Fuse ratings?

[Darren]

I'm looking at putting some driving lights on the front of my surf, I found some 130w bulbs to put in them and realise this will need substantial wiring and relays etc to handle the current. With 2 x 130w bulbs I work that out as 21 amps through the relay, so I figured a fuse rated at 30 amps and relays and wiring rated at 40amps ought to be plenty strong enough.
I am quite happy these figures are right but when it came to buying the bits the fuses on vehiclewiringproducts.co.uk have 2 ratings continous and fused so do I want a
30 amp continous (fused 60amps) fuse or a
15amp continous (fused at 30amps)?
Thanks in advance
Darren

[marky.b]

you will need to look at the continous figure, the fuse figure is what it will take to blow the fuse.

[Darren]

Thanks Marky, But if the 30amp (continous) fuse doesn't fuse until 60amps then surely it won't protect my 40amp relay from getting fried or am I missing something.

[marky.b]

as long as the fuse is before the relay you should be fine ( I can't think of anything that will just cause a 20 amp increase that would blow the relay and not the fuse)
If your still not sure just phone them up to place the order and check with them first.

[Darren]

Thanks Marky I will check with them when I order.
Darren

[Silversurfer]

I'm looking at putting some driving lights on the front of my surf, I found some 130w bulbs to put in them and realise this will need substantial wiring and relays etc to handle the current. With 2 x 130w bulbs I work that out as 21 amps through the relay, so I figured a fuse rated at 30 amps and relays and wiring rated at 40amps ought to be plenty strong enough.
I am quite happy these figures are right but when it came to buying the bits the fuses on vehiclewiringproducts.co.uk have 2 ratings continous and fused so do I want a
30 amp continous (fused 60amps) fuse or a
15amp continous (fused at 30amps)?
Thanks in advance
Darren

Really is confusing isn't it. I believe the continious rating is what the fuse will carry continiously without degrading not what it will blow at. The load you intend to carry is 21amps (actually a little less as nominal battery voltage is 12v but actual voltage is 13.2v - 6 x 2.2v lead acid cells and the alternator is giving out more than that). If you fuse at 60amp you will have to use cable that will carry that and that will be BIG cable. otherwise the fuse won't protect the cable.
A general rule of thumb is to use a fuse of 1.5 times the rating of the load to be carried provided it also protects the cable used.
In the event of a short circuit, which is what you are protecting against the fuse will blow very quickly before the cable is damaged.