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I live in a very hot climate where temperatures regularly touch 45 deg C and the heat in the engine bay area means that under-hood wires, especially the thinner ones, endure high levels of thermal fatigue.

As a result, when working on older cars (10+ years) I am very apprehensive about flexing any wiring in the engine bay area in case I introduce a break (and a new headache). This includes wires for things like coolant temp sensor, MAF sensor, coil and injector power supply

Is there anything that one can do to delay the onset of wire embrittlement? Would it help if manufacturers use a different grade/gauge of wire to help with longevity?

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    Negative ground DC systems on vehicles are prone to this, not much you can do, negative ground systems leach copper out of the wiring to anything metal that is grounded in the vehicle (electroplating process), it is a slow process. Heavy equipment used in mining operations (and other uses) use a positive ground which elevates this issue. – Moab Aug 11 '16 at 20:26
  • @JasonC The engine bay is hotter because the ability for heat to be absorbed by the environment is reduced. That said, I wasn't aware that electrolysis could play a role here. – Zaid Aug 11 '16 at 20:32
  • Am I correct in suspecting that most of your observed cracks (for insulated wiring), are at or near connection points? – Jason C Aug 11 '16 at 21:26
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    @Moab elevates or alleviates? Opposite meanings. – user253751 Aug 12 '16 at 6:46
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    Hot Network Questions listed this, I misread it as "wine embrittlement" and thought "you should drink the wine before it goes off" – Criggie Aug 12 '16 at 9:37
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The very short answer is that heat isn't a significant problem for the wire. Heat is a problem for the insulation.

So if you have wires failing I think there is something else going on.

The problem with dealing with failing insulation is that there isn't much you can do about it – short of rewiring the affected parts. In a modern car that could be a non-trivial project. As Agent_L mentioned there are good high temperature insulation materials, but retrofitting them isn't really practical in many cases.

There are a few practical things you can do:

  1. Heat shrink tubing, though this usually requires reterminating the wire since most heat shrink won't shrink enough to fit over most connectors.

  2. Electrical tape, it's not elegant, but it can be effective and is relatively easy to do.

  3. "Paint on" electrical tape – I've never used the stuff, but it is available and it might work well in some applications, I think it would do a better job of excluding moisture than tape.

  4. If the affected area is small and the car is old one of the easier approaches might be to replace the section of the wiring harness, or to rebuild it. If the car is old then I wouldn't even worry too much about getting better wire, unless it is a classic, as the replacement wire will last for many years before it fails.

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    I think you've hit the nail on the head... it is the insulation that suffers most. Could you elaborate on possible ways to reduce the impact of heat on insulation? – Zaid Aug 12 '16 at 7:18
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    @Zaid You can't, but you can rewire with silicone-insulated wires. Those are often rated well over 100°C. Or pull silicone sleeves over existing cables at least. It's rarely done at the factory because it's overkill - such wires will outlast the car. – Agent_L Aug 12 '16 at 12:12
  • @Zaid Outdoor rated, high temperature range, good flexibility. Silicone nails it. You've probably seen it before on nice ignition wires. – Jason C Aug 12 '16 at 12:55
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    @Zaid Another thing you could do: Use self fusing tape. You could also use additionally a resealable cable protection fraenkische.com/en/Cable-protection/Resealable-cable-protection/… – Martin Aug 28 '16 at 23:01
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Heat and thermal stress, even at engine bay temperatures, is not generally the cause of copper wire failure. There is "hydrogen embrittlement" for copper but that only really comes into play at 400C+ in hydrogen-rich environments, usually during annealing. Also, the 20-30C higher range from your hotter climate is pretty negligible from copper's point of view.

Copper's linear thermal expansion coefficient is roughly 17 micrometers / meter / degree C (off the top of my head, you can look it up though). It's very negligible in the lengths found in most wiring applications and especially negligible when the wire is just hanging there with plenty of space to expand/contract without inducing mechanical stress.

Copper is also specifically chosen for wiring because of its high tensile strength and great durability, not just its high electrical conductivity. Aluminum, for example, is a lot cheaper, makes perfectly fine wiring, but has a lower tensile strength and higher thermal expansion coefficient, meaning extra care needs to be taken to managing it (aluminum's big advantage is cost and weight; they use it in a lot of long distance overhead wires).

