Overcurrent protection for continuous load

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How do you know the load is 16.7 amps if you can't read the label? Most water heaters- I assume residential- are 4500 watts and are 240 v--- that means 18.75 x 1.25 = 25 amps minimum conductor and 30 amp max overcurrent protective device.

If this is commercial and the load is rated 4500 watts at 240 and the actual voltage is 208 then you could use #12
 
I would post a picture but am having trouble.

It is a point of use 120v water heater and I am just able to see part of the label that says it is a 2000w element which is how I derived my load calc.


My main question irregardless of this example is if my load Calc for a single load comes out at 21A and I have thhn #12 and 75 degree terminals is the 20A overcurrent protection permissable or does 240.4(d) not allow that?

as for my particular example it does not matter as I know the wire is NMS and can only be rated at 20 amps. Just throwing out a theoretical example if the wire were thhn.
 
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Mustwin351 said:
I would post a picture but am having trouble.

It is a point of use 120v water heater and I am just able to see part of the label that says it is a 2000w element which is how I derived my load calc.
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:slaphead:

An on-demand or tankless water heater is not a continuous load. Does it have a tank or not?
 
JFletcher said:
Okay. All posts above apply then.

It's an existing installation, and even though it was not installed up to code, that is not your problem (unless you were the original installer), if you are only being paid to replace the element.
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I am am not the original installer but am asked by my company (a college) to troubleshoot and fix. Putting the same 2000w element is not to code and I don’t feel right about that.
 
Mustwin351 said:
I am not the original installer but am asked by my company (a college) to troubleshoot and fix. Putting the same 2000w element is not to code and I don’t feel right about that.
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The element did not fail because the branch circuit is on a 20 amp breaker or has number 12 conductors. Corroded connections, mechanical or stress fatigue causing cracks, scaling up and overheating, thermostat failure... that will all happen even with a completely code compliant installation.

There are a ton of options depending on how far they want to go correcting a code violation. Repull the wire and use a larger breaker, a 1500 watt element as you suggested, replacing the water heater with a smaller wattage or 240 volt unit, or perhaps a tankless, etc.

422.13 is another code section that could stand some revisions or exceptions. A small 5 or 10 gallon tank water heater supplying a single point of use is probably never going to be a continuous load. I would let your company know of the violation, however I would not lose any sleep over replacing the element and letting it fly.
 
Mustwin351 said:
Agreed.

Also the bad element in this heater is a 2000w element which is how I came up with my load calc.

This water heater is protected by a 20a breaker. I wanted to just throw a 1500w element in it and call it a day but have been overruled in doing so.

Back to my original question, is it permissible to protect it with a 20A breaker or does 240.4(D) not allow that?
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240.4(D) allows a 20 amp breaker on 12 AWG but is more complex than that. With a required 125% factor for continuous load minimum required conductor is still more than 20 amps per 210.19(A)(1), and overcurrent protection is also required to be more than 20 amps per 210.20(A). If conductor is allowed to be 75C, then 12 AWG is enough, until 240.4(D) comes along. If this were a "tap conductor" or supplying something mentioned in 240.4(G), then other areas of code trump what is in 240.4(D).

"on demand" style water heater changes everything said before this was mentioned.

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 for the purposes of sizing branch circuits.
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This doesn't say the unit must have a tank, it says "fixed storage-type ... capacity of ...120 gallons or less." If it is intended to only heat on demand, it doesn't fit that description and this section doesn't apply to it.
 
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