Is circuit breaker necessary if fuse is present

Hi there-

I’m wiring up 8 3A accessory to a fuse block with 5A fuse for each accessory- it’s powered by car battery that goes into a relay- is it necessary to