Use temp to find current?

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drbond24

drbond24

Senior Member
Location
Barboursville, West Virginia
I am curious to know if there is an equation that will allow me to calculate the current in a conductor if I know its operating temperature. I have looked on the internet and through the books I have but can't find anything.

I came across some cable last week that was operating at 194 F and would like to be able to figure about how overloaded it was.
 
This is what I use.
FLUKE_336.jpg
 
I agree with Barbeer. It is so easy to use a clamp-on amp meter, why go through the calculation that you propose? There are so many variables that would need to be accounted for:
ambient temperature
air movement
number of conductors in raceway/cable
raceway material (if in raceway)
proximity of other conductors and their load/temperature
termination torque and material
effect of sunlight adding to heat load
and many others

The best you could hope to do is get a very rough approximation.
Clip on an amp meter and know the exact load in seconds.
 
I have one of those too, but I am in West Virginia and the wire I'm talking about was in Houston, Texas. My arm isn't that long. :)

I know I couldn't get an exact answer from an equation but it's the best I can hope for at this point.
 
I would think my degree was obvious at this point from the way I am overcomplicating this situation. Engineers aren't happy unless there is some math involved, hence I am looking for an equation. If I can't find one I'll just get out the old calculus book and get my fix that way.
 
You want the inverse of the Neher-McGrath equation. See 310.15(C)

Unfortunately, I have to agree with haskindm; the number of unknowns that you would need to feed into your inverse equation is probably far to massive to get a real grasp on the current being carried.

On the other hand, 194F is 90C, which is the temperature _expected_ for a 90C conductor fully loaded in the ambient conditions assumed by table 310.16...so you would seem to be in the ballpark of 'fully loaded'.

-Jon
 
That is logical, and I am embarassed for not looking at it that way. :(

I don't have a lot of details on the situation, but I do know that our electrician had installed 262 MCM cable from a 600 A breaker and that is only half of the ampacity needed. I just assumed that when the cable was hot it was extremely overloaded, but at that temperature it must have been loaded pretty much 100% or about 300 amps.

The temp rating of the cable was 90 C, but the terminations on the cable were only rated 75 C.

Yea, I know, that is a weird cable size. It is DLO cable which has sizes slightly larger than standard. I had to look it up when I heard it the first time. :) Electrician said DLO cable was more flexible and therefore easier to install. Same guy didn't know he had to derate the ampacity when he put cable in conduit, so I don't listen to him anymore anyway. :)
 
drbond24 said:
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The temp rating of the cable was 90 C, but the terminations on the cable were only rated 75 C.

Yea, I know, that is a weird cable size. It is DLO cable which has sizes slightly larger than standard. I had to look it up when I heard it the first time. :) Electrician said DLO cable was more flexible and therefore easier to install. Same guy didn't know he had to derate the ampacity when he put cable in conduit, so I don't listen to him anymore anyway. :)
Click to expand...

I am curious
Where did you physically take the temperature reading?

The reason I ask, is the lugs for termination for the equipment are most likely Class "B" lugs
The DLO is not rated to be terminated to those type lugs (DLO is most likely Class "C") and maybe the temperature is the premature signs of a termination failure.
 
Rise or actual

Rise or actual

winnie said:
You want the inverse of the Neher-McGrath equation. See 310.15(C)

Unfortunately, I have to agree with haskindm; the number of unknowns that you would need to feed into your inverse equation is probably far to massive to get a real grasp on the current being carried.

On the other hand, 194F is 90C, which is the temperature _expected_ for a 90C conductor fully loaded in the ambient conditions assumed by table 310.16...so you would seem to be in the ballpark of 'fully loaded'.

-Jon
Click to expand...
Hmmmmmmmm.

Isn't 90Centigrade is the RISE above ambient?
 
I am curious
Where did you physically take the temperature reading?


Someone in Houston took an infrared photo of the cable indicating its temperature. The terminations were not in the photo.

The reason I ask, is the lugs for termination for the equipment are most likely Class "B" lugs
The DLO is not rated to be terminated to those type lugs (DLO is most likely Class "C") and maybe the temperature is the premature signs of a termination failure.

I didn't receive any information on the terminations other than someone saying they were rated 75 C. With the other trouble I've had with this particular electrician, I'll have someone check on the class rating you're talking about.
 
weressl said:
Hmmmmmmmm.

Isn't 90Centigrade is the RISE above ambient?
Click to expand...
No, 90 C is the maximum temperature the conductor can handle. It gets as high as 115 F inside the plant I'm at during the summer. That is about 46 C. Another 90 C above that puts you around 275 F which isn't far from the point at which the insulation would melt (approximately 300 to 350 F).
 
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