Fiesta Brake Lights keep blowing

Hi
Wonder if anyone can help me with this?
I am forever burning out my rear brake lights.
This time all 3 went.
I have looked at the fuse panel and it has a 15 amp fuse.
If the 2 side brake lights are 21W and the high level is 15W that makes a total of 57 W - 57 /12 is about 5 Amp by my reckoning.
If I fit a smaller Fuse say 10 amp will that protect the bulbs from blowing?
Car electrics not my strong point in fact I hate them !!!!!

G.

if one bulb blows, it is a good idea to replace the whole set (on the grounds that they are all equally old); also the Voltage Drop will be less once one load on the circuit fails, so the voltage goes upon the others and shortens their lives.

More often, by the time an old lamp fails, it will be quite dark due to silvering or blackening of the glass, so it is no bad thing to replace all these small bulbs in one go.

I had this on my old Fiesta, convinced the alternator was providing too much voltage but I measured it and it was fine. In the end I just replaced all the bulbs and it never happened again.

A few years back the fiesta I drove at work had a healthy appetite for stop lamps. Turned out the alternator regulator was stuffed. I found out after it wouldn't start one day- The alternator was delivering about 18 Volts, and the over-voltage had eventually reduced the contents of the battery to lead soup!

Cheap lamps can be a false economy. Some have a very short life expectancy. A decent lamp should be able to handle the highest voltage the car electrics can deliver, unless there is a problem with the charging circuit.
Vibration is a big killer of auto lamps. If the lamp holders can rattle about a bit, or if there is excessive vibration (wheel balance problems etc. etc.) that can reduce the lamp life.

De-rating the fuse won't change anything except for the frequency of fuse failures - the fuse is already matched to the load on the circuit, which may power more than just the brake lights.
A fuse doesn't limit the current that can flow - it just cuts the circuit completely when the current flow is too high, hopefully some time before the wiring loom catches fire.

Are the contacts on the lamp holder nice and clean? Is there plenty of pressure on them with the lamp installed? Are you using branded lamps?

Thanks a lot.
All 3 have been replaced and still working. Think it is probably the quality of the bulbs even though we pay through the nose for them - built in failure rate.
Latest set from Charlie Browns nearly had a heart attack when he asked for £2.50 for a bulb!!
But it is next door to my work and I needed to get home - it was raining and I was desperate.
Heres hoping
g.