Copper also has a high thermal conductivity, meaning "hot spots" in wires tend to even out quickly, reducing internal stresses due to uneven expansion. It also has a fairly high resistance to corrosion from environmental factors like moisture and oxygen, although of course it is not immune and this can be an issue over time.

In a way, copper is the solution to the types of stresses that wiring generally undergoes in an environment like an engine bay.

The primary failure modes of wiring are usually:

  • Insulation failure: Cracking insulation leading to shorts that can get hot enough to melt copper. Insulation failures can be caused by heat, UV, environmental factors and degradation, etc.

  • Galvanic corrosion at connection points: Aluminum, steel and zinc will generally corrode when in electrical contact with copper. Copper will generally corrode when in contact with stainless steel or brass. This can weaken connections over time.

  • Oxidation: While copper is pretty resistant to oxidation, it still happens over time (e.g. the Statue of Liberty is green, and you can certainly spot lots of corrosion in exposed copper wires in aging vehicles) (actually copper oxides aren't green but ultimately they turn into green copper carbonates/sulfates like malachite and such). The corroded copper is much more brittle than the metal. Obviously, wiring with more surface area, such as stranded wiring, will have a bigger issue. Also if the insulation is cracked or permeable to oxygen, that's another potential source.

  • Edit - I guess mechanical fatigue is worth mentioning too, perhaps due to past abuse during maintenance or fatigue due to vibrations in really unfortunately placed wiring. Copper's pretty resilient to this but certainly not immune.

Oxidation is probably the issue that'll cause you the most problems over time.

So from all that I would say:

  1. Manufacturers are already doing the right thing just by using copper to begin with.
  2. You might be able to slow it down by e.g. spraying good insulation onto exposed points but... there may not be much to improve.
  3. In a vehicle that already has heavily corroded wiring there's nothing you can do to reverse it.
  4. Get used to old wiring breaking as a fact of life and budget time for wiring replacements after a visual inspection of the state it's in.
  5. Using thicker wire wouldn't hurt but may not help. More importantly, use wiring with good, flexible insulation rated for outdoor environments and engine bay temperatures. Silicone is a good choice.
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    See my comment to dlu's answer... I think he's right that heat affects insulation more than the copper itself – Zaid Aug 12 '16 at 7:20
  • @Zaid Yep. I tried to say that too but he said it much more clearly. Point is, you either aren't actually observing copper failure, or what you're observing isn't due to heat. Visible cracks mid wire, probably insulation only. Actual copper failure is more likely to happen near connection points. From your description you've probably run into both. Silicone insulation is a good choice for that. Any connection point copper failure you see, though, are just one of those inevitabilities. I couldn't tell what you meant by "introduce a break" so I just took the shotgun approach. – Jason C Aug 12 '16 at 13:10
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    Silver would make for better wire... the thermal expansion coefficient for silver is 7.8, for copper it's 16.6, and for aluminum (shudder) 23. Also, silver is the best known conductor. – Ben Welborn Aug 13 '16 at 1:12
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When you say "embrittlement", are you talking about this?

enter image description here

This is a common issue with PVC used as insulation: PVC is brittle by nature an needs plasticizers to be added to it when making cable insulation. Over time, plasticizers evaporate (remember that "new car smell" which gradually goes away?), brittling the PVC.

Is there anything that one can do to delay the onset of wire embrittlement?

You could keep your car in the garage and not under sunlight / rain. That will reduce both copper corrosion and insulation embrittlement.

Would it help if manufacturers use a different grade/gauge of wire to help with longevity?

Obviously, using bigger copper gauge and thicker insulation would make wires more robust. Also, replacing PVC insulation with a more robust one (XLPE or similar) would also help. Since it would also be more expensive at no benefit to the manufacturers, they wouldn't bother.

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If you can't find silicone insulated wire, try nylon coated pvc insulation. it's very common at any electrical supply place. It's rated for higher temperatures and the nylon coating helps protect the pvc from degrading. A little more expensive than regular wire, but less expensive than silicone insulated wire.

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