We have an aerospace application and are having trouble defining EEE-INST-002's current derating guidelines in regards to wire.
The issue that I have isn't about stranded versus solid. MIL-W-22759 does seem to classify tinned, copper strands as one wire so long as the outer diameter of said wire is of its declared gauge. Similarly, we don't have any cause for concern when it comes to multi-conductor, shielded, jacketed cable in the sense of, say, Ethernet or something similar.
What I'm having trouble understanding is what exactly does NASA consider to be a "wire bundle," and when should we use the derating guideline for bundled wires/multi-conductor cables?
Consider this portion of our application:
We need a power input of something like 85A @ 70V. INST-002 would dictate that we need 8x 12AWG wires since, according to the derating table, 12AWG wire is rated at 25A, and 4x would cover the positive bus and 4x the negative (with some margin to spare).
But wait! We just tie-wrapped these wires inside of our box, making this a harness (bundle) of 8x wires. So….if these are now part of a bundle, did our derating of each 12AWG wire in this bundle suddenly drop to 11.5A according to INST-002? Why?
I found an article titled "Derating Standards and Thermal Modelling Tools
for Space Harness Designs" which boils down to "why in the world do multi-conductor cable ratings vary so widely between different space agencies?" But I was not able to glean any sort of reasonable answer from it regarding my own application.
Any thoughts? And more importantly, are my boss's fears that we suddenly need to double the current rating for every cable in our unit unfounded, or at least able to be allayed?
Best Answer
Imho the 25A is for a single instance of the cable in an open situation . As soon as you group 4 well 8 together into a box the cooling is reduced so the current capacity has to be reduced.
This explanation is the same as cables laid open in a loft or attic or wall with an air space. But as soon as the cables are covered or surrounded by insulation the current capacity is reduced.