Cable current rating advice

There are SAE cable standards (Society of Automotive Engineers) standards for cables in cars and so on  that may be a better design standard for this sort of application.

( SAE_ AS50881  for example, but it is not cheap..)


Actually the data you would get is the same as scroll to page 47 of 84 of the old "MIL-W-5088L - Wiring, Aerospace Vehicle" that is free..  the line for 6AWG is pretty close to 16mm2

Now depending how hard you are being shot at, and your appetite for hot cables, you can decide on the rating of the cable yourself.


I have screenshotted the graph below. It really  is that spotty in the original, sorry. Now this graph is not quite what you  need but let us slide along the 6AWG line to show the principles of how it works.


So at the bottom of the line  a 30 degree rise (40C ambient, 70C core temp) gets you 75A.


A 50C rise between air and core temp is more like 100A.  - so 20C ambient 70C core, or 30C ambient 80c core.


150A is a 110C rise, so really the cable now needs running water cooling or a different sort of insulation, as for anything other than outside  in the arctic you will be damaging PVC, and can run it like that for a few hours at most before it will need replacement. The graph carries on up, but really that is for PTFE, mineral and silicone rubber cables, which you will not have.







If it was a commercial aircraft, you'd be looking at AC.13-1B , slightly different figures,  but the rules are fiddly as you then consider whole bundle current for looms, as well as the current in any one wire. (the grouping rules in BS7671 do not handle large bundles in the same way ) There you'd be looking a something like a 50 A fuse (75A cable rating)


The very fact that BS7671 makes no distinction between sheathed and non-sheathed cases tells you that for that sort of 'zip cord' the current rating in BS7671 will be a significant under-estimate. It also does not mention clipped vertically being different to horizontally, but a moment of thought about convection tells you there has to be a difference. The ratings are just conservative.


We are not that interested in where the insulation starts to degrade, as that has us looking at lifespans of half a million years for very cool running (only out of sunlight of course).  What we need is to exceed the maximum lifetime of the installation by  a factor of perhaps 2 or 3.


In reality the PVC internals are is quite happy with short flights to perhaps 120C, but the cable loses its flexibility when repeatedly heated that hot and cooled and if you need it to remain flexible then  a maximum  of 105C is a better choice. Now, what current gets the core of the wire to that temp when the environment is at say 30C is a very variable thing, depending on airflow, bunching with other wires, mounting points and orientation.


In short, 16mm zip cord will be OK for 100 amps, most of the time, but not in poorly ventilated places or in bundles of other hot cables.

150 A almost never OK.

70-90A pretty conservative (which is still too hot to touch by the way..)

Mike.

There are SAE cable standards (Society of Automotive Engineers) standards for cables in cars and so on  that may be a better design standard for this sort of application.

( SAE_ AS50881  for example, but it is not cheap..)


Actually the data you would get is the same as scroll to page 47 of 84 of the old "MIL-W-5088L - Wiring, Aerospace Vehicle" that is free..  the line for 6AWG is pretty close to 16mm2

Now depending how hard you are being shot at, and your appetite for hot cables, you can decide on the rating of the cable yourself.


I have screenshotted the graph below. It really  is that spotty in the original, sorry. Now this graph is not quite what you  need but let us slide along the 6AWG line to show the principles of how it works.


So at the bottom of the line  a 30 degree rise (40C ambient, 70C core temp) gets you 75A.


A 50C rise between air and core temp is more like 100A.  - so 20C ambient 70C core, or 30C ambient 80c core.


150A is a 110C rise, so really the cable now needs running water cooling or a different sort of insulation, as for anything other than outside  in the arctic you will be damaging PVC, and can run it like that for a few hours at most before it will need replacement. The graph carries on up, but really that is for PTFE, mineral and silicone rubber cables, which you will not have.







If it was a commercial aircraft, you'd be looking at AC.13-1B , slightly different figures,  but the rules are fiddly as you then consider whole bundle current for looms, as well as the current in any one wire. (the grouping rules in BS7671 do not handle large bundles in the same way ) There you'd be looking a something like a 50 A fuse (75A cable rating)


The very fact that BS7671 makes no distinction between sheathed and non-sheathed cases tells you that for that sort of 'zip cord' the current rating in BS7671 will be a significant under-estimate. It also does not mention clipped vertically being different to horizontally, but a moment of thought about convection tells you there has to be a difference. The ratings are just conservative.


We are not that interested in where the insulation starts to degrade, as that has us looking at lifespans of half a million years for very cool running (only out of sunlight of course).  What we need is to exceed the maximum lifetime of the installation by  a factor of perhaps 2 or 3.


In reality the PVC internals are is quite happy with short flights to perhaps 120C, but the cable loses its flexibility when repeatedly heated that hot and cooled and if you need it to remain flexible then  a maximum  of 105C is a better choice. Now, what current gets the core of the wire to that temp when the environment is at say 30C is a very variable thing, depending on airflow, bunching with other wires, mounting points and orientation.


In short, 16mm zip cord will be OK for 100 amps, most of the time, but not in poorly ventilated places or in bundles of other hot cables.

150 A almost never OK.

70-90A pretty conservative (which is still too hot to touch by the way..)

Mike.