Hot Water Heater

[davidwigler]

According to online sources, applying 240 volts to a 120 volt element quadruples the watts.

If this formula is correct, you can’t just divide the labeled wattage of the element by the voltage available to get amperage. In other words, as the available voltage changes, the change in watts is not linear.

[Briankinley2004]

The generator is probably lower voltage. Higher would result in even more amps.

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Unless I have been using OHMs law wrong for 40 years which is possible this statement is backwards. Higher voltage should result in amp draw being lower.

Power (watts)= Impedence (amps) x Electromotive Force (volts)

1000 W/110volts = 9.09 amps
1000W/125 volts = 8.0 amps

Thus higher voltage from generator should pull less amps or I have been taught wrong by more than one "qualified" professor. My teachings were that appliances with higher wattage use are made to run on 240 volt to reduce amperage which in turn reduces wire size/costs. Three phase power does the same using higher voltages and allows large equipment to be run on smaller more manageable wiring

[Photolomy]

Unless I have been using OHMs law wrong for 40 years which is possible this statement is backwards. Higher voltage should result in amp draw being lower.

Power (watts)= Impedence (amps) x Electromotive Force (volts)

1000 W/110volts = 9.09 amps
1000W/125 volts = 8.0 amps

Thus higher voltage from generator should pull less amps or I have been taught wrong by more than one "qualified" professor. My teachings were that appliances with higher wattage use are made to run on 240 volt to reduce amperage which in turn reduces wire size/costs. Three phase power does the same using higher voltages and allows large equipment to be run on smaller more manageable wiring

Ohms law has nothing to do with power. It states the relationship between current, voltage and resistance. Namely, I = V/R. If you apply a voltage V across a resistance R, then the current will be I. I am not trying to be funny, but what you are missing in your analysis is actually OHM's law.

For example...

You design a stove to have 2000 watts of power at 120 volts. As you stated, power is voltage times current, and in order to achieve 2000 watts at 120 volts, you will need 16.67 amps of current (120 * 16.7 = 2000).

Thus, you need to design the heating element such that when you apply 120 volts across it, you get 16.7 amps of current. This is where OHM's law comes into play. You take 120 and divide it by 16.7 and you get a RESISTANCE of 7.2 ohms. Thus, you create the heating element of such material and length that it has a resistance of 7.2 ohms. When you plug it into a 120 volt socket, 16.7 amps will flow through it (because of OHMs law) and the power will be 2000 watts.

If you plug that same element into a 240 volt socket, because of OHM's law, twice the current will now flow through it and the power will be four times as much. Of course, your element and/or the wiring will fry, so instead, you make the element with twice the resistance of the 120 volt element, and then because of OHM's law, the current will be the same (16.7 amps), but the power will be double because now the voltage is 240 instead of 120.

The heating element has a fixed resistance, period. The current is entirely determined by OHM's law, I = V/R. More voltage, more current, less voltage, less current. More resistance, less current, less resistance, more current.

The mistake people have been making is thinking that the power in the equation (P = V*I) is constant, but it isn't. The only thing constant is the resistance. You can use P = V*I to determine the current, but you then must apply OHM's law to determine the resistance, and then you can change the voltage and determine what the new current and power will be. So, when I see a 120 volt water heater with a 1500 watt element, I can use P = V*I to determine that it has a current of 12.5 amps. I then determine, using OHM's law, that the resistance of the element must be 9.6 ohms. Now I can see what that same element will do under 240 volts. The current at 240 volts will be 25 amps, and the power will now be 6000 watts. But of course, that element would fry with that much current and power, since it was only designed for 1500 watts.

[Briankinley2004]

Most of my learning was for sizing wire thus we were given the wattage. For example the name plate rating of a stove may be 4000 watts at 120 volts so we would back out the amps from the equation to determine wire size. I havent worked it from the resistance end but do know voltage is stepped up in design to reduce amperage so that wire size can be reduced. I wasn't factoring in on an element with fixed resistance. My bad

[davidwigler]

Ohms law has nothing to do with power. It states the relationship between current, voltage and resistance. Namely, I = V/R. If you apply a voltage V across a resistance R, then the current will be I. I am not trying to be funny, but what you are missing in your analysis is actually OHM's law.

