Heat trace resistance

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Dsg319

Dsg319

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Going to test and see in the morning but figured somebody would know here. At a job there’s some heat trace we’ve installed and today had to go around with the the inspector and measure resistance values on the heat trace runs.

Wanted ohms readings from line to line on the heat trace and we got on the topic of wether the length of the heat trace would change the resistance (ohm reading).

Example, 75ft of 15watt per foot heat trace 120 vac. When measured resistance we got 4ohms. Say If we had double the amount would the reading change ? Never tested this theory before and wasn’t sure if it measured the the resistance of the whole run, or just at the point of contact.
 
Dsg319

Dsg319

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Wv Master “lectrician”
Cow said:
I can understand wanting to know the line-ground resistance via a megger test, but not a line-line resistance reading.
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Yeah none of us really understood it, just something the company wanted him to do lol.
 
synchro

synchro

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EE
If you double the length then the line-to-line resistance resistance will be approximately one half of the previous value. Think of it as a ladder where the side rails are the two line conductors and each rung is a resistor across the two conductors. So if you double the length, then there are twice as many resistors in parallel and so the resistance goes down by a factor of 2.
In practice the resistance is distributed along the heat trace cable and not in individual resistors, but doubling the length will have the same effect. This ignores the resistance of the two line conductors themselves, but this should be a relatively small effect for shorter lengths. But eventually the line-to-line resistance will not keep going down by a factor of two as you keep doubling the length because this conductor resistance starts coming into play.

As drcampbell mentioned, if it's a self regulating cable when you apply 120VAC the resistance will go up as the temperature of the cable rises such that the power is reduced and the local temperature is stabilized. That means that areas of the heat trace that are in contact with something like a pipe that's conducting the heat away will tend to keep a lower resistance, draw more power, and therefore deliver more heat to maintain the target temperature for the cable.
 
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alimardani

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synchro said:
If you double the length then the line-to-line resistance resistance will be approximately one half of the previous value. Think of it as a ladder where the side rails are the two line conductors and each rung is a resistor across the two conductors. So if you double the length, then there are twice as many resistors in parallel and so the resistance goes down by a factor of 2.
In practice the resistance is distributed along the heat trace cable and not in individual resistors, but doubling the length will have the same effect. This ignores the resistance of the two line conductors themselves, but this should be a relatively small effect for shorter lengths. But eventually the line-to-line resistance will not keep going down by a factor of two as you keep doubling the length because this conductor resistance starts coming into play.

As drcampbell mentioned, if it's a self regulating cable when you apply 120VAC the resistance will go up as the temperature of the cable rises such that the power is reduced and the local temperature is stabilized. That means that areas of the heat trace that are in contact with something like a pipe that's conducting the heat away will tend to keep a lower resistance, draw more power, and therefore deliver more heat to maintain the target temperature for the cable.
Click to expand...
thanks. it really help me too.
 
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