Originally Posted by RichardV6
You will need to uprate fuse 12 to 15 amp though (as you've found!) given this is nearest common value to allow a load margin.

By way of an update on this one I did actually stick with the 7.5a o/s side light fuse in the end and with no blowing of fuse 12 at all, I figured it would work fine as two 5w side lights and two 45w fogs totals 100w and if you divide 100w by 13.6v you get 7.35a, admittedly a bit on the edge but the 7.5a fuse definitely refused to blow.

However, more recently and after one very enthusiastic acceleration event the plastic insulation around fuse 12 got hot and started to melt, this should not be confused with overloading a fuse. In my experience if a fuse melts but fails to blow it's an indication there's a bad connection somewhere, so I went hunting and with the prime suspect being the Marshals it didn't take long before I found the culprit.



As we can see in my above video the bulb is a very loose fit in the bulb holder, the BA21S bulbs used by Marchal are of the single contact type so the bulb body is the ground, clearly the poor fit and subsequent bulb chattering within the bulb holder was causing a make and break condition so I’m confident this is why the plastic body of fuse 12 melted last weekend, it also makes sense why the violent acceleration event triggered the issue.

The solution to stop the chattering was simply to put a bit more preload on the sprung contact, this pulls the bulb tightly into the holder against its bayonet pins, now, although I'm confident I've solved the melting fuse issue and proved the standard 7.5a fuse is fine I’ve still decided to follow what I did with my Lucas PL700 headlights and fit LED bulbs in my fog lights too.

I was pretty much convinced this would involve converting the bulb holders to H11 but to my amazement Classic Bulbs offer the rare Marchal BA21S bulbs in LED, they're not cheap but they will last indefinitely and according to Classic Bulbs will also draw over 75% less current than the original incandescent bulbs.

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https://www.classicbulbs.co.uk/prod...m-6-12-24v-led-head-spot-lamp-ba21s-base

Just like my headlight LEDs they'll live under these yellow glass globes.

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When the fogs are off this will give the correct golden glow the original Marchal globe style yellow bulbs produced back in the day.

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