Constant resistance wire

[ptonsparky]

Educate me please.

I have a short section of constant wattage heat trace cable labeled as 8 watts per foot.

Measuring the resistance of that section definitely calculates out higher than 8 watts. Ok, resistance of it increases as it heats. That works for me, but the info for the product indicates it uses constant resistant wire which isn’t how I would have defined it. Obviously I’m wrong again.

Is it constant resistance at applied voltage?

 

[ptonsparky]

I was expecting constant resistance at any voltage,

 

[dkidd]

but the info for the product

Can you post a link?

 

[ptonsparky]

https://www.briskheat.com/products/...stant-wattage-heating-cable-fe-2821.html#tab3

Fixed resistance, not constant. Sorry

 

[winnie]

I think that by 'constant wattage' they mean that the watts per foot of the cable doesn't change with the length of the cable.

Imagine a heat Trace made with simple resistance wire. If you double the length of the trace, you double the resistance, and thus get lower current and lower watts per foot.

With 'constant wattage' cable you have the main circuit do conductors supplying current to small local resistance wire sections. All of the resistance wire sections are in parallel, and there is very little voltage drop in the main conductors.

I would expect resistance to change with temperature as with most conductors, there is nothing in this cable to regulate or compensate for the conductor tempco.

Jon

 

[winnie]

Oh, I think 'constant wattage' is also distinguished from 'self regulating' where there is an intentional large resistance change to control temperature.

Jon

 

[retirede]

Oh, I think 'constant wattage' is also distinguished from 'self regulating' where there is an intentional large resistance change to control temperature.

Jon

This is important part. ‘Constant resistance’ is the alternative to ‘self regulating’.

 

[ptonsparky]

I measured the 28” piece of heating section @ 1080 ohms. 38.6 ohm per inch. 463 per ft.
That turns into 12.5 watts per foot. The only way for the cable to be 8w/ft is if the resistance of the element changes as it warms. Not what I would consider a fixed resistance. ??

 

[winnie]

I measured the 28” piece of heating section @ 1080 ohms. 38.6 ohm per inch. 463 per ft.
That turns into 12.5 watts per foot. The only way for the cable to be 8w/ft is if the resistance of the element changes as it warms. Not what I would consider a fixed resistance. ??

I agree, the resistance is not absolutely fixed. It just has the normal variation with temperature of a metal.

I think that in this case the resistance is simply much more stable than the intentional resistance change designed into the self regulating cables.

-Jon

 

[winnie]

I measured the 28” piece of heating section @ 1080 ohms. 38.6 ohm per inch. 463 per ft.
That turns into 12.5 watts per foot. The only way for the cable to be 8w/ft is if the resistance of the element changes as it warms. Not what I would consider a fixed resistance. ??

If 28" has a resistance of 1080 ohms, then 1" would have 28x the resistance. The resistance elements are in parallel, so the fewer the elements the _higher_ the resistance.

Also the elements are made in sections, and any section cut in the middle is no longer connected; so 28" might not actually have 28" of active element.

-Jon

 

[ptonsparky]

I am between the contact points so the section would have been heating.

the element wire is single and wraps from one contact point, L1, to another contact point on L2 about 3’ away. Those points alternate so each section would be an independent heater. My section of 28” had 1080 ohms and is free from contact of either power lead.

 

[ptonsparky]

Could it be that the wrapping of the element wire around the power conductors gives me that added impedance to reduce the current flow?

 

[retirede]

Two kinds of heating cable. One is called fixed temperature or self-regulating. The other is (technically incorrectly) called fixed resistance simply to differentiate from the former.

Self-regulating will increase in resistance rapidly above a certain temperature, limiting the temperature rise.
Under the same conditions, the ‘fixed resistance ‘ variety will continue to heat, sometimes to the point of destruction.

 

[ptonsparky]

Two kinds of heating cable. One is called fixed temperature or self-regulating. The other is (technically incorrectly) called fixed resistance simply to differentiate from the former.

Self-regulating will increase in resistance rapidly above a certain temperature, limiting the temperature rise.
Under the same conditions, the ‘fixed resistance ‘ variety will continue to heat, sometimes to the point of destruction.

Thank you, (technically incorrectly) helps.

The difference in watts per foot calculated and the actual rating still has me wondering why.

 

[steve66]

I was expecting constant resistance at any voltage,

I think the word "constant" is relative. Meaning relatively constant over the expected temp. range as compared to self-regulating heat trace.

Since it says its made for intermediate temps. around 400 deg F, I wouldn't expect to measure the same resistance at room temp. as it would have when its operating.

So I'm assuming as it heats up the resistance rises enough to meet the specs. And since the heat is I squared R, the resistance only has to rise by about 25% to hit the spec.

 

[Tony S]

This may help.

https://www.youtube.com/watch?v=jKbMFtUfgx8

I've installed loads of it but never questioned how it works.

The last lot I installed was during a gale 200 Ft in the air, all I wanted to do was get the job and go where I could get warm.

 

[steve66]

This may help.

https://www.youtube.com/watch?v=jKbMFtUfgx8

I've installed loads of it but never questioned how it works.

The last lot I installed was during a gale 200 Ft in the air, all I wanted to do was get the job and go where I could get warm.

During my short stint as an electrician eons ago, I remember agreeing stay late to help replace some heat trace on the coldest night of the year. I think it was something like 10 or 20 below zero.

One time was enough to leave me with the impression that heat trace always goes bad at 5 PM on a Friday on the coldest night of the year. And its always in a location up in the air where there is nothing to block the wind.

 

[ptonsparky]

During my short stint as an electrician eons ago, I remember agreeing stay late to help replace some heat trace on the coldest night of the year. I think it was something like 10 or 20 below zero.

One time was enough to leave me with the impression that heat trace always goes bad at 5 PM on a Friday on the coldest night of the year. And its always in a location up in the air where there is nothing to block the wind.

I'll have to agree. Colder than a well digger's behind every time we have to work on it.