Watt, Amp & Volt: Jargon-Free Guide to Water Heater Wiring

A high school physics teacher was telling me that in her experience, the topic that students struggle with the most is electricity. For many of us, the confusion about electricity doesn’t end with high school, and wiring an electric water heater as an adult can still be a challenge.

The basic electrical requirements of water heaters may be described in terms of wattage, voltage and amperage. A water heater’s power rating label gives its required volt input and maximum watt output. Amperage, or current drawn by the unit may be calculated by dividing wattage by voltage.

But what’s the difference between a volt and an amp? And how are the two different from a watt? Why do you need to know all this, just to wire a simple water heater? If you find jargon to be a pain, and definitions like, “voltage, also called electromotive force, blah blah…”, give you a headache, then this guide is just for you.

In less than the time it takes to down a cup of coffee, you’ll know all the concepts you need to wire a water heater without anxiety. Plunge in!

Electricity Simplified: Volt, Amp, Watt  

You may already be familiar with the metaphor of flowing water, used to explain electricity. But unless the explanation gave you a clear understanding of the three important terms – volt, amp, and watt, it may have left you high and dry. So, let’s work with that analogy a bit more to make these terms easier to understand.

We can think about the electricity going through a wire similarly to water flowing through a pipe. To really get the concepts of voltage, amperage, and wattage, imagine the following scenario:

Think of a hill with a reservoir at the top. At the bottom of the hill is a garden, with a pond containing a water-wheel. The water from the reservoir is let out through a gate, into a pipe going downhill, all the way to the garden below. The end of the pipe is positioned exactly over the water-wheel so that when water flows out, it turns the wheel.

You can already see that:

• The force of water reaching the garden from the reservoir depends mainly on two things: (i) the amount of water in the reservoir, and (ii) the girth or diameter of the pipe.
  
• Voltage can be compared with the amount of water stored in the reservoir. The more the water in the reservoir, the greater the amount of energy it possesses. Similarly, the higher the voltage of the electricity supply source, the greater the amount of energy it has.
  
• Amperage is like the flow of water through the pipe, which depends on the diameter of the pipe; a larger pipe allows more water to flow, while a smaller pipe restricts the amount of flow. Likewise, the amp rating of electrical wires depends on their thickness. Thicker wires allow more current to flow through them than thinner wires.*
  
• Wattage can be thought of simply as the work done by electricity. Going back to our example, water from the reservoir flows through the pipe and turns a water-wheel. The turns or rotations per minute (RPM) of the water-wheel can be used to measure the amount of work done by the water. In the same way, the wattage rating on a water heater tells you the amount of work done by electricity as it heats the water.

*Keep in mind, though, that the thickness of electrical wires is typically denoted by their AWG or American Wire Gauge rating. Smaller AWG numbers denote thicker wires, while larger AWG numbers denote thinner wires.

Making Sense of Electricity in Wiring a Water Heater

The previous section helped us translate the three most important electrical terms, voltage, amperage, and wattage, into concepts that we can actually relate to. Now let’s put that information into context, and understand how each of these aspects of electricity plays out in the real-world, while wiring your electric water heater.

What do you need to know to get your appliance working perfectly? What are the precautions you need to take to ensure that your wiring project goes off safely and smoothly? Let’s find out…

1. Voltage

• A standard, dual-element electric water heater typically requires a 240-volt electricity supply.
  
• Because electricity supplied to American homes comes in at 120 volts, a special, ‘double leg circuit’ is usually set up for a water heater, so that the two legs make up 120V + 120V = 240V.
  
• If you install a 240-volt water heater on an ordinary 120-volt circuit (the kind that has a single circuit breaker on the mains panel or breaker box), it will either not work at all, or will not be able to heat water to the desired temperature.
  
• Conversely, installing a 120-volt (single-element) water heater on a 240-volt circuit will cause the appliance to blow a fuse (or pop the ECO button), and may even permanently damage the unit.
  
• When using a multi-meter to measure the voltage supplied to your water heater, you might see a number like ‘238’ or ‘241’. For a 240-volt circuit, this is normal fluctuation. Generally speaking, a couple of volts on either side is okay, but it’s best to avoid an overvoltage situation because excessive voltage can damage your appliance.

2. Amperage

Circuit Breakers:

• Each circuit breaker on your home’s mains panel (breaker box) has an amperage or amp rating embossed on it.
  
