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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?

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3 Answers 3

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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.

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  • \$\begingroup\$ True, and that's what a lot of these specs talk about - making sure your insulation holds up due to heating, and how really, the harness itself is best derated on a case-by-case basis based on the temperature rise of the wires and the temperature rating of the insulation. \$\endgroup\$
    – depwl9992
    Jul 5, 2017 at 21:54
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Bundle these wires ----- flat ----- against the chassis, so the wires dump their heat into the metal. Much better than air cooling.

At high altitudes there is no air, anyway.

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  • \$\begingroup\$ Does this imply that at high altitudes where there is little or no air, all wires (even singles) should be derated as if they were in bundles? \$\endgroup\$
    – brhans
    Jul 6, 2017 at 11:51
  • \$\begingroup\$ Agreed that sinking them to the chassis would help in low atmosphere situations. Talking to my boss, however, all of these wires are internal to our box, and less than 4-in per, some even being shorter. We have calculated power dissipation per wire already, and so I think our solution for now will be to collate this data and propose to simply prove that we're not going to exceed insulation ratings. I'll post an answer on our findings though.. \$\endgroup\$
    – depwl9992
    Jul 6, 2017 at 15:40
  • \$\begingroup\$ @brhans take a look at the derating curve I posted for bundled wires. Turns out single wires all converge at a derating of 1.0 (according to MIL-W-5088). \$\endgroup\$
    – depwl9992
    Jul 6, 2017 at 16:31
  • \$\begingroup\$ @depwl9992 - but doesn't that derating curve assume that there's air around the wire(s) for convection cooling? In a low-atmosphere environment all you have is conduction (along the length of the wire into whatever it's connected to) and radiation (nowhere near as efficient as convection). \$\endgroup\$
    – brhans
    Jul 6, 2017 at 16:37
  • \$\begingroup\$ @brhans Hmm.. it does look like it's a factor in the entire test procedure in section 6.7, but if they use Figure 3 (correlating current, temperature rise and wire gauge) to calculate allowed current of a single wire in free air, I would assume (hope) that it would take care of the capability of convection cooling by itself, and that it would not be a factor in the bundle derating curve (Figure 4). Plus EEE-INST-002 (in comparison with the two examples in 6.7.1a of MIL-W-5088) is WAY more conservative in its ratings table, so it may take conduction-only-cooling into an account ahead of time. \$\endgroup\$
    – depwl9992
    Jul 6, 2017 at 17:33
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Proposed these comments, plus my research, to my boss yesterday and came up with an okay proposed solution to our predicament. It turns out that yeah, INST-002's "bundled wires" table is a fairly shallow interpretation of how current ratings of wires should be handled.

With a bit of research from him, we found an older MIL spec (MIL-W-5088) which addresses this very issue quite well, and includes the following derating curves for bundled wires! MIL-W-5088 Bundle Derating Curves

So considering that I could have a 2 bundles of 4x 12-AWG wires, each with a single-wire derating of 25A, and all of them handling 85A total, I can easily calculate the "total bundle loading" to 85%, giving me about 0.53 on the derating curve and a new current derating per wire of $$0.53\cdot25A = {13.25A}$$

which, if we do the math according to INST-002 is much better than 11.5A per wire: $$\frac{11.5A}{25.0A}=0.46$$

Of course, one idea to make this even better was to bundle positive and negative separately, bringing our wires per bundle down to 4x and our derating factor up to 0.68.

But this is all a bunch of idealized math, and really, the proposal that we've come up with for now is, "well, these are all internal wire harnesses, with wire lengths less than 6-in everywhere (low effective series resistance), and we have already calculated the power dissipation per wire based on measured efficiency calculated throughout the unit, so let's just propose that from this information, plus tying wires to the chassis for cooling where needed, we should be okay."

2023 Edit: Ran across this picture in my archives today and decided to transcribe it manually into Excel for no reason. For future reference, I made a Gist of the resulting CSV file.

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  • \$\begingroup\$ You can also measure the wire temperature when the wires are in service to verify they are below the insulation temperature. \$\endgroup\$
    – user57037
    Jul 7, 2017 at 7:00
  • \$\begingroup\$ So to be clear, you were originally assuming 8 wires all together in one bundle (4 power and 4 return)? Would you consider this any differently that 8 power wires all together in one bundle? And if you split into two bundles, why wouldn't you make it one bundle of 2x ground and 2x power and another bundle of 2x ground and 2x power (maybe even twisted pair)? That would surely be better in terms of EMI, yes? \$\endgroup\$
    – scuba
    Jul 26, 2018 at 22:35
  • \$\begingroup\$ Well, we're also looking at a lot of space constraints in this whole exercise. The entire box is 9-in x 4-in x 5.5-in, and we're going to really only be using the top half of that for harnessing this bundle of low-gauge wire to an MS27497 connector on one of the 4-in x 5.5-in sides. Additionally, this is a straight DC feed from a solar array, so the noise on the bus power should already be relatively low. But separating them even a little bit would help in the cross-conduction of heat, so that's not a bad idea... \$\endgroup\$
    – depwl9992
    Jul 30, 2018 at 15:55

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