Hello:
I have a 1500 watt coil water heater and it is hooked up to an AC generator. The coil heater is designed to work with 120 volts but the generator is supplying 136 volts. The company that makes the heater said that with 136 volts, the heater will produce 1250 watts instead of 1500 watts. I understand volts x amps = watts but also I thought that the resistance should not change, and if that is the case this defies Ohms Law. R= volts/current. does the resistance change with the higher voltage, and if so does that mean Ohms law only refers to the voltage drop across the resister, which has gone down instead of up with the higher voltage?

 

Hello:
I have a 1500 watt coil water heater and it is hooked up to an AC generator. The coil heater is designed to work with 120 volts but the generator is supplying 136 volts. The company that makes the heater said that with 136 volts, the heater will produce 1250 watts instead of 1500 watts. I understand volts x amps = watts but also I thought that the resistance should not change, and if that is the case this defies Ohms Law. R= volts/current. does the resistance change with the higher voltage, and if so does that mean Ohms law only refers to the voltage drop across the resister, which has gone down instead of up with the higher voltage?

Welcome to the forum.

Maybe that assumption, underlined, is incorrect. It is quite common for resistance to change with temperature. In fact, one might say that is the rule rather than the exception. An incandescent light bulb is another example. Without the increase in resistance as the filament gets hot, it would burn out quickly. Some materials show a decrease in resistance with temperature. Others show an increase.

John

 

The generator is supplying 136 volts.

Is the generator supplying 136 V RMS or peak voltage?

 

The water heater element is maintained at a near constant temperature by the surrounding water so is unlikely to have a significant resistance change with an increase in input voltage and power.

I don't see how the power could go down with an increase in voltage unless there was an active circuit controlling the heater voltage to do that.

 

The company that makes the heater said that with 136 volts, the heater will produce 1250 watts instead of 1500 watts.

I'm declaring BS until further proof is offered. As crutschow intuits, higher voltage will not reduce current and power. We don't dim lightbulbs by providing them higher voltage.

 

I'm declaring BS until further proof is offered. As crutschow intuits, higher voltage will not reduce current and power. We don't dim lightbulbs by providing them higher voltage.

We do.Result is permanently dimmed light bulb.

 

The water heater element is maintained at a near constant temperature by the surrounding water so is unlikely to have a significant resistance change with an increase in input voltage and power.

The physical form of a Calrod type heater is a nichrome rod surrounded by aluminum oxide, inside a steel tube. The nichrome part gets quite a lot hotter than the outside steel because of the AlO2 insulation layer.

 

Is the generator supplying 136 V RMS or peak voltage?

The generator is a 120 Kva Diesel powered unit. The reason i have 136 volts on the 120 volt circuit is because the generator is set to provide 460 volts to a 30 hp pump. There is plenty of capacity on the 136 volt circuit to provide as much current as the 1500 watt aka 1250 watt at 136 volts heater coil requires. As far as I know, there is not circuitry with the heater at all.

 

Perhaps the guy who said 1250W was thinking "110V" (or simply not thinking at all )?