5kW heater load current rating at 208-240V

[Pitt123]

This is the same heater I posted in another posted but I had more of a theory question so I thought I'd start another thread.

The heater is a 5kW 208-240V heater. With this heater since it has a fixed resistance wont the heater have (2) differnt power ouputs depending on voltage applied. I'm assuming the current is calculated using whatever voltage we apply.

For example for a 5kW heater at 240V using V^2/R I get 11.2ohms. However if 208V is applied then 208^2/11.2ohms = 3862W. So will applying the lesser voltage result in less of a power ouput? Or will the heater change its resistance due to heating between the two voltage values and thus always output 5kW? Is there a way to calculate this?

 

[LarryFine]

You are correct.

 

[winnie]

In theory, _either_ setup is possible. You could certainly design a heater that would have constant output with different supply voltages, or a heater that varied its output with different supply voltages. The calculation of how heat output changes with voltage will depend upon the specifics of the heater.

In practice, the heater is almost certainly a simple resistive heater, possibly with an associated fan. In this case your V^2/R calculation is spot on.

-Jon

 

[Npstewart]

Yep, always when im selecting an AHU you have to find the specific voltage, then the KW so you can find the correct model number.

 

[dbuckley]

It won't be "spot on", because the temperature of the heater will change its resistance, and the heater temperature will vary with voltage applied, but the simple sums and assumptions are close enough for rock and roll.

 

[boboelectric]

You get less BTUs at 208 than 240 volts = less heat.

 

[Pitt123]

You are correct.

For which one :-?

The fact that at 208V the power (wattage) will be decreased on such a heater.

Or the fact that the heater resistance will change for lower 208V so the output at 208V will still be 5kw, and the heater will put out 5kW as long as voltage is is 208-240V range?

 

[Dennis Alwon]

For which one :-?

The fact that at 208V the power (wattage) will be decreased on such a heater.

Or the fact that the heater resistance will change for lower 208V so the output at 208V will still be 5kw, and the heater will put out 5kW as long as voltage is is 208-240V range?

The lower the voltage the less wattage the heater will use. The higher the voltage the more wattage the heater uses. So your example of 5000 watts at 240v and 3862 watts at 208v. I didn't check your math.

 

[kwired]

You are correct with the formula but may have made a math error.

I come up with heater resistance of 11.52 ohms and wattage @208 volts of 3755.55555~

3750 is the wattage usually marked on units that have a 5000 watt rating @240 volts, this is what made me double check the math.

 

[Pitt123]

You are correct with the formula but may have made a math error.

I come up with heater resistance of 11.52 ohms and wattage @208 volts of 3755.55555~

3750 is the wattage usually marked on units that have a 5000 watt rating @240 volts, this is what made me double check the math.

Makes sense. Thank You.

 

[LarryFine]

For which one :-?

The former. The heater will develop about 75% of its 240v power on 208v.