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Inrush current for water heater elements
2

Inrush current for water heater elements

Inrush current for water heater elements

(OP)
Do residential style water heater elements have significant inrush when first energized?

I have seen warnings about huge inrush currents for incandescent lamps damaging contactors. For switching water heater elements, can they be treated as purely resistive or would I need to account for inrush to a water heated similar to switching incandescent elements?

RE: Inrush current for water heater elements

There is a substantial inrush (~9x rated) current for lightbulbs because of the difference in resistance between operating temperature and at ambient temperature ~2000°C. Tungsten also has a high temperature coefficient of resistance, 10-25x higher than common heating element alloys (NiCr, CuNi, NiFeAl). Even with a 500°C difference between ambient and operating temps, you would only see a 2.5x change in current.

I believe heating elements are rated at their max (cold) current while lightbulbs are rated at their running (hot) current. In this case, the heating element should be considered strictly resistive when sizing controls.

RE: Inrush current for water heater elements

(OP)
Thanks Tug. I had not realized the ratings for heater elements were based on the cold temperature reading.

This also gives me a bit insight into cold load pickup after a long utility outage. I had assumed cold load was 100% controlled by thermostatic devices, but I guess very small portion of tapering of cold load is due to the temperature coefficient of resistance in various heating elements.

RE: Inrush current for water heater elements

Mr. bacon4life (Electrical)(OP)25 May 23 16:59
"...#1 Do residential style water heater elements have significant inrush when first energized? #2. .....I have seen warnings about huge inrush currents for incandescent lamps damaging contactors. For switching water heater elements, can they be treated as purely resistive or would I need to account for inrush to a water heated similar to switching incandescent elements?"
I have the following opinion for your consideration.
1. a) The water heater name-plate rated current value (A) MUST be the running current, NOT the starting surge current.
b) The heating element resistance when off (cold) MUST be lower than during running (hot). Therefore, a short duration of starting surge, which is higher than the running current; is inevitable.
c) contactor selection based on AC-1 (see IEC 60947-4-1 on utilization category for more detail).
2. a) for incandescent lamps, the element resistance MUST be lower in off (cold) state and increases to a higher value when lighted up in (hot) state. A short duration of starting surge, which is higher than the running current; is inevitable.
b) contactor selection based on AC-5b (see IEC 60947-4-1 on utilization category for more detail).
FYI: the same contactor can be used for say AC-1 or AC-5b etc.... depending on the breaking current value and the required number of operation cycles.
Che Kuan Yau (Singapore)

RE: Inrush current for water heater elements

Quote:

ratings for heater elements were based on the cold temperature reading.

The resistance of the water heater element is dictated by the material composition of the heating element, which is tailored to produce a relatively flat resistance vs. temperature characteristic, specifically to minimize inrush current. The graph from here shows a minimal amount of inrush because the resistance is relatively constant over temperature. Nevertheless, the rating would be of the heater in operation, and not during startup.






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RE: Inrush current for water heater elements

Thanks IR, that clarifies some things. From what I have seen, tungsten has a thermal coefficient of resistivity of 0.0045 and the NiFeAl is as low as 0.00017 ohms/°C. With the much flatter curve and lower temperature differential there is still an "inrush" but it's not so significant when compared to a tungsten filament light bulb.

RE: Inrush current for water heater elements

Using:
0.00017 Ohms/degree C
15 Ohms (16 Amp at 240 Volts)
60 degree delta T (20 degree C to 80 degree C.
I get a whopping 16.165 Amps of cold current. The surge is +0.165A
I am not really comfortable calling 1% a surge.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Inrush current for water heater elements

Starting current on heating elements is all about the temperature coefficient of resistance, which relates to how the resistance changes with temperature. “Ideal” heating elements have a very low temperature coefficient of resistance, meaning it changes as little as possible, and also you want it to have a slightly positive temperature coefficient (PTC) of resistance, meaning the resistance increases as temperature increases, preventing thermal runaway.

But incandescent lamps are not intended to be ideal heating elements, their end goal is emitting light. So they have a very high temperature coefficient of resistance and are thus more PTC, so the resistance is very low when “cold”, which is why there is an apparent initial surge in current. That is why most incandescent lamps appear to burn out when switched on.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Inrush current for water heater elements

The incandescent lamp exhibits nonlinear resistance because the resistance changes with temperature. All the heating elements are linear devices and exhibit linear voltage and current characteristics.

Therefore, the rated current of the bulb is in hot condition. The resistance of the bulb is low when it is cold, and increases as the filament temperature increase. The V-I characteristics of the incandescent lamp are given below.

RE: Inrush current for water heater elements

All good and valid information, but let's not forget the title of this thread;

Quote:

Inrush current for water heater elements

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Inrush current for water heater elements

Correct Bill, but we were addressing this:

Quote (Bacon4life)

I have seen warnings about huge inrush currents for incandescent lamps damaging contactors. For switching water heater elements, can they be treated as purely resistive or would I need to account for inrush to a water heated similar to switching incandescent elements?
I was explaining why incandescent lamps do have that, but resistance heaters do not.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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