Applying Voltage to a Heater Other Than What It’s Rated For

If you look closely at the small print on a power brick for a laptop you’ll see that it can accept anywhere from 100V to 240V AC. That’s a huge range for the allowable input voltage. That brick contains circuitry that takes that input AC voltage and converts it to 19V DC that powers your laptop (or something like that). Heating elements are nothing like that. They have a fixed resistance and there is no fancy circuitry in them that will alter the incoming voltage as just described. Think of a heating element as a glorified light bulb (the old fashion incandescent kind) consisting of the element and that’s it. When applying a different voltage it follows a square law as to how much power you’ll get out of the heater. Double the input voltage and you’ll quadruple the power output. Quadruple in the input voltage (think of 480V applied to a 120V heater) and you’ll get 16 times the power output. 16 times!!

Here’s the rule for calculating what you’ll get when you don’t apply the expected voltage.

Actual Output Power = (Rated Power) x (Applied Voltage)Ç / (Rated Voltage)Ç

The ‘Rated’ power and voltage are what a heater is expected to produce (rated power) at the expected (rated) voltage.

Here are a few examples:

Take a heater designed to produce 500 watts when 115 volts is applied. 

If 110 volts were applied, you would get 500 x 110²/115² = 457.5 watts. That’s not bad. It’s a little less than what you want but you can probably live with it. 

If 120 volts were applied, you would get 500 x 120²/115² = 544.4 watts. That’s also not bad. It’s a little more than what you want but you can probably live with it too. 

If 208 volts were applied, you would get 500 x 208²/115² = 1635.7 watts! Things are getting toasty, and this is most probably NOT a good idea. You’re over three times the rated power output. If you’ve exceeded the watt density limit of the product line, do not proceed. 

If 480 volts were applied, you would get 500 x 480²/115² = 8710.8 watts! This is a huge no. No need to explain this one. Don’t do it. Don’t even think about it. 

If 575 volts were applied, you would get 500 x 575²/115² = 12500.0 watts! This is also a huge no, but even bigger. 

If 24 volts were applied, you would get 500 x 24²/115² = 21.8 watts! Is it even worth it? Would you even be able to tell if the heater were turned on?

In these instances of excessively high-power output, they might be OK if there’s enough surface area of the heater to be able to get the heat away from the heater. All different types of heating elements have a watt density rating which is the wattage output per square inch (or square whatever) of surface area. They all have a limit and if you exceed that limit, you’re asking for trouble. 

Here’s a case in point. Let’s say you have a 3/4” OD, 24” long cartridge heater, designed to produce 100 watts at 120 volts. Its purpose is to sit inside an ATM to make sure that the paper bills don’t stick to each other when it’s bitterly cold outside. That heater has a low watt density of only about 2 W/in². At that watt density you can almost hold the energized heater in your hands (please notice the word ‘almost’). What would happen if you applied 480 volts to that heater? It would produce 1600 watts of power, giving you a watt density of about 32 W/in². That’s perfectly fine for a heater of that variety and wouldn’t cause any problems at all, for the heater. What about the application though? Could those paper bills handle a hot heater sitting a few inches away from them? I don’t know but I don’t want to find out. You’ve got to consider the effect on the heater but also how it will affect the application. 

A 3/4” diameter heater was used in that example because I didn’t want to introduce another variable that needs to be considered, and that’s the voltage itself. If you’re going to increase the voltage to a heater you’ve got to make sure it can handle that increased voltage from an insulation point of view. A 3/4” heater can take 480V without batting an eye. A 1/4” diameter heater? Not at all. A 1/4” diameter heater would fail when a higher voltage is applied not because of the higher temperature it would experience but the higher voltage it has to be insulated against. That applied voltage is constantly trying to jump from the heater circuit the sheath and that insulation strength is less when there’s less distance between life and grounded components and decreases with temperature. When applying a higher voltage to a heater, make sure the heater itself can take the higher voltage. 

In that example earlier of a 3/4” OD x 24” long cartridge heater designed to produce 100 watts at 120 volts, let’s compare it to a heater with the same dimensions producing 1600 watts at 480 volts. Their internal construction would be absolutely identical; same components, same resistance wire, same winding pitch, same everything. With the exception of the marking on outside of the heaters, they would otherwise be indistinguishable from each other. Heaters don’t ‘know’ if they’re going to be connected to 120 volts or 480 volts, just that their resistance is fixed so the wattage produced will be determined by the voltage applied. Just something to think about. 

 We’ve discussed a lot about applying a higher voltage to a heater than it’s rated for. The general rule in that situation is to simply not do it. Don’t try to reason yourself into it and that it’ll be fine. There are liability issues that need to be considered as well as the safety issues mentioned earlier. Even if you’re SURE that everything will be fine, it’s best just not to do it. Speaking of liability, there’s also the issue of applying a higher voltage to a heater than what’s marked on it. Can you explain that satisfactorily to your customers? Think carefully about it and the consequences. 

 Applying a lower voltage? Have a good time. That’s perfectly safe as long as there’s no other circuitry that might be affected.

To summarize: 

- Always consider the safety of what you’re doing. This is key not only with the heaters involved, but the application in which they will be used. 

- Using a lower voltage than a heater is rated for is always safe. Make sure all precautions used for the rated voltage are also followed when using lower voltage. 

- Using a higher voltage should be done cautiously. A small increase in the applied voltage (like applying 120V to a 110V heater) is probably fine, but you should always check. Something like doubling the voltage is generally not a good idea, but possible if certain criteria are met. Be sure the heater can withstand the higher voltage. Be sure the heater can withstand the higher watt density and temperature. Be sure the application can withstand the higher temperature. 

- The power output of a heater changes by a square law. Double the voltage and you’ll get four times the power. Halve the voltage and you’ll be a quarter of the power. 

To close, as stated several times, keep it safe. Check your work. If you’re unsure of the results of your actions, just don’t do it.