water heater name plate

[chevyx92]

So I have a water heater with a name plate saying: total wattage 4500 @ 240V. Next column says total wattage 3380 @ 208V. I'm confused, how can the wattage value drop when the voltage drops? This is a single phase water heater.

 

[Besoeker]

So I have a water heater with a name plate saying: total wattage 4500 @ 240V. Next column says total wattage 3380 @ 208V. I'm confused, how can the wattage value drop when the voltage drops? This is a single phase water heater.

With a resistive load, lower voltage means lower current and lower power.
And it's square law. So you get 4500*(208/240)^2 which is 3380W.

 

[Cavie]

ohm's law 101

 

[iceworm]

ohm's law 101

For zero temperature cooeficient resistive heating elements.

ice

 

[Dennis Alwon]

So I have a water heater with a name plate saying: total wattage 4500 @ 240V. Next column says total wattage 3380 @ 208V. I'm confused, how can the wattage value drop when the voltage drops? This is a single phase water heater.

With resistive heat there is a difference of about 75% between the wattage of a 240 v element and the same element wired at 208v. With less voltage the element will not get as hot.

We are so used to saying if the voltage is increased then the current decreases but with resistive heat the amperage actually increases.

 

[Besoeker]

With resistive heat there is a difference of about 75% between the wattage of a 240 v element and the same element wired at 208v. With less voltage the element will not get as hot.

Agreed. Power is down square law. See post #2.

We are so used to saying if the voltage is increased then the current decreases

It's one of those generalisations of debatable merit and often used in the context of motors. True if fully loaded at nameplate power rating.
Not necessarily so at reduced power and definitely not so on light load.

 

[GEMoravek]

Power (watts) =Volts x Amps (current)
Voltage = Current (amps) x Resistance (ohms) Ohm's Law

 

[ron]

In some cities, in the heat of the summer, they drop the voltage in an attempt to reduce power consumption, since a lot of load traditionally was resistive (lighting etc).

 

[iceworm]

... It's one of those generalisations of debatable merit and often used in the context of motors. True if fully loaded at nameplate power rating.
Not necessarily so at reduced power and definitely not so on light load.

I agree, really debatable merit. And not so for motors if the voltage goes up very far. As soon as the voltage goes up enough to saturate the iron, the current goes way up.

The only place this works is constant power loads as in tranformers.

I'd like to see this one go the same way as "total current" and "amps per phase"

ice

 

[chevyx92]

With a resistive load, lower voltage means lower current and lower power.
And it's square law. So you get 4500*(208/240)^2 which is 3380W.

What is square law?

 

[don_resqcapt19]

What is square law?

P=E?/R. For the water heater the resistance of the heating element does not change so the change when the voltage changes, so the power will change with the square of the change in voltage.

 

[Smart $]

What is square law?

Actually its more commonly referred to as the inverse-square law:

In physics, an inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity.

Source: https://www.google.com/search?q=squ...rome..69i57&sourceid=chrome&es_sm=93&ie=UTF-8

In this case, it is not distance but the ratio of voltages.

 

[iceworm]

P=E?/R. For the water heater the resistance of the heating element does not change so the change when the voltage changes, so the power will change with the square of the change in voltage.

Actually its more commonly referred to as the inverse-square law:

In physics, an inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity.

In this case, it is not distance but the ratio of voltages.

Don has it right. The power is porportional to the square of the voltage -- not the inverse of the square of the voltage

ice

 

[Smart $]

Don has it right. The power is porportional to the square of the voltage -- not the inverse of the square of the voltage

ice

I was implying the power change is proportional to the inverse voltage ratio squared, or the inverse of the voltages squared ratio, makes no difference... not what you said. Regardless, I was in error in referring to the inverse-square law which involves power dissipation over distance from a point source.

Nonetheless, P=E?/R is not a law (or even known as the square law in the scientific community). It is commonly mistaken as Ohm's Law, but in fact is a derivation of Ohm's Law (E=IR) and Joule's Law (Q=I??R?t), where the derived power formulas (P=IE=I?R=E?/R) are credited to Joule. In fact, the P unit of Watt is defined as one Joule per Second.

 

[Besoeker]

I was implying the power change is proportional to the inverse voltage ratio squared,

Disagree. Double the voltage, four times the power in a resistive load. 2²
Inverse square law is applicable to things like sound pressure - double the distance, quarter of the power. (1/2)2

 

[iceworm]

I was implying the power change is proportional to the inverse voltage ratio squared, or the inverse of the voltages squared ratio, makes no difference... not what you said. Regardless, I was in error in referring to the inverse-square law which involves power dissipation over distance from a point source.

Nonetheless, P=E?/R is not a law (or even known as the square law in the scientific community). It is commonly mistaken as Ohm's Law, but in fact is a derivation of Ohm's Law (E=IR) and Joule's Law (Q=I??R?t), where the derived power formulas (P=IE=I?R=E?/R) are credited to Joule. In fact, the P unit of Watt is defined as one Joule per Second.

I'm at a loss for a response. It's either:

Huh?

or

Okay

or

Thanks for cleaning up my nits - they were starting to get itchy.

ice
yes, I'm actively jerking your chain :roll:

 

[Smart $]

...yes, I'm actively jerking your chain :roll:

As a consequence, I must insist that you get over your loss... :happyyes:

 

[Smart $]

Disagree. Double the voltage, four times the power in a resistive load. 2²

And that's is what I was implying (whether accurately or not )

P₂ = P₁ ? (V₂/V₁)? = P₁ ? V₂?/V₁?...if you want...

P₂/P₁ = V₂?/V₁? = (V₂/V₁)?

Inverse square law is applicable to things like sound pressure - double the distance, quarter of the power. (1/2)2

I already stated I was in error in referring to the inverse-square law. Are you trying to rub it in?

Anyway, I posted that because I took the post I was replying to at face value rather than in the context of the thread. It was shortly after I woke this morning.

 

[Besoeker]

I already stated I was in error in referring to the inverse-square law. Are you trying to rub it in?

Chill!!
It's simply not my style, sunshine.
I just missed your comment about being in error.

No problem. As my late, great friend Tommy was fond of saying:
"The man who never made a mistake never made anything."

And, FWIW, I'm currently probably in the same time zone as you.
GA.

 

[iceworm]

As a consequence, I must insist that you get over your loss... :happyyes:

Yes ........

..............

..............

ok - over it

ice