Pushing 240V through a 208V heating element

[electrofelon]

the only time increasing voltage means less current is when the equipment is designed and built to consume a given power with a different applied voltage.

A good example of that is a multi voltage ballast or LED driver. Seems like most now can take anywhere from 100-277V and the wattage is the same (probably efficiency varies a bit over that range, but ignoring that). But a resistive load is too dumb to change its resistance, although there are some special cases of things like certain "self regulating" heating cables made of special alloys where the resistance changes significantly enough with temperature to alter the power consumed.

 

[Another C10]

That is very incorrect. Except for motors, which attempt to be constant-power loads, the only time increasing voltage means less current is when the equipment is designed and built to consume a given power with a different applied voltage.

In other words, for a constant impedance load, current increases proportionately with increased voltage, not inversely.

Thank you for the correction, I suppose I should refresh my studies on that law a bit .. Sincerely, thanks.

 

[Another C10]

In other words, for a constant impedance load, current increases proportionately with increased voltage, not inversely.

I finally brushed away a few dusty brain cells this morning and did come to the understanding that yes I understand the reasoning on how a metal element would get hotter with a higher voltage and being that there is no restrictive load basically the heat or wattage should increase as well, I imagined a fixed object like a potato with 240 running through it, yes it would cook but then imagined running 480 through it and bingo, it would cook a heck of a lot faster .. not sure why I thought the element had any real form of resistance. Thanks again Larry.

 

[Another C10]

1.3x the power of the elements.

I get it now, makes perfect sense, and if 480 was applied to it then the element would probably be white hot instead of the typical orange.

by the way to those that may of wondered, I never have or would subject an element or motor to any voltage not mentioned on the nameplate.
just saying one thing for certain I always confirm a theory I may not completely be positive of with a written fact to fall back on.

 

[LarryFine]

As a rule-of-thumb, the power of a 240v resistive load on 208v is almost exactly 75% of the power at 240v.

And of course, the inverse is true: the power of a 208v load on 240v is 4/3, or 133% of that when on 208v.

 

[drcampbell]

... Except for motors, which attempt to be constant-power loads, the only time increasing voltage means less current is when the equipment is designed and built to consume a [constant] power with a different applied voltage. ...

It's also applicable to transmission & distribution. Step up the voltage and the same load can be served with less current. (and less voltage drop)

 

[Todd0x1]

Office I had a few years ago (I was first tenant in new bldg) had an electric instant water heater under the bathroom sink. The hot water was REALLY hot. After 6 months or so the water heater failed. I checked it out and found the heating element open. After removing the unit I discovered it was a 208v model supplied with 240v. The heating element couldn't take the extra current and eventually burned up.

 

[kwired]

It's also applicable to transmission & distribution. Step up the voltage and the same load can be served with less current. (and less voltage drop)

But if you would double the voltage you would have to change supplied transformers or change taps if designed with that option, you don't just double voltage and do nothing else in this situation.

 

[kwired]

I finally brushed away a few dusty brain cells this morning and did come to the understanding that yes I understand the reasoning on how a metal element would get hotter with a higher voltage and being that there is no restrictive load basically the heat or wattage should increase as well, I imagined a fixed object like a potato with 240 running through it, yes it would cook but then imagined running 480 through it and bingo, it would cook a heck of a lot faster .. not sure why I thought the element had any real form of resistance. Thanks again Larry.

I get it now, makes perfect sense, and if 480 was applied to it then the element would probably be white hot instead of the typical orange.

by the way to those that may of wondered, I never have or would subject an element or motor to any voltage not mentioned on the nameplate.
just saying one thing for certain I always confirm a theory I may not completely be positive of with a written fact to fall back on.

Heating element in this case is about as real of resistance you will encounter in higher power resistors. It's actual resistance may change a little bit as it's temperature changes but it still remains close enough to same that we typically disregard that change. Incandescent lamp elements however are one thing that has much lower resistance when cold than when heated.

Motors - are a different ball game, especially AC induction motors. Voltage and frequency together determine how that motor is going to respond to what you input, though you still typically not going to put over both rated volts and/or rated frequency to a particular motor unless it was designed to handle it for any extended period of time. At same time you won't apply significantly less volts/frequency unless motor or other features of application were designed for it as the shaft mounted cooling fan won't be as effective at low speeds as it is at nameplate speed.

 

[drcampbell]

The only problem with appliance 240 volts on an appliance rated for 208 volts ...is the appliance may not last or work for a very long period of time.

The "only" problem?
I daresay the heating elements operating at a significantly higher temperature during the "on" portion of the cycle and compromising the insulation, melting something, or setting something afire is also a problem.

 

[kwired]

The "only" problem?
I daresay the heating elements operating at a significantly higher temperature during the "on" portion of the cycle and compromising the insulation, melting something, or setting something afire is also a problem.

They get even hotter if you accidentally connect a 120 volt appliance to 208 or 240

 

[retirede]

They get even hotter if you accidentally connect a 120 volt appliance to 208 or 240

Normally not for long

 

[A/A Fuel GTX]

It appears the manufacturer of this convection oven won't supply the 240 volt elements needed to correctly resolve this dilemma. Therefore, I've decided to go the buck/boost transformer route. Since the 10 KW load is mainly inductive with a small motor for air circulation, should I size the transformer at 125% even though this isn't a continuous load? This is uncharted territory for me.

 

[iceworm]

.... Since the 10 KW load is mainly inductive with a small motor ...

The load will be primarily resistive. I suspect you likely just mistyped.

Transformers are 100% devices. I'd consider going with one sized for the load,

 

[A/A Fuel GTX]

The load will be primarily resistive. I suspect you likely just mistyped.

Transformers are 100% devices. I'd consider going with one sized for the load,

 

[A/A Fuel GTX]

Yes, you are correct. I meant resistive. So you're thinking a 10 KW transformer would be sufficient?

 

[iceworm]

Keep in mind that at full load, the transformer will be pretty hot. However, as you (and others) mentioned, after it heats up, it is not a 100% to maintain.

The transformer is pretty small. You are bucking 28V at 50A - about 1.5kw

100% should be fine. I wouldn't undersize.

 

[LarryFine]

So you're thinking a 10 KW transformer would be sufficient?

The BB need not be sized for the load, just the boost. So, in theory, you only need ((10000 / 208) x (240 - 208)) a tad over 1.5kva.

So, look for a 2kva unit.

Buck and Boost Transformer Calculator - Schneider Electric United States

www.buckboostcalculator.com

 

[iceworm]

Larry -
You sure about that?

 

[LarryFine]

Larry -
You sure about that?

Let's see:

10000 / 208 = 48a.
240 - 208 = 32v to drop
32 x 48 = 1536va

What did I do wrong?