25 or 30 Amps?

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kbsparky

kbsparky

Senior Member
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Delmarva, USA
When dealing with something as common as a residential water heater, there can be some confusion as to how much overcurrent protection is needed.

Case in point: Take your standard 4500 watt electric water heater. Operating at 240 Volts, it requires 18.75 Amps. On the surface, that would appear to require a 20 amp circuit. But wait!! Doesn't the Code require a continuous load to be figured at 125% capacity when determining feeder and branch circuit sizing? That translates into 23.44 Amps, more than your typical 20 Amp circuit can supply.

Mobile and modular home manufacturers have responded by installing 25 Amp breakers in their panels to satisfy this load and requirement.

For years, I've always used a 30 Amp circuit with a #10 AWG wire for such a load. It's just the way we have always done it one might assume.

But am I required to use a 25 Amp breaker? Or does it not make any difference if I choose to install a 30 instead?
 
Baring any specific instructions from the manufacturer stating otherwise, 422.11(E) is your solution (and what you install is a good design, as it allows expansion to a 5500W heater). It allows the following:

Single Nonmotor-Operated Appliance. If the branch circuit supplies a single non?motor-operated appliance, the rating of overcurrent protection shall:
(1) Not exceed that marked on the appliance;
(2) Not exceed 20 amperes if the overcurrent protection rating is not marked and the appliance is rated 13.3 amperes or less; or
(3) Not exceed 150 percent of the appliance rated current if the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.
 
kbsparky said:
When dealing with something as common as a residential water heater, there can be some confusion as to how much overcurrent protection is needed.

Case in point: Take your standard 4500 watt electric water heater. Operating at 240 Volts, it requires 18.75 Amps. On the surface, that would appear to require a 20 amp circuit. But wait!! Doesn't the Code require a continuous load to be figured at 125% capacity when determining feeder and branch circuit sizing? ?
Click to expand...

This seems to be a common mistake in a lot of installs. My brother in law is a HI in Jersey. He sees this in most of his inspections. I haven't seen it much in my area.
 
Use 422.13 for the minimum: 4500 / 240 x 125% = 25A breaker.

Use 422.11(E) for the maximum: 4500 / 240 x 150% = 30A breaker.

Either would be compliant...
 
JES2727 said:
Where does the code specify that an electric water is considered a continuous load?


John
Click to expand...

Article 100 (I). The water heater decides if it runs for three hours or more straight. Do laundry all day, and that water heater is going to be running a bit more than 3 hours straight. Also, 422.13 says it explicitly.
 
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JES2727 said:
Where does the code specify that an electric water is considered a continuous load?


John
Click to expand...
Come to my house,4 females and 2 males and washer going constantly.A look at my electric bill will tell you it`s a continuous load :)
 
422.13 Storage-Type Water Heaters
A fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load.
 
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