Indeed, and I've come across more than one situation where it's had an effect.
The most recent was a colleague at work fitting an electric fridge (compressor type) into a camper van. He'd needed to extend the cables provided with the fridge, and found that it tripped out when starting due to the volt drop making the fridge detect a low battery condition when there wasn't - the manual did warn of this. I'd have suggested replacing the whole run with a larger size, but he'd already bought a short length of much larger cable as the extension piece.
I'd not trust unsheathed "fixed wiring" cable, no matter how multi-stranded and flexible, in an automotive environment because of the thickness of the insulation. Or rather the thinness of it.
If you looked at the wiring in a Skoda Fabia you would see that it is entirely unsuitable for it's application.
Too stiff, breaks too easy (very common fault in the door looms).
I wonder how many DIY applications of PVC cable are out there working perfectly? must be loads.
To run some lights (assuming LED) then 1mm flex is very likely going to work fine.
Naysayser!
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