• Your water heater is typically on a dedicated circuit with a double-pole circuit breaker (which looks like two singles tied together). The entire water heater circuit is typically rated for 30 amps, though each individual breaker may have ‘30A’ embossed on it.

Electrical Outlets:

• A standard, dual-element water heater is typically wired directly to an incoming power supply cable, usually a 10/2 AWG cable coming through a metal flex line, whereas a compact water heater (point-of-use unit) might be plugged into an electrical outlet. 
  
• When plugging a water heater into an electrical outlet, make sure that the outlet is rated at or above the amp rating for the appliance. For example, you can use a 20-amp outlet for a water heater rated 15 amps, but not the other way around. (A 20-amp water heater plugged into a 15-amp outlet will cause the wiring to overheat and melt, and could even cause a short-circuit and start a fire!)

Wiring:

• As explained previously, the AWG number of a wire indicates its amp rating, with smaller numbers denoting higher current carrying ability (termed ‘ampacity’).
  
• A standard, dual-element electric water heater is typically wired using a 10/2 AWG cable.**
  
• Wires with higher amp ratings may be used for an appliance with a lower amp rating, but not vice versa. For example, a wire rated 50 amps may be used to connect a 30-amp water heater,  but using wire rated 20 amps with a 30-amp water heater will cause overheating and possibly, a fire.

**Keep in mind that the amp ratings described here are for standard copper wire used in modern circuits. Some older homes constructed in the 1960’s or 1970’s may have aluminum wiring with different amp ratings. If you live in an old structure, be sure to check for relevant standards.

3. Wattage

• The wattage rating for a standard, dual-element water heater is provided on a label near the top of the water heater.
  
• When replacing either heating element, ensure that the new one is rated for the same wattage as the old one. (Replacing a heating element with one that has a higher wattage, for example, replacing a 3500-watt element with a 5500-watt element will cause the element to draw too much current, resulting in overheating that could potentially start a fire.)
  
• Some experts suggest switching out the lower element in a standard water heater for one that has a lower wattage rating (for example, replace a 4500-watt element with one rated 3800 watts). The new element will heat water slower, but will also likely last longer.

Now that you feel right at home with the electrical requirements of a water heater, you could go ahead and get a start on wiring yours. (Also, readers love our step-by-step guide to wiring an electric water heater.)

Water Heater Electricals: FAQs

Is my hot water heater 208 or 240 volts?

Many residential water heaters’ power rating labels read, “240V 208V”. This means that the heater will work on both, 240-volt, and 208-volt circuits (found in homes with a three-phase electrical supply). Do NOT connect a water heater to a higher voltage supply than mentioned on its power rating label (like a 277-volt circuit), as it will burn out.

Can you use a 50-amp breaker for a water heater?

The safe amp rating for a circuit exceeds 125% of its amperage (amount of current drawn), calculated by dividing wattage by voltage. For a water heater rated 4500 watts & 240 volts, 4500W ÷ 240V = 18.75A. 125% of this works out to 23.44A. So, anything over 24A is safe, like a 30- or 50-amp breaker.

Can I replace my 4500-watt water heater with a 5500-watt water heater?

A circuit with 10/2 AWG wiring and a 30-amp circuit breaker can be safely used to run a 5500-watt water heater. However, a circuit with 12/2 AWG wire and a 20-amp circuit breaker violates NEC requirements for 5500W water heater wiring and poses a serious fire hazard.

The wattage rating of a water heater determines how quickly the heating element can heat water. Typically, a water heater rated 5500 watts has a more powerful heating element that will produce more hot water, faster than one rated 4500 watts. 

Technically, a dedicated, double-pole 240-volt circuit rated for 30 amps that serves a 4500-watt water heater should be adequate for a water heater rated 5500 watts. However, be warned that in older homes, the wiring may conform to earlier standards. 

For example, an older, 4500W water heater may be on a 20-amp circuit breaker, with 12/2 AWG wiring which is likewise rated 20 amps. Such a circuit will not support a 5500 watt water heater.

In case you’re interested, here’s the math: A 5500-watt water heater will draw 5500W ÷ 240V = 22.92A, that’s roughly 23 amps of current through the wires. The recommended ampacity for this circuit should exceed 125% × 22.92A = 28.65A. The modern 240-volt, double-pole circuit uses a standard 10/2 AWG cable (which is rated 30 amps) as well as a 30-amp circuit breaker, both of which exceed the 29-amp requirement.