more current than everything else in the house except the air conditioner. This post explains how it heats water, what is inside the tank, and how it is wired to the panel.
How an Electric Water Heater Works
Cold water enters the tank through the dip tube, which carries it to the bottom. Two heating elements warm the water, and hot water leaves from the top outlet. Because hot water rises, the hottest water is always at the top, ready to flow to the taps. The tank is insulated and the thermostat keeps the water at the set temperature, usually 120°F.
The Parts Inside the Tank
The tank. A steel tank, usually 40 or 50 gallons in a home, lined with glass to resist corrosion.
Heating elements. Two resistance elements, usually 4500 watts each, one near the top and one near the bottom. They never run at the same time.
Thermostats. One for each element. The upper thermostat controls the upper element first, then hands over to the lower thermostat.
The anode rod. A magnesium or aluminium rod that corrodes in place of the tank, protecting the steel.
The T&P relief valve. The temperature and pressure relief valve. It opens if the temperature or pressure gets dangerously high.
The dip tube. Carries cold water to the bottom of the tank so it heats fully before rising to the top.
Diagram: Cross-section of an electric water heater: dip tube, two elements, thermostats, anode rod and T&P valve.
240-Volt Wiring and the Breaker
An electric water heater is a 240-volt load, so it is fed by a double-pole breaker. A typical 4500-watt heater draws 4500 / 240 = 18.75 amps. The circuit is usually a 30-amp double-pole breaker with 10 AWG copper wire.
Because a water heater can run for three hours or more, the NEC treats it as a continuous load, which may use only 80% of the breaker rating, so a 30-amp breaker can supply at most 24 amps continuously, plenty for 18.75 amps. The circuit needs two hot wires and a ground; there is no neutral, because a pure 240-volt load does not need one.
Thermostats and Elements: How They Take Turns
Only one element runs at a time. This is called non-simultaneous operation. When the tank is cold, the upper thermostat sends power to the upper element first, heating the water at the top so hot water is available quickly. Once the top is up to temperature, the upper thermostat switches power to the lower thermostat, which runs the lower element until the whole tank is hot. Because the elements never run together, the circuit only has to carry one element's current, which is why a 30-amp breaker is enough for two 4500-watt elements.
Diagram: Wiring of an electric water heater: 30-amp double-pole breaker, disconnect switch, thermostats and elements.
The Disconnect
Under NEC Article 422.31, a water heater must have a disconnecting means. If the breaker is within sight of the heater and no more than 50 feet away, the breaker itself can serve as the disconnect. If not, a separate disconnect switch must be installed within sight of the heater.
Key Takeaways
An electric water heater uses two 4500-watt elements that take turns heating the water, controlled by two thermostats. A typical unit draws 18.75 amps at 240 volts on a 30-amp double-pole breaker with 10 AWG wire, and it counts as a continuous load. The T&P valve protects against excess temperature and pressure, the anode rod protects the tank from corrosion, and the NEC requires a disconnect within sight of the heater.
Objective Questions
1. A 4500-watt electric water heater runs on 240 volts. How much current does it draw?
a) 9.4 A
b) 18.75 A
c) 30 A
d) 37.5 A
Answer: (b) 18.75 A
Explanation: Current I = P / V = 4500 / 240 = 18.75 A.
2. What does non-simultaneous operation mean in an electric water heater?
a) Both elements run at the same time
b) Only one heating element runs at a time
c) The heater runs only at night
d) The thermostats never switch on
Answer: (b) Only one heating element runs at a time
Explanation: The upper thermostat powers the upper element first, then hands over to the lower thermostat, so the two elements never run together.
3. What is the usual breaker and wire size for a 4500-watt residential water heater?
a) 15-amp single-pole breaker with 14 AWG wire
b) 20-amp single-pole breaker with 12 AWG wire
c) 30-amp double-pole breaker with 10 AWG copper wire
d) 50-amp double-pole breaker with 6 AWG wire
Answer: (c) 30-amp double-pole breaker with 10 AWG copper wire
Explanation: A 4500 W heater draws 18.75 A at 240 V, and the usual circuit is a 30-amp double-pole breaker with 10 AWG copper wire.
4. What is the job of the anode rod inside the tank?
a) To heat the water faster
b) To measure the water temperature
c) To corrode in place of the tank, protecting the steel
d) To carry cold water to the bottom of the tank
Answer: (c) To corrode in place of the tank, protecting the steel
Explanation: The magnesium or aluminium anode rod corrodes instead of the tank, protecting the steel from corrosion.
5. Under NEC Article 422.31, when can the circuit breaker itself serve as the water heater disconnect?
a) When it is within sight of the heater and no more than 50 feet away
b) When it is a 30-amp breaker
c) When the heater is in the garage
d) When the heater has two elements
Answer: (a) When it is within sight of the heater and no more than 50 feet away
Explanation: Article 422.31 allows the breaker to serve as the disconnect if it is within sight of the heater and no more than 50 feet away; otherwise a separate disconnect switch is needed.
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