For example...

You design a stove to have 2000 watts of power at 120 volts. As you stated, power is voltage times current, and in order to achieve 2000 watts at 120 volts, you will need 16.67 amps of current (120 * 16.7 = 2000).

Thus, you need to design the heating element such that when you apply 120 volts across it, you get 16.7 amps of current. This is where OHM's law comes into play. You take 120 and divide it by 16.7 and you get a RESISTANCE of 7.2 ohms. Thus, you create the heating element of such material and length that it has a resistance of 7.2 ohms. When you plug it into a 120 volt socket, 16.7 amps will flow through it (because of OHMs law) and the power will be 2000 watts.

If you plug that same element into a 240 volt socket, because of OHM's law, twice the current will now flow through it and the power will be four times as much. Of course, your element and/or the wiring will fry, so instead, you make the element with twice the resistance of the 120 volt element, and then because of OHM's law, the current will be the same (16.7 amps), but the power will be double because now the voltage is 240 instead of 120.

The heating element has a fixed resistance, period. The current is entirely determined by OHM's law, I = V/R. More voltage, more current, less voltage, less current. More resistance, less current, less resistance, more current.

The mistake people have been making is thinking that the power in the equation (P = V*I) is constant, but it isn't. The only thing constant is the resistance. You can use P = V*I to determine the current, but you then must apply OHM's law to determine the resistance, and then you can change the voltage and determine what the new current and power will be. So, when I see a 120 volt water heater with a 1500 watt element, I can use P = V*I to determine that it has a current of 12.5 amps. I then determine, using OHM's law, that the resistance of the element must be 9.6 ohms. Now I can see what that same element will do under 240 volts. The current at 240 volts will be 25 amps, and the power will now be 6000 watts. But of course, that element would fry with that much current and power, since it was only designed for 1500 watts.

Thanks for explaining it so well. Now, back to the question posed by the very first post. We now know he had a 2000 W 120 V fixed resistance heating element. It was popping a 15 amp breaker on 108 V shore power but not on 120 V generator power. Does this make any sense? Why?

[KPTIM]

Thank goodness someone reset the post.

Here is a new take on what might be happening. In this case assuming the breaker is tripping after the water heater has been energized for a while and not tripping as soon as it is turned on at 108 VAC. Also assuming the breaker is not fatigued from old age and there are no other unusual circumstances going on on not being explained in the post.

The original breakers used in our Hatteras' are pretty complex. They were made by Airpax and are "thermal-magnetic" type units. They can/will trip on two different types of events. The Magnetic section will cause a trip from a short burst, fast rise time of a very high over current event, like a locked AC compressor rotor or dead short circuit. The Thermal part is designed to give the breaker a delayed trip action which keeps it from nuisances tripping when operating at slightly OVER or NEAR it's rated load capacity for short to medium time durations. This compared to what is/were called time-delayed fuses.

When the water heater is operating on the generator at 120 VAC the water heater is going to take less time to heat the tank of water to the desired temperature. This SHORTER amount of time drawing full or even slightly over current will not cause the Magnetic or Thermal part of the breaker to react.

When the water heater is operating on the lower 108 VAC dock voltage (about 11% lower then the generator) it is taking a much LONGER time to make a tank of hot water. Because it is taking so long to heat up the water the Thermal part of the breaker could be causing the breaker trip. Which is what it is supposed to do.

[Photolomy]

The slips at the end of the dock (where my boat is) are 240v, not 208v. I pointed that out later in the thread, and when I measure the voltage at the water heater, it is 118v. In the first post I assumed that my slip was 208v because that is what all the marked pedestals say, but that was incorrect, it is 240v. Sometime over the weekend I will start the generator and see what its voltage is. I suspect that it will be less.

[Boatsb]

118 at the the 120 volt circuit is fine. It doesn't make the other feed 240. It can still be 208.

[Photolomy]

118 at the the 120 volt circuit is fine. It doesn't make the other feed 240. It can still be 208.

True, but I measured the stove and it was 238-239.

[Boatsb]

Measure at the pedestal.

Then theres no transformers involved on board.

Thats the real test.