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Wattage Equivalents for Different Voltages

12/18/2011 9:36 PM

I want to run a 120v. heater element on 12v. I want to know how much wattage (amperage) it will draw on 12 volts. I know the wattage will be lower, but I seem to remember that it will not decrease by a simple factor of 10 just because the voltage is 1/10th. Is this correct? And how do I figure what the drop will actually be? Is there a formula for this type of conversion? Thanks for the help.

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#1

Re: wattage equivalents for different voltages

12/18/2011 9:58 PM

For a first approximation Google up "Ohm's Law".

The heater element will probably have a change in resistance as temperature changes so using Ohm's Law on available data will be of limited value, possibly very limited value. You are so far outside the intended application of the heating element that a data sheet is probably useless.

For this simple question the best answer is probably "You will not be happy with what you get if you try this".

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#3
In reply to #1

Re: wattage equivalents for different voltages

12/18/2011 10:29 PM

GA This is exactly what will happen with the most likely heating element, nichrome. The resistance of the heating element increases with its temperature.

Now how the OP might generate a formula will be convoluted and iterative but it can be done. Personally I wouldn't bother trying to derive a curve because there will be a variety of other items that affects your results (i.e. mass of filament, surface area of filament, thermal mass contacting filament, bimetallic regulator, air temperature, air velocity). I'd just do a hand wave expectation of a 20% wattage and then attach the 12V and measure the current from start to equilibrium, done.

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#2

Re: wattage equivalents for different voltages

12/18/2011 9:58 PM

Yes, Ohm's Law. At 12V, the heater will draw 1% of what it does at 120V.

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#16
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Re: wattage equivalents for different voltages

12/20/2011 6:05 AM

Here, and in 4, wouldn't that be 10%, and (roughly approximated) 15%-20%?

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#17
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Re: wattage equivalents for different voltages

12/20/2011 6:11 AM

Never mind. No coffee yet, this morning. Dumb strikes daily!

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#18
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Re: wattage equivalents for different voltages

12/20/2011 9:52 AM

yes it would.

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#21
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Re: wattage equivalents for different voltages

12/20/2011 7:02 PM

Hi Tornado

you missed a zero - it would be 10% and it will only apply to the inrush current before the element starts to warm up and thus increase it's resistance

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#22
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Re: wattage equivalents for different voltages

12/20/2011 9:40 PM

Sorry, but you are wrong on both counts. See also several other correct posts.

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#24
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Re: wattage equivalents for different voltages

12/21/2011 12:14 PM

Incorrect. Review Ohms Law and the power formula.

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#4

Re: Wattage Equivalents for Different Voltages

12/18/2011 10:47 PM

Good points from 1 and 3. At the lower running temperature, the heater's resistance will be somewhat less than Ohm's Law alone dictates. (I'm too lazy to look up the curve; WAG 1.5-2% of the original current at 120V.)

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#5

Re: Wattage Equivalents for Different Voltages

12/18/2011 10:47 PM

Not enough information. The question was about WATTAGE, not amps (although later you say them as if they are the same, they are not). So no it would not be 1/10 the wattage. For those of you who said so, take a look at your Ohm's law again. Wattage is Power, or P. In Ohm's Law, if all you know is that you want P and you know E, you must use P= E2/R.

You don't know R, but assume it will not change. All you are changing is the volts (E). So really, without knowing the wattage of the heater you don't know the answer. Here's why:

Lets say for simplicity that you have a heater rated 1200W at 120V. That means the resistance is 1200/120 = 10Ω.

Now you apply 12V and use the above folmula. 122/10Ω = 144/10 =14.4W

14.4/1200 = 0.012, or 1.2% of the original wattage

But now let's say it is a 500W heater at 120V. 500/120 = 4.16666667Ω.

You still have 12V so that part is still 144, but now it is divided by 4.16666667Ω = .006912, or 6.9% of the original wattage.

So again, you cannot answer this question without the wattage to start with.

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#8
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Re: Wattage Equivalents for Different Voltages

12/18/2011 11:47 PM

True! If the resistance never varied with temperature the expected wattage would be 1% of the original wattage. But depending on how hot the nichrome filament gets a 10% increase in resistance is not unheard of.

So was my 20% wattage estimate guaranteeing that the 12V supply would have plenty of reserve current for testing? You betcha!

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#10
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Re: Wattage Equivalents for Different Voltages

12/19/2011 4:35 AM

Huh? Sorry but your math is worse than mine and there is more than enough info to come up with at least a percentage of power ratio!

For example.

A 500 watt 120 volt heater would have current load of 500/120 = 4.167 amps not a resistance of 4.167 ohms.

Its resistance would be 120/4.167 = 28.8 ohms.

at 12 volts 28.8 ohms of resistance would allow 12/28.8 = .4167 amps of current flow.

