water heater breaker size

[kwired]

Absolutely. We used to be required to have a 25 amp breaker but I am wondering if there was a rule change to allow overcurrent protective device at 150% and allowing next size up rule.

Good question. I looked in 1987 NEC and not seeing this 150% rule. Though I know we were installing 30 amp breakers on nearly all water heaters at that time and was happening long before then as well.

But at same time I wasn't finding anything prohibiting say a 40 or 50 amp circuit either, but not many would run the larger conductor that would be necessary to do so, so 30 amps just becomes the normal since the conductor can be protected by 30 amp device.

 

[ramsy]

Jump to 6000 watts and.. you need more than 10 AWG and min of 35 amp breaker..

Where does code prohibit 30A OCP on 6000w/240v=25A load, or on #8cu wire?

 

[mbrooke]

How does #12 work on a 25 amp breaker?

#12 is good for 20 amps at 60*C.

Typically a run from the panel to the water heater does not exceed 75 feet.

#12 is about 0.002 ohms per foot, so 0.3 ohms total at 75 feet.

120volts/0.3 ohms= 400 amps of ground fault current.

400 amps opens both a 25 amp and 30amp breaker in 2 cycles or 0.03 seconds.

According to 240.92 B, it takes 0.92 seconds to raise #12 being from 60*C to 150*C.

Thus IMO we can conclude that the #12 conductor is protected from thermal damage.

How I humbly view it

 

[Dennis Alwon]

Where does code prohibit 30A OCP on 6000w/240v=25A load, or on #8cu wire?

Because you need to multiply by 125% for continuous loads. A water heater is considered a continuous load-422.13

 

[ramsy]

Thank you. Found 210.20(A) & 230.208(B) for sizing OCP 125% of continuous loads, except where listed for 100% of its rating.

Found UL 489, and DIVQ prohibits loads that exceed 80% of molded case circuit breaker rating, unless otherwise marked.

Example of a 100% rated circuit breaker marking:

https://library.e.abb.com/public/dfd81f0d5f1f4f09b89a42b4b67d1fc4/61914333_circuit_breaker_ratings3-5.pdf

Suitable for operation at 100% of rating only if used in a cubicle space 520 x 420 c 200 mm and where used with 90°C wire. The wire size shall be based on the ampacity of 75°C rated wire.

 

[Frank DuVal]

Can you image the demand on the power company if all water heaters used 9000 watts instead of 4500 watts.

I imagine it every winter in heat pump territory when the 10 kW strip heaters come on. Most houses around here have TWO 10kW heat strips, because they have two heat pumps.

I have two 54 kW water heaters at work for a project. Gets 100 gallons of water to 180 ° F really fast! Yes, 480 volt three phase. Twelve 4500 watt elements and thermostats per water heater.

I do not recall having to inform the power company when I turn on both ovens in the kitchen...

 

[ramsy]

Please disregard post #15's use of 20A breaker.

To answer the original post #1, while 20A meets the requirement of 422.11(E), for a continuous load per 422.13 the (4500w / 240v) = 18.75A load on is way over 80% of 20A OCP rating.

UL requires OCP to have special marking & enclosure conditions listed to exceed 80% of its rating, under continuous loads.

 

[kwired]

I imagine it every winter in heat pump territory when the 10 kW strip heaters come on. Most houses around here have TWO 10kW heat strips, because they have two heat pumps.

I have two 54 kW water heaters at work for a project. Gets 100 gallons of water to 180 ° F really fast! Yes, 480 volt three phase. Twelve 4500 watt elements and thermostats per water heater.

I do not recall having to inform the power company when I turn on both ovens in the kitchen...

15, 20 and 25 kW heat strips are more common here than 10 kW.

But on demand water heaters are even higher kW for a whole house unit, probably going to be 30+ kW most the time. If on your own well you might be trying to raise the water temp from 55F to ~110-115F. Takes a lot of instantaneous kW if you want any flow rate to speak of.

 

[EngineerPlayinElectrician]

Absolutely. We used to be required to have a 25 amp breaker but I am wondering if there was a rule change to allow overcurrent protective device at 150% and allowing next size up rule.

