heat trace cable..... a question....

[Fulthrotl]

if you have a heat trace cable that is drawing 24 amps on
120 VAC, and you hook it up on 240 VAC, how many amps
will it draw?

 

[Dennis Alwon]

if you have a heat trace cable that is drawing 24 amps on
120 VAC, and you hook it up on 240 VAC, how many amps
will it draw?

It will draw twice as much-- 48 amps. resistive element will increase as voltage increases. The heat cable may not be rated 240V either.

 

[Dennis Alwon]

V= I x R

120= 24 x R

R= 5

V= I x R

240 = I x 5

I= 48 amps

 

[Sierrasparky]

Do I get a prize for the correct answer?

 

[Sierrasparky]

Darn Dennis....
I bet it will make a bit of a fire too. Most cables are not designed for twice the heat.

 

[zbang]

Most cables are not designed for twice the heat.

Four times the heat: P = I^2 * R (double the voltage, double the current, 2*2=4)

 

[Fulthrotl]

Four times the heat: P = I^2 * R (double the voltage, double the current, 2*2=4)

thank ya all... there was something written in a manufacturers guide that was a bit
confusing, and the resistance of something is not a function of the voltage applied,

and if ya double the voltage, the current doubles as well....

had a length of heat trace cable, supposed to be 5 watts a foot nominally... was
initially going to be 100', turned out to be 190' or so... now, if it's self regulating
cable, 5 watt a foot will draw up to 8 watts per foot until the temperature comes up
to 100 degrees or so....

so initially, 8 x 190' would be 1,590 watts, tapering down to 950 watts at operating range....

said cable, on 117 volts, is drawing 24.5 amps, or 2866 watts. current flow is staying constant.

something to investigate.....

 

[hurk27]

thank ya all... there was something written in a manufacturers guide that was a bit
confusing, and the resistance of something is not a function of the voltage applied,

and if ya double the voltage, the current doubles as well....

had a length of heat trace cable, supposed to be 5 watts a foot nominally... was
initially going to be 100', turned out to be 190' or so... now, if it's self regulating
cable, 5 watt a foot will draw up to 8 watts per foot until the temperature comes up
to 100 degrees or so....

so initially, 8 x 190' would be 1,590 watts, tapering down to 950 watts at operating range....

said cable, on 117 volts, is drawing 24.5 amps, or 2866 watts. current flow is staying constant.

something to investigate.....

Most conductors will change their resistance as they get hotter, so the hotter they get the higher the resistance, this is the quality that make self controlling heat trace cable work, it also makes the 48 amps answer wrong, there are other factors that will have to be considered such as the material the element is made from, how much thermo coupling there is to dissipate the heat away from the element, which would require knowing the temp the element would run at at 240 volts which would give the correct answer.

 

[kwired]

Most conductors will change their resistance as they get hotter, so the hotter they get the higher the resistance, this is the quality that make self controlling heat trace cable work, it also makes the 48 amps answer wrong, there are other factors that will have to be considered such as the material the element is made from, how much thermo coupling there is to dissipate the heat away from the element, which would require knowing the temp the element would run at at 240 volts which would give the correct answer.

Correct. If this were a simple fixed resistance heating element the current will be four times higher if you double the voltage.

Since it is self regulating heat trace cable there is more that needs considered. This kind of cable is simply two buses with semiconductive material between them, and not a simple fixed resistance. As it heats up the resistance gets higher, but it will have to be able to withstand the higher voltage or it will break down.

 

[Dennis Alwon]

I know nothing of self regulating cable so I answered based on what I was given. Obviously other factors enter into the equation. I did learn something new.

 

[kwired]

I know nothing of self regulating cable so I answered based on what I was given. Obviously other factors enter into the equation. I did learn something new.

Almost all heat trace cables are like this. You can cut them to any length and they still work - just at a lower total wattage.

Now buried floor heating cables are usually a fixed resistance and you can not change the length or you will effect the resistance of the whole thing. You also have to bring each end to the power source and can not just end the cable - otherwise you have an open circuit.

