# Effects on a cable at sea level compared to 35,000 feet

1. Jul 30, 2013

### joe465

Hi Guys,

Struggling to find any information on the changes to a cable at altitude compared to at sea level. The initial task was to calculate the resistance of generator wires and then change them from copper to aluminium cables to save weight. I have done this leading me onto describing the effects of the cable at 35,000 feet compared to at sea level.

All i can think of is a decrease in temperature causing a reduction in resistance but would ideally like to provide an example but struggling to find a formula to fit. I know copper has a temperature coefficient of resistance of 3.9 * 10^-3 and aluminium is the same.

Are there any other major effects to the cable at altitude?

Cheers,

Joe

2. Jul 30, 2013

### Staff: Mentor

The temperature depends on the environment. 35000 feet above sea level is a typical altitude for an airplane, so temperature will depend on the position of the cable in the airplane.
You also get a bit more radiation, but that should be negligible for cables, and I don't see any effect of a changed air pressure.

3. Jul 30, 2013

### joe465

Thanks, it is for an aircraft, it is generator cables for a 747. No environment stated so i could possibly just go with the 2 degree decrease every 1000 feet?. What would you recommend?

4. Jul 30, 2013

### Staff: Mentor

If temperature is important, I would recommend to check the design, and the expected temperature range for the cable.

5. Jul 30, 2013

### Carl Pugh

I don't have any answers, but do have some questions.
At 35,000 feet the breakdown voltage of air is much less than at sea level.

Would there be a problem with cable terminations breaking down at 35,000 feet?
Would cable have corona at 35,000 feet? (Very short life)
Cooling of cables will be less at 35,000 feet. What is temperature rise?
Will cables outgas at 35,000 feet? Is this a problem?

6. Jul 30, 2013

### Staff: Mentor

It is impossible to answer those questions in general.

7. Jul 30, 2013

8. Jul 30, 2013

### joe465

9. Jul 30, 2013

### Staff: Mentor

10. Jul 30, 2013

### AlephZero

If this is a "homework" exercise, I think using the International Standard Atmosphere would be defensible set of design conditions. Or use ISA + or - 20C, whichever is the worst design conditions.

If it is "for real", you need to find the relevant section of the Joint Airworthiness Regulations (JARs) - sorry, I'm not an "electrical" guy so I can't tell you which section you want, but don't expect it to be easy bedtime reading!

Remember your generator probably has to work after sitting on the ground all day in the Middle East in air temperatures of +50C, as well as at -80C on a cold day at 35,000 ft!