It looks like the rule change was in 2020 NEC 422.13. The new code, instead of reading that we have to treat a water heater as a continuous load, says to size the "branch-circuit overcurrent device and conductors . . . not smaller than 125 percentage of the rating of the water heater". So this just puts a minimum on the conductors AND the breaker. So 422.13 is minimum for breaker with 422.11(E) as the maximum.

 

[Dennis Alwon]

It looks like the rule change was in 2020 NEC 422.13. The new code, instead of reading that we have to treat a water heater as a continuous load, says to size the "branch-circuit overcurrent device and conductors . . . not smaller than 125 percentage of the rating of the water heater". So this just puts a minimum on the conductors AND the breaker. So 422.13 is minimum for breaker with 422.11(E) as the maximum.

That is not what I was talking about. I was thinking section 422.11 (E) which limits us to 150% of the appliance rating.

The section you mentioned may have changed the wording but it is essentially the same. A continuous load must be figured at 125% for overcurrent protective device and conductor so it is the same calculation as it was before.

 

[hillbilly1]

Up until the past 10-20 years, a 25 amp breaker was basically non-standard, so the next size up was allowed. Can’t remember ever seeing a 4500 watt WH on a 25 amp breaker. Must be a local thing.

 

[david luchini]

Up until the past 10-20 years, a 25 amp breaker was basically non-standard, so the next size up was allowed. Can’t remember ever seeing a 4500 watt WH on a 25 amp breaker. Must be a local thing.

25A was a "standard" OCPD size in the 1968 NEC, which is the earliest version on the NFPA website.

 

[Dennis Alwon]

Up until the past 10-20 years, a 25 amp breaker was basically non-standard, so the next size up was allowed. Can’t remember ever seeing a 4500 watt WH on a 25 amp breaker. Must be a local thing.

What does basically mean? If it was made then you would have to use it. I know Durham, NC enforced a 25 amp overcurrent protective device. I am guessing that code section changed to allow 150% of the rating which would then allow 30 amps.

I wrote this yesterday but forgot to submit it. haha

 

[electrofelon]

Ok for more fun, how about a 4500W (@240) WH used on 208 volts. Please discuss conductor and ocpd sizing

 

[Dennis Alwon]

My guess is #12 on a 20 amp overcurrent protective device .

208 is approx 75% less wattage. If I worked it out closer it may require #10 with 25 amp or a 30 amp overcurrent protective device .

 

[electrofelon]

My guess is #12 on a 20 amp overcurrent protective device .

208 is approx 75% less wattage. If I worked it out closer it may require #10 with 25 amp or a 30 amp overcurrent protective device .

So it comes out to 16.25A X 1.25 is 20.3125. what I don't remember, is there an allowance for dropping the fraction or do I have to round up to 21?

 

[Dennis Alwon]

220.5(B) fraction of an amp

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes
of calculating branch-circuit and feeder loads, nominal system
voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480,
600Y/347, and 600 volts shall be used.

(B) Fractions of an Ampere. Calculations shall be permitted
to be rounded to the nearest whole ampere, with decimal fractions
smaller than 0.5 dropped.

 

[electrofelon]

220.5(B) fraction of an amp

Yay! So #12/20A it is!

 

[WA_Sparky]

No need for a state rule of 30 amps, 422.11(E)(3) allows a 4500 watt 240 volt water heater to be protected by a 30 amp breaker.

Jump to 6000 watts and 30 amp breaker generally does hold, but since this is supposed to be considered a continuous load you need more than 10 AWG and min of 35 amp breaker - though it can also be on as much as 40 amp breaker.

"though it can also be on as much as 40 amp breaker." Can you point me to a reference for this if you know one? I always though it would have been FLA*1.25 =##A, next size up would be MOCP. Already answered

 

[hillbilly1]

What does basically mean? If it was made then you would have to use it. I know Durham, NC enforced a 25 amp overcurrent protective device. I am guessing that code section changed to allow 150% of the rating which would then allow 30 amps.

I wrote this yesterday but forgot to submit it. haha

Basically means nobody stocked them. Didn’t start seeing any on the shelf until the late 90’s. Still have not come across any used on a water heater, just A/C units.