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What Size Wire Do I Need for a 4500 Watt Water Heater?

Fact checked by Stephen Conklin

what size wire do i need for a 4500 watt water heater

It’s not unusual for greenhorn water heater owners to ask, “What size wire do I need for a 4500 watt water heater?” You risk electrical fires and other disastrous consequences if you don’t pick the correct wire size for an appliance.

The ideal wire gauge for 4500 watt electric hot water heaters is 10 American Wire Gauge (AWG), although some say a 14 AWG cable is enough.

Please continue reading to learn the more accurate answer.

Table of Contents

Determining Wire Size for a 4500-watt Water Heater

Two schools of thought exist about the correct gauge wire for water heater units with 4500-watt elements.

A 10/2 wire (10 AWG cable with two wires) is sufficient for a 4500-watt electric water heater. On the other hand, some water heater experts recommend a 14 AWG wire for such an appliance.

So, how do you pick the correct wire for hot water heater units with a 4500-watt rating?

1. Water heater amp draw


A parameter you should assess when choosing the correct 4500 watt water heater wire size is the amp draw or amperes (current) the water heater pulls from the circuit and moves along the wires.

Larger-diameter wires can carry more electricity than small-diameter options.

Folks with limited wire size knowledge tend to assume that the higher the cable’s AWG value, the thicker or larger its diameter. The correct interpretation is the reverse: wire diameter increases with a decrease in AWG values.


For instance, a 6 AWG wire has a diameter of 0.162 inches, while a 14 AWG cable has a 0.0641-inch diameter.

2. Wire size or AWG wire ampacity

A closely related parameter to water heater amp draw is the wire’s ampacity or amperes the cable can “safely” accommodate.

Although the calculation for these figures is too technical, you can observe one rule. The wire’s ampacity should always be greater than the device’s amp draw.

We created a table for you to understand the relationship between wire sizes and ampacity. Moreover, we took the liberty of recommending water heaters for such wire sizes.

Wire Size Wire Ampacity Recommended for
6 AWG 65 amps >100-gallon water heaters
8 AWG 50 amps 90 to 100-gallon water heaters
10 AWG 35 amps 90 to 100-gallon water heaters
12 AWG 25 amps 90 to 100-gallon water heaters
14 AWG 20 amps 30- to 90-gallon water heaters
16 AWG 17 amps 10- to 20-gallon water heaters
18 AWG 14 amps 10- to 20-gallon water heaters

Note that these are estimates, and we wouldn’t usually generalize water heater amperage requirements based on their capacities. Still, water heater sizes can help minimize the guesswork when complying with wiring requirements.

For example, larger-capacity water heaters (i.e., 90 to 100 gallons) might require 10 to 12 AWG wires. On the other hand, a 14 AWG cable is enough for a 50 gallon water heater.

But what if you want a more conclusive wire size? In that case, you might want to brush up on your math.

How to Calculate Wire Size


The formula we share relates to determining the correct breaker size for water heaters. However, the calculations should still be valid because these wires connect the water heater to the circuit breaker.

Here’s the formula.

\[ \text{Wire size (in amps)} = \left( \frac{\text{watts}}{\text{volts}} \right) \times 1.25 \]

Our water heater runs on 4500 watts at 240V. Dividing 4500 watts by 240 volts will give us 18.75 amps. We must multiply 18.85 amps by 1.25 (or 125%) to produce 23.4375 amps.

Using the table above, the closest wire ampacity to 23.4375 amps is a 12 AWG wire.

If you have a 220V hot water heater, you can substitute it for the 240 volts and get 20.45 amps. Multiplying this by 1.25 gives us 25.56 amps.

Although 12 AWG would still be somewhat adequate, its ampacity (25 amps) is lower than the water heater’s amp draw. Hence, we can get the next higher-value AWG cable – the 10 AWG.

The question is why you must multiply the ampere value by 1.25 or 125%. This rule underscores the National Electric Code guideline, requiring circuit breakers to cap their load capacity to 80% of their amperage ratings.

You can use 1.0 or 100%, but most electricians are more comfortable capping the load at 125%.

Factors to Consider When Choosing Wire Gauges for Water Heater


You can also use the above formula to identify the wire size of a tankless water heater. But, what other factors should you consider when choosing water heater wire gauges?

1. Voltage requirements

Although 4500-watt water heaters require the same power, they might differ in voltage needs.

For instance, most units run on 240-volt systems, while others are 220 volts. The water heater’s voltage requirements impact the wire size by affecting the amperage value.

2. Wire material

Most electricians working on residential projects recommend copper wire over aluminum materials because it conveys electrical currents better. Copper is also more robust, allowing installers to pull the wire to the hilt without breaking.

On the other hand, if you’re on a tight budget, aluminum wires should be more suitable. These cables are also lighter than copper, making them ideal for DIY projects.

3. Circuit breaker capacity or size

As mentioned, wires connect the water heater to the home’s electrical circuit breaker panel. Ideally, your water heater wire gauge should match the breaker’s amperage rating.

For example, a 25-amp water heater needs a 31.25-amp breaker after applying the 125% NEC rule. The nearest wire ampacity to a 31.25-amp breaker is 35 amps for a 10 AWG cable.


How long should the wire be for a 4500 watt water heater?

The wire should be long enough to connect the 4500-watt water heater to the residential electric circuit breaker panel. It’s worth noting that cable length doesn’t affect amperage but can produce voltage drops by increasing resistance. However, the rule of thumb is to increase the wire size by one for every 100 feet.

Installation of wire for a 4500 watt water heater

Wiring a 4500-watt water heater involves the following steps.

  1. Switch off the home electrical circuit panel.
  2. Access the wiring connections by opening the electrical junction box.
  3. Connect the right size wire to the electrical circuit breaker and link the other end to the water heater.
  4. Use wire nuts to link cables with identical colors (i.e., black with black and red with red).
  5. Secure the ground cable to the junction box’s green screw.

We recommend checking your water heater manual’s electrical connection instructions for more detailed and device-specific information.

What happens if I choose the wrong wire size for a 4500 watt water heater?

Choosing the wrong size wire for a 4500-watt water heater can produce disastrous consequences. Too small, and you can say goodbye to your property as electrical fire consumes it. Too large in comparison to the circuit breaker, and you can have a frequently tripped circuit.


Determining what size wire do I need for a 4500 watt water heater isn’t as complicated as many beginner water heater owners imagine. If we adhere to the NEC guideline, a 4500-watt, 220-volt water heater will work best with a 10 AWG cable with a 35-amp ampacity.

On the other hand, factoring in a 240-volt heater and disregarding the 125% NEC rule shows that a 14 AWG cable is sufficient. After all, its 20-amp wire ampacity adequately covers the 18.75-amp draw.

If you’re still confused, consult a professional to look at your water heater setup.

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