Fuse size for re-wired headlights?

[toopy]

I will soon be re-wiring my headlights to run via relays

Im using thin wall 16.5Amp cable for each connection, so obviously two cables per headlight/bulb via a 30amp relay
for each main and high beam circuit.

60w is 5A, so 10Amp per circuit max, is a 15A fuse suitable bearing in mind the cable is rated 16.5A
im guessing the cable overload rating is probably twice the max rating but ive no idea at what point the fuse should blow! 16,20,25Amps?

The fuses are the standard blade type.

 

[toopy]

Having done a bit more research it would appear that 25A cable would be better suited for use with a 15A fuse
as said fuse could take 10mins to blow at a 20A draw

 

[andrew muir]

I was going to say 15amp cable was too light, the job of the fuse is to protect the wiring loom so should be a lot lower than the cable rating, so 25 amp cables with a 10-15amp fuse per lamp should be right.

 

[toopy]

Thanks for your reply Andrew

I have bought some 25A cable which im sure would be fine, but when you compare that to whats already on the car it still looks way to small!

The new stuff is modern thinwall cable, compared to the original wiring, but even so it looks to be approx half the diameter

 

[andrew muir]

The thin wall stuff is better and can carry a higher load for a specified cross section compared with standard wiring, it also is lighter with a better heat resistance etc.
It is what I use if I am doing any re-wiring so good call!

 

[toopy]

The thin wall stuff is better and can carry a higher load for a specified cross section compared with standard wiring, it also is lighter with a better heat resistance etc.
It is what I use if I am doing any re-wiring so good call!

Agreed, just from a visual point of view it looks holy inadequate!

 

[toopy]

Fuse ratings

• Continuous rating

Fuse are marked with the current that they will continuously pass (at a standard temperature) without blowing, known as the continuous rating. It is good practice not to allow the continuous current to exceed 75% of the fuse's rated value to accommodate momentary current surges that might cause the fuse to fatigue over time or blow unnecessarily (nuisance blow).
​

• Blow rating

In simplified terms this is the current rating at which the fuse will blow. Most fuses have a blow rating around twice that of the marked continuous rating. So a 5A continuously rated fuse will have a blow rating of 10A.
​

Some older style fuses may still be marked with the blow rating rather than the continuous rating so care should be taken when replacing an older style fuse with a modern one that the ratings are understood and the correct fuse used.
Selecting the correct fuse rating
If replacing a blown fuse in a manufacturer-designed (factory) application, e.g. in a vehicle fuse panel, then the same type and rating of fuse should be used. If a fuse continues to blow then there must be a fault with the circuit and a higher rating fuse should never be fitted to overcome this, even temporarily. Doing this creates a high risk of component failure and electrical fire.
When specifying a fuse for an after-market application, the key consideration is that the fuse should be the weakest point (i.e. lowest rated component) so that it always blows before any damage occurs to other parts of the electrical circuit. However, you also do not want the fuse to keep blowing under normal operation (known as a nuisance blow), so the two elements to consider are:

1. The current rating of the smallest cable in the circuit
2. The current draw on the circuit under normal expected operating conditions

The fuse rating should lie somewhere between these two values to allow normal operation but blow on overload. For example, if the normal expected current draw is 10A and the cable size is 25A, then a fuse rated at 15A would be appropriate.

 

[MikeB]

I have one of the Aussie relay kits you get on eBay fitted to my car (currently for sale BTW ), it has a 30 amp fuse on the line from the battery feed to the starter