Engineer Insight

Ask Ian - Applied Voltage vs. Rated Voltage?

by Ian Renwick

The ratio between applied and rated voltage is a square relationship. That means that applying twice the rated voltage to a heater does not produce twice the power (wattage). It actually produces 4 times the power. Applying half the rated voltage to a heater does not produce half the power,but only a quarter of the power.

Here’s the general formula:
Vr = Rated Voltage
Va = Applied Voltage

A few examples:

For a heater rated at 1000W and 240V, applying 208 volts will generate the following power. Output Power = 1000 x 2082/2402 = 751.1 Watts
For a heater rated at 1000W and 208V, applying 240 volts will generate the following power. Output Power = 1000 x 2402/2082 = 1331.4 Watts

And to show what was discussed earlier:
For a heater rated at 1000W and 240V, applying 480 volts will generate the following power. Output Power = 1000 x 4802/2402 = 4000 Watts !
Double the voltage and you quadruple the power !

Sometimes it’s perfectly fine to apply a voltage to a heater that’s different from its rated voltage. Sometimes it’s not. Be aware and always think of what you’re doing, making sure that it’s safe.

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