Q. How are the branch circuit conductors and overcurrent protection sized for electric water heaters (120 gal or less)? Please provide an example.
Find the Answer
A. An electric water heater having a capacity of 120 gal or less is considered a continuous load, for the purpose of sizing branch circuits [422.13]. The conductors are sized per 422.10 and overcurrent is sized per 422.11(E)(3).
Let’s work through an example so you see how to make these calculations.
What’s the calculated load for conductor sizing and overcurrent protection for a 4,500W, 240V water heater, using 75°C rated terminals?
We start with the formula: I = P ÷ E
In our example, we know P = 4,500W and E = 240V.
Therefore, I = 4,500W ÷ 240V = 18.75A (continuous load)
Conductor Sizing = 18.75A x 1.25 = 23.44A [422.10(A)]
As per Table 310.15(B)(16), 10 AWG conductor is rated 35A
Overcurrent Protection = 18.75A x 1.50 = 28A [422.11(E)(3)]
As per 240.6(A), the next size up circuit breaker is 30A.