How long will a cable last at its limit?

[marko333]

Hello,

If you have a cable that is rated to, say 100amps, how long will it last for, running at 100A??
Will it run forever, or is there some sort of degredation as it is run at its limit?

Does a cable have a let through energy rating of sorts that can be calculated from current and time?

Thankyou.

Marko T

 

[Midos]

Marko,
The cable vendor would be the best person to speak to on this subject, but I would say having it run fully loaded is not good engineering practice and I have known cable to be changed out in the region of 25 years for private dwelling properites.

Cables have a type of let through energy before destructing, it's called short circuit rating and you can get this from from the cable vendor. it will tell you what it can handle fully loaded at the time of short circuit and it will tell you in time/seconds as well.

Midos

 

[RAMConsult]

The Onderdonk equation is used to determine the fusing time versus current of a section of copper wire. Check out this link:

http://www.bussmann.com/library/docs/small wire paper.pdf

for the actual parameters. Another thing that will limit the life of wire is annealing caused by heating due to extended or repeated overload currents below the Onderdonk thresholds.

 

[rbulsara]

Yes, practically it should last forever and will. Now here forever means for the duration of its useful life, which is normally 30 yrs for electric cables. As with any piece of equipment or material, if you stretch them to their limits at all the time, you can expect some reduction in useful life. As the useful life is an average number based on the past experience.

There are 'damage curves' available for all cables that shows how much current a cable can sustain for how long before getting damaged. All short circuit calculation softwares include these curves. These are essentially time inverse curves, obviously lesser current can be carried on for longer time for a given size.

It also varies by type of insulation, PVC, rubber etc. as the insulation, which gets damage first, long before the metal (copper or aluminum) starts mellowing.

But in general a cable can sustain current several times its 'rated' current for a long time.

 

[marko333]

I agree.

How can you then calculate how long a cable will last being overloaded but not at a short circuit fault level.

For example, back to our 100amp rated cable (lets say pvc insulated), can you determin how long it will last for running continuously at say 140amps?
I know of a way to estimate rise temp. but the tables of constants and information says it is only valid for time periods of up to 5 seconds. The system does not work for times longer than this as it is really for fault level currents only.

What i want to know is,theoretically, how many minutes/hours/days i can overload this cable for at a certain overloaded current rating.

 

[jbartos]

Suggestion: The cable insulation aging is governed by the Arhenius Curve. Visit

http://www.silicavalley.com/archives/Reliability.pdf

for the reliability theory behind it
There are different aspects of cable aging. Visit

http://www.adventengineering.com/service/research/insul/H-OPaper-VoidsforCableAgingAsstIEEE-EIDP.pdf

http://www.adventengineering.com/service/research/insul/NPIC086.pdf

http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1998/in98021.html

(for standards)

http://www.bnl.gov/est/files/pdf/ECTF-FactSheet.pdf

(for Labs testing aging)
etc. for more info

 

[rbulsara]

Mark:

The 'damage' curves shows exactly what you are looking for! Ask manufacturers or get a curve from an electrical friend with a short circuit software.