Water Heater

[k947]

I'm a little confused with the proper feeder and over current device for an electric water heater. Referring to 2014 NEC (used in my state now) article 422.11(E)(3) states that nonmotor appliance rated over 13.3 amps overcurrent not exceeding 150 % can be used (then you go to next size up rule if it doesn't match standard OVD). But in 422.13 it says that storage type water heaters 120 gal or less are considered a continuous load (which then would require 125% calculation).
Mike Holts book "Understanding the NEC Volume 1 article 90 - 480" in both of the sections listed shows water heaters and both calculations (150% and 125%).

So I am confused, what is the intent, difference of these two sections and the proper calculation for the OCD (and thus feeder) for electric water heaters for the normal dwelling (keeping in mind the standard 4500 watt or the newer 5500 watt units)

5500 Watt/240 V = 22.9 amps (23) X 150% = 34.5 Amp (OCD 35 amp breaker with # 8 wire) based on 422.11(E)(3)

or

5500 Watt/240 V = amps (23) x 125% = 28.75 amps (30 amp breaker with # 10 wire) based on 422.13

Thanks,

K947

 

[Little Bill]

422.11 is for overcurrent protection. (E)(3) is for when the ocp is not listed on the nameplate. It says you can go up to 150 % if the rated current is over 13.3A. However, the amp rating can be calculated from the watts on the water heaters so 422.11(E)(3) would not apply
422.13 is saying to add the 125% for continuous load for sizing the branch circuit.

 

[Dennis Alwon]

Your first equation gives you the max overcurrent protective device however your second equation is all you need for a compliant install.

 

[infinity]

Your first equation gives you the max overcurrent protective device however your second equation is all you need for a compliant install.

I agree and a 30 amp circuit with #10 conductors is more than adequate for a 23 amp load.

 

[k947]

Thanks to all in helping me understand. I really appreciate it and always look to you for good advice

 

[Electromatic]

Watch out for an OCPD size called out by the manufacturer either on the nameplate or in the installation manual. Lots of times they'll state a 25A breaker for a 4500W water heater though you could use a 30A per the 150% rule. I've had inspectors ask to see the manual.

 

[Little Bill]

Watch out for an OCPD size called out by the manufacturer either on the nameplate or in the installation manual. Lots of times they'll state a 25A breaker for a 4500W water heater though you could use a 30A per the 150% rule. I've had inspectors ask to see the manual.

What's wrong with a 25A breaker?
4500/240=18.75A
18.75x125%=23.44A

25A breaker would be fine.

 

[LarryFine]

What's wrong with a 25A breaker?

Availability?

 

[Little Bill]

Availability?

My SH keeps them in stock. But to be honest, they are the same price as a 30A so I keep 30A. I was just pointing out to the poster that said...

"Watch out for an OCPD size called out by the manufacturer either on the nameplate or in the installation manual. Lots of times they'll state a 25A breaker for a 4500W water heater though you could use a 30A per the 150% rule. I've had inspectors ask to see the manual."

that a 25A is ok.

 

[Electromatic]

I didn't mean there was anything wrong with a 25A breaker. It's a standard size and meets the 125% for a 4500W unit.
But also what's really standard is people running #10 off a 30A breaker to a residential water heater. I just wanted to state a friendly warning that a lot of water heaters these days specify a 25A OCPD, and I've had inspectors (rightly so if it is a manufacturer's instruction) insist on the 25.

 

[tortuga]

It comes up fairly often.

 

[kwired]

I didn't mean there was anything wrong with a 25A breaker. It's a standard size and meets the 125% for a 4500W unit.
But also what's really standard is people running #10 off a 30A breaker to a residential water heater. I just wanted to state a friendly warning that a lot of water heaters these days specify a 25A OCPD, and I've had inspectors (rightly so if it is a manufacturer's instruction) insist on the 25.

If instructions say so I think that trumps the 150% rule.

If not for the 150% rule a 4500 watt @ 240 volt unit should be required to have 25 amp breaker IMO.

OP's 5500 watt unit can be on 10 AWG and 30 amp breaker. Can also be on 8 AWG and 35 amp breaker. 240.4(D) would prevent it from being on 10 AWG and 35 amp breaker.