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In the UK, household wiring uses 3-core flexible cables. Core size for some applications might be 2.5mm^2 and rated for 24a at 5.75kW at 240v.

This means the cable exceeds the current draw of most in car circuits, such as the 12v outlet or horn. I have used it for both of these applications because the flex is thicker than the existing OEM wire in both circuits. It is also rated at higher current, voltage and power. Connections can still be made reliably using solder or crimp connectors.

Its been suggested to me that this isn't safe or proper for automotive use. Why? Surely the fuse is still the weakest point in these circuits and would break the circuit long before any problem with the wire?

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    I can't say for sure about wire in the UK, but household wiring in the U.S. often lacks the chemical and heat resistant insulation used in automotive wiring.
    – mikes
    Dec 11 '16 at 23:15
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    possibly weight concerns are a factor. there's a lot of wiring in cars.
    – Ben
    Dec 12 '16 at 0:53
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Automotive cables are normed (ISO 6722 ?). The insulation of automotive cables offer a superior resistance to heat, cold, chemicals and vibration. A common FLYR-cable should guarantee its insulation for a temperature range from -40°C to 105°C. Nobody will give you the same guarantee for a common household flex cable

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    Totally agree, household cables are usually rated for 70°C, see e.g. page L7 of this document, which is a typical, stranded cable for household use. And this is environmental temperature plus temperature from the current flow. 70°C can easily be reached under the hood and also in the cabin. Due to vibrations, I'd say installation in a car is not fixed, so the lower limit is more -5°C than -40°C.
    – sweber
    Dec 12 '16 at 11:16
  • Even -40° C to 105° C isn't really going to cut it in the engine compartment. Use something like GXL to TXL (or equivalents in other parts of the world) which will take you up to 125° C.
    – dlu
    Dec 20 '16 at 4:27
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Suitable and optimal are a bit different.

For automotive purposes, solid core wire is rarely used because vibration, heat cycling, and impact will cause separations over time. These breaks will often be intermittent faults, making them hard to track down.

Braided wire on the other hand, is less likely to crack and separate, and is self-healing. That makes it more suitable for automotive use.

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    Flex is stranded core, not solid. Solid would definitely be unsuitable.
    – DizzyFool
    Dec 11 '16 at 22:01
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It depends where you're going to use it. The main failure modes are an open circuit and a short to ground (though intermittent connections are always interesting). The latter could be caused by the insulation melting/cracking/rubbing away.

In the cabin I have used domestic wiring without worrying (including in the living space of a camper conversion).

In the engine bay I wouldn't trust the insulation. For a run strapped securely away from heat/oil/coolant/screenwash you could probably get away with it indefinitely (e.g. to repair corroded lighting wiring) but it isn't a great idea especially if the circuit is even remotely safety-related. You also have to consider abrasion.

Other more-or-less exposed bits of wiring such as powering a reversing camera off the reversing light circuit are an intermediate case. Realistically the cable can probably withstand the conditions but I wouldn't splice a length into something important because of the risk of abrasion somewhere you wouldn't notice it.

When making this judgment call remember that many accessories use whatever cable was going cheap when they were designed (and not just the cheap add-ons). Your brake lights (for example) don't.

Don't forget that if you add a length of cable in parallel with something important and that length shorts, it will blow your fuse. Taking the brake lights example again, say you add a high brake light to a vehicle without one, and the insulation cracks when cold where it flexes for the boot hinge. It then shorts to the bodywork, next time you use the brakes, the fuse goes and you have no brake lights at all. The failure is unlikely, but the effect of the failure is severe enough to not want to take the risk.

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    I had a big fuse that would blow ocassionally on a 12 year old car, it would mean the engine wouldn't turn over and none of the dash lights would come on. No mechanic was able to repair it. In the end I traced it to a wiring loom that was being run between the engine and trans, right past the exhaust manifold... the insulation had become brittle and the wire was shorting out on the engine block. I replaced that section of the loom with new wire, used lots of heatshrink and put the entire thing in a conduit cable tied snug.
    – DizzyFool
    Dec 12 '16 at 12:57

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