When wiring a water heater, you have to nail the breaker size. If you don’t adequately determine what size breaker you need for your water heater, the whole circuit can go up in flames. That’s why we will explain exactly how to size a water heater breaker. You can also consult the water heater breaker size chart further on for 10-100 gallon water heaters.
Namely, for sizing a water heater, we will need to account for two factors:
- Water heater amp draw. Water heaters can use as little as 8.3 amps to above 40 amps to run. Most water heaters will draw 18.8 amps (this is for 4500 watt 240V heaters in 30-gallon, 40-gallon, 50-gallon, 60-gallon, 70-gallon, and 80-gallon water heaters). You can check out how many amps do water heaters draw here.
- NEC 80% rule. According to the National Electric Code (NEC), you can load the breaker up to only 80% of its maximum amperage. Example: You can only put a 16 amp water heater on a 20A breaker because 16A is 80% of the maximum 20A breaker capacity.
In the US, we mostly use 15A, 20A, and 30A breakers. Most water heaters (4500 watt heaters) will use 18.8 amps to run at 100% output. This doesn’t mean that you can use a 20A breaker for most water heaters. That’s because we also need to apply the 80% NEC rule like this:
Breaker Size (18.8 Amp Heater) = 18.8 Amps / 0.8 = 23.5 Amp Breaker Or Bigger
As you can see, for most water heaters we will need an amp breaker with at least 23.5 amps. 23.5A breaker would be perfect but we don’t have breakers in that size. Instead, we most commonly use a 30 amp breaker for 4500 watt water heaters.
Obviously, the size of the amp breaker also depends on the hot water heater gallon capacity (and corresponding water heater wattage and voltage).
Here is the full chart of how big amp breaker you need for different water heater sizes:
Water Heater Breaker Size Chart
|Water Heater Size:||Breaker Size:|
|10 Gallon Water Heater||20 Amp Or 30 Amp|
|20 Gallon Water Heater||20 Amp Or 30 Amp|
|30 Gallon Water Heater||30 Amp Breaker|
|40 Gallon Water Heater||30 Amp Breaker|
|50 Gallon Water Heater||30 Amp Breaker|
|60 Gallon Water Heater||30 Amp Breaker|
|70 Gallon Water Heater||30 Amp Breaker|
|80 Gallon Water Heater||30 Amp Breaker|
|90 Gallon Water Heater||30 Amp Breaker Or Bigger|
|100 Gallon Water Heater||30 Amp Breaker Or Bigger|
As you can see, most water heaters will require a 30A breaker. For example, a 40-gallon hot water heater requires a 30A breaker because it uses a 4500 watt 240V heater. The same is true for 30-gallon, 50-gallon, 60-gallon, 70-gallon, and 80-gallon water heaters.
The smallest 10-gallon and 20-gallon hot water heaters require 20A or 30A breaker, depending on the heater size. The most common small water heater wattage is 1600 watts which runs on a 120V circuit. This means it draws 13.3 amps.
You can’t use the small 15A breaker here because we still have to apply the NEC 80% rule. Accounting for this, we need a breaker with at least 16.6 amp capacity. The 20A breaker will be perfect for most of these smaller water boilers.
Bigger 80+ gallon water heaters can require bigger breakers. These heaters can have a wattage far above the standard 4500 watts; up to even 10,000 watts. At 10,000 watts, for example, you will need a breaker that can handle at least 52.1 amps. In this case, two 30A breakers are the go-to choice; they have a combined capacity of 60 amps.
All in all, most standard 4500 watt water heaters will require a 30A breaker. For smaller hot water heaters, a 20A breaker might just be enough. For bigger heaters (100-gallon, for example), the go-to breaker size is double 30A.