422.11 E 3

[Greg1707]

I saw this question in the EC Magazine:

Example Problem: What’s the maximum size overcurrent protection device for a 4,500W, 240V water heater?
Solution: Protection Size = 4,500W ÷ 240V = 18.75A × 150% = 28A
Answer: 28A doesn’t correspond to a standard ampere rating, so use 30A which is the next size up [Sec. 240.6(A)].

I was not familiar with this part of the code. I would have approached this question from the idea of what would be the minimum over current protection for a water heater. I thought the installation of a water heater would be: 18.75 amps x 125% = 23.4 amps and use a 30 amp breaker because that is what is available at the supply house!

 

[Dennis Alwon]

You can use a 25 amp breaker. Our supplier stock it

 

[Dennis Alwon]

Max is 150% so that is 30 amps since 28 is not a standard size

 

[mopowr steve]

Where do you get the 150% from?
Water heaters under what150 gallons are figured at 125% continuous load.
18.75 x 125% = 23.4A 25A Breaker

Sorry I see you were talking about MAXIMUM allowable circuit.

 

[LarryFine]

This has been debated a bunch; the consensus is that you can use either a 25a or a 30a breaker, and must use #10.

 

[infinity]

This has been debated a bunch; the consensus is that you can use either a 25a or a 30a breaker, and must use #10.

Yup that's it, no need to make it any more complicated than that.

 

[Greg1707]

I understand the idea of a maximum OCP for motors. But what is the issue regarding a water heater? What would be the problem of installing a WH on a 40 amp circuit?