12 volts at .4167 amps equals 5 watts or 1% of the 120 volt wattage and yes a slight fudge factor for the heater element resistance could be included also but still even then its doubtful that the actual wattage at 12 volts would still be over 2% of what it would be at 120 volts.

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#13
In reply to #5

Re: Wattage Equivalents for Different Voltages

12/20/2011 12:44 AM

P=E^2/R

Therefore R=E^2/P

in your example R= 120^2 / 1200 = 12 ohm

OR, since E is 1/10, P~ 1/100 - simple squares.

Resistance will drop with the colder wire - my 1500 watt heater (R=120^2/1500 = 9.6 ohms) reads 10.5 ohms cold(wrong direction - limit of meter accuracy or the name plate on the heater is "within manufacturing tolerances") -- therefore at 12 volts it would provide only 13 watts of heat. -- hmm, resistance essentially didn't change, only a percentage point or two.

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#6

Re: Wattage Equivalents for Different Voltages

12/18/2011 11:16 PM

Check out this website:

http://www.wiretron.com/nicrdat.html

...and look at the table below where you'll find that the resistance goes up by a maximum of 6.3% at the wire's operating temperature. Therefore it's safe to assume that at 12V the heater will draw 1/10th of the current it would draw at 120V and consequently the power will be 1/100th its original value, or 10 Watts instead of 1,000 Watts. Ohm's law triumphs again!

Ni Cr A - Increase in Resistance with Temperature

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#7

Re: Wattage Equivalents for Different Voltages

12/18/2011 11:38 PM

Heating elements are generally rated in kw...A typical portable room heater is around 1500 watts...The watts equal the voltage times the amperage....If you have a 1500 watt heater running on 120v that equals 1500 divided by 120 which would be 12.5 amps..Any variation in the voltage will effect the amperage...So if you have a 1500 watt heater running on 12v that would be 1500 divided by 12 which equals 125 amps...This is not a practical setup....Resistance heat is one of the largest consumers of electricity, so the higher the voltage the better....It is the least efficient heat source, speaking monetarily...Household electric heat is generally between 10 and 20 kw running on 240v and when on, fully consumes nearly half the available power to the home...

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#9

Re: Wattage Equivalents for Different Voltages

12/18/2011 11:55 PM

If the heater is immersed in liquid, the hotter/colder temperatures won't differ much, and the resistance will remain nearly the same.

If immersed in gas, there will be a greater temperature difference, along with a resistance difference as in the previous table.

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#11

Re: Wattage Equivalents for Different Voltages

12/19/2011 10:33 AM

It won't be so much of a heater on 12V, more a slight warmth-emitting curiosity.

How about selling tickets to see it? It might draw a crowd....

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#12

Re: Wattage Equivalents for Different Voltages

12/19/2011 8:07 PM

Thanks for all the generous and quick replies. I think I get the picture- forgot about Ohm's law. I'm not expecting too great a temp. rise, so I don't think the change of resistance will be significant (but good to know the principle). The 1% figure is really helpful. I knew it wasn't a straight line reduction, but didn't know about the squared thing- quite a difference! Thanks so much for all the help and time to post, and Merry Christmas to all.

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#14
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Re: Wattage Equivalents for Different Voltages

12/20/2011 5:09 AM

I'm not expecting too great a temp. rise, so I don't think the change of resistance will be significant

Just to clarify: the heater's power is specified at the elevated temperature you'd expect if you ran it at 120V, so, the change of resistance running it at just above ambient will be significant. But the 1% does still get you in the right ball park.

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#15

Re: Wattage Equivalents for Different Voltages

12/20/2011 5:24 AM

Measure the resistance of the cold heater. The resistance will go up less than 10%, as indicated by another answer. Now use V * V / R for Watts.

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#19

Re: Wattage Equivalents for Different Voltages

12/20/2011 10:28 AM

Lots of nice theories, You say you want to run it on 12 volts? AC or DC ? This is a practical application?? So connect your 12 Volts to it and measure the current and apply Ohms Law. Why would you want to do this in the first place?

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#20

Re: Wattage Equivalents for Different Voltages

12/20/2011 6:13 PM

One time I picked up a 3000W 220V heater that someone had placed 110V plug on the cord. I figured it still put out 750W which was plenty of heat to keep my feet warm under the kitchen table and with fan running at a slower speed it was quiet too.

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#23

Re: Wattage Equivalents for Different Voltages

12/20/2011 10:08 PM

I have a baseboard heater rated at 3000 watts that I use in the bedroom at 115 volts instead of its rated 230 volts. Works perfect, doesn't get very warm, but has a thermostat. I figured it at one quarter the heat output. You can do the math.

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#25

Re: Wattage Equivalents for Different Voltages

12/21/2011 12:59 PM

It is unbelievable the number of mistakes and misconceptions by so many on simple ohms law and power calculations for a resistor.

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