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Code Q&A: Electric Water Heaters

Sept. 25, 2014

NEC guidelines on sizing branch circuit conductors for water heaters

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.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.