 

[Dennis Alwon]

Almost all heat trace cables are like this. You can cut them to any length and they still work - just at a lower total wattage.

Now buried floor heating cables are usually a fixed resistance and you can not change the length or you will effect the resistance of the whole thing. You also have to bring each end to the power source and can not just end the cable - otherwise you have an open circuit.

Yeah I knew about floor heat since I have installed many of them but I had no idea about heat tape. I have seen it installed but have never used any. Not much need for that here in NC.

 

[weressl]

Most conductors will change their resistance as they get hotter, so the hotter they get the higher the resistance, this is the quality that make self controlling heat trace cable work, it also makes the 48 amps answer wrong, there are other factors that will have to be considered such as the material the element is made from, how much thermo coupling there is to dissipate the heat away from the element, which would require knowing the temp the element would run at at 240 volts which would give the correct answer.

Tungsten, constant wat(CW)t resistance type heaters would not be significantly effected by the temperature difference. Variable, self-regulating(SR) cables are variable resistance cables and the resistance change is exponential in relation to the temperature change. Normally SR cables are designed to operate on a single voltage and the W/F is given at the designed optimum operating temperature. The thermal aging of SR cables are much more severe than the CW cables.

 

[weressl]

Correct. If this were a simple fixed resistance heating element the current will be four times higher if you double the voltage.

Since it is self regulating heat trace cable there is more that needs considered. This kind of cable is simply two buses with semiconductive material between them, and not a simple fixed resistance. As it heats up the resistance gets higher, but it will have to be able to withstand the higher voltage or it will break down.

i

Doubling the voltage is usually not a problem from the insulation breakdown viewpoint, the temperature rise is.

 

[Dennis Alwon]

i

Doubling the voltage is usually not a problem from the insulation breakdown viewpoint, the temperature rise is.

Yeah but they are related. Higher voltage higher temp.

 

[weressl]

Almost all heat trace cables are like this. You can cut them to any length and they still work - just at a lower total wattage.

Now buried floor heating cables are usually a fixed resistance and you can not change the length or you will effect the resistance of the whole thing. You also have to bring each end to the power source and can not just end the cable - otherwise you have an open circuit.

There are constant resistance cables of series and parallel resistance. Series resistance cables can be used for different length if the voltage is variable and parallel resistance cables are current/length limited - max. current depends on the bus-wire size - and can be cut to length at marked intervals of 2-3'. If it is cut away from the mark, the remainder length becomes an inactive, non-heating segment.

 

[weressl]

Yeah but they are related. Higher voltage higher temp.

Nobody said they aren't and for those familiar with the SR technology it self-evident.

 

[jim dungar]

Correct. If this were a simple fixed resistance heating element the current will be four times higher if you double the voltage.

Based on a fixed resistance:
Double the voltage is double the current.
Double the current is four times the power.
Therefore double the voltage is also four times the power.

 

[Dennis Alwon]

Nobody said they aren't and for those familiar with the SR technology it self-evident.

No but you said

Doubling the voltage is usually not a problem from the insulation breakdown viewpoint, the temperature rise is.

My point was doubling the voltage raises the temp. so it is a problem for the insulation breakdown. NO?

 

[Fulthrotl]

Based on a fixed resistance:
Double the voltage is double the current.
Double the current is four times the power.
Therefore double the voltage is also four times the power.

this was the clarification i was seeking... not into the subtleties
of of the system design, or anything like that, but just into the
basic ohms law.... the question came up, and one opinion that
came up was that if you doubled the voltage, the current would
be halved, and i knew that wasn't gonna happen, and i had a
cheeseburger bet hanging in the balance here, so i posted it up
here... i knew before the horse was beaten to death, i'd get a full
primer on heat trace cable, which will be helpful come tuesday morning.

thank ya all for the input, and it'll be a fun day tuesday finding out why
a couple hundred feet of 5 watt cable, which should pull maybe 1 kw, is
pulling almost 3x that amount.

i'll let ya know how it turns out...