Overcurrent protection for continuous load

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Mustwin351

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Correct me if I’m wrong.

For a single continuous load of 16.7 amps (16.7 X 1.25 = 20.8 amps) is it still permissable to size the overcurrent protection at 20 amps using 12 awg thhn conductors with 75 degree terminals (conductor is then rated for 25 amps)?
 
Mustwin351 said:
Correct me if I’m wrong.

For a single continuous load of 16.7 amps (16.7 X 1.25 = 20.8 amps) is it still permissable to size the overcurrent protection at 20 amps using 12 awg thhn conductors with 75 degree terminals (conductor is then rated for 25 amps)?
Click to expand...

No, that is not permitted. See 210.20.
 
Mustwin351 said:
Correct me if I’m wrong.

For a single continuous load of 16.7 amps (16.7 X 1.25 = 20.8 amps) is it still permissable to size the overcurrent protection at 20 amps using 12 awg thhn conductors with 75 degree terminals (conductor is then rated for 25 amps)?
Click to expand...

What is the load? If it is an a/c you may be able to use a 25 or 30 amp overcurrent protective device. Nm cable would not apply as that is rated 60C
 
My understanding is that the 125% sizing rule is intended to protect the _breaker_ from excessive heating caused by continuous operation near its trip point. So if you have a 100A continuous load you need a minimum of a 125A breaker _or_ a 100% rated breaker.

However after you increase the size of the breaker to protect _it_, you must also increase the size of the conductors so that the breaker will properly protect them.

As you note, the ampacity of 12AWG copper can be 25A if you have the proper insulation; however an additional rule prohibits protecting 12AWG copper conductors at more than 20A for general purpose circuits.

-Jon
 
Load is a water heater that is tucked so tightly into a cabinet I cannot read the manufacturers label.

winnie said:
As you note, the ampacity of 12AWG copper can be 25A if you have the proper insulation; however an additional rule prohibits protecting 12AWG copper conductors at more than 20A for general purpose circuits.

-Jon
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That is exactly where I was going with it Jon. Does 240.4(D) for small conductor limit my 12 awg thhn for this application?
 
Mustwin351 said:
Correct me if I’m wrong.

For a single continuous load of 16.7 amps (16.7 X 1.25 = 20.8 amps) is it still permissable to size the overcurrent protection at 20 amps using 12 awg thhn conductors with 75 degree terminals (conductor is then rated for 25 amps)?
Click to expand...

Here is the progression... You have a 16.7 amp continuous load. Since you can only load the breaker to 80%, you must use a 25 or 30 amp breaker. Once you use that 25 or 30 amp breaker, 240.4 comes in and requires you to use #10.

Eta: if you cannot read the label, you could always put an amp clamp around a conductor to get an idea. Most residential water heaters are going to be on a 30 amp circuit, wired with number 10.
 
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Sounds like a 4000 watt water heater so your minimum conductor size is #10 and you can use a 25 amp OCPD. You cannot use the #12 at the 75° C ampacity.
 
JFletcher said:
Expected to be at its maximum current for 3 hours or longer.

Water heaters are almost never a continuous load, however per 422.13, 120 gallon water heaters are continuous loads for the purpose of sizing the branch circuit.
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OK. Just a hypothetical question.
Suppose you have a 55kW motor and run it rated power for 2hr 55min, rest it for 5min and repeat that cycle forever
Does that mean you could, theoretically reduce the rating of things like supply conductors, overload protection, and breaker?

Yes, I know that you are constrained to discrete steps in component ratings so you'd have to size them as per continuous. As I said, it was a hypothetical question, not a practical example.
 
Besoeker said:
OK. Just a hypothetical question.
Suppose you have a 55kW motor and run it rated power for 2hr 55min, rest it for 5min and repeat that cycle forever
Does that mean you could, theoretically reduce the rating of things like supply conductors, overload protection, and breaker?

Yes, I know that you are constrained to discrete steps in component ratings so you'd have to size them as per continuous. As I said, it was a hypothetical question, not a practical example.
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In the NEC motors have different rule. In general the motor supply conductors are already rated at a minimum of 125%.

Regarding a continuous load by definition it's the maximum load and must continue at that maximum level for 3 or more hours. Any break in that 3 hour time limit even for a second would make it a non-continuous load.
 
infinity said:
In the NEC motors have different rule. In general the motor supply conductors are already rated at a minimum of 125%.

Regarding a continuous load by definition it's the maximum load and must continue at that maximum level for 3 or more hours. Any break in that 3 hour time limit even for a second would make it a non-continuous load.
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Yes, that's kinda what I was getting at.
My 55kW motor would run at about 100A so I'd rate everything continuously for for that - the 100A. Even it ran for one hour a day.
Would your rules allow me to derate them and, if so, by how much?

Assume that the application is a centrifugal pump that will run at 100 kW input when in operation.
Please understand that I'm just trying to get an understanding of the practicalities of how the three hour rule was arrived at.
 
JFletcher said:
Expected to be at its maximum current for 3 hours or longer.

Water heaters are almost never a continuous load, however per 422.13, 120 gallon water heaters are continuous loads for the purpose of sizing the branch circuit.
Click to expand...

I think you need to read that again. It says 120 gallon OR LESS are considered a continuous load.
Most household water heaters would fit that description.
 
Little Bill said:
I think you need to read that again. It says 120 gallon OR LESS are considered a continuous load.
Most household water heaters would fit that description.
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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?
 
Besoeker said:
OK. Just a hypothetical question.
Suppose you have a 55kW motor and run it rated power for 2hr 55min, rest it for 5min and repeat that cycle forever
Does that mean you could, theoretically reduce the rating of things like supply conductors, overload protection, and breaker?

Yes, I know that you are constrained to discrete steps in component ratings so you'd have to size them as per continuous. As I said, it was a hypothetical question, not a practical example.
Click to expand...

Your example has been answered.

A more practical example which people have tried to do is with places like grow operations, where the lights would be on for 12+ hours. Per the NEC, these are continuous loads.

Now say I put these lights on a timer so that they only run for 2:55, and are off for 5 minutes, then back on. Not a continuous load.

While my example is legal, I could see the NEC rewording the definition of a continuous load, or changing certain lighting applications, such as grow lamps, to always be considered continuous, like water heaters are.

A 175 minute on / 5 minute off cycle has no practical purpose in that case, it would only be implemented to skirt the rules of continuous loads, which would allow the installer to put more lights on a circuit, using fewer branch circuits.
 
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?
Click to expand...

Your idea of using a 1500 watt element would reduce the current below 16 amps, which would allow the number 12 with a 20 amp breaker. Even though you have been shot down on this presumably by your boss, if repulling the number 12 conductor would be a significant expense, the customer may appreciate your idea... You would just have to let them know that they are working with a water heater that is putting out roughly 12% less wattage than a brand new 4kw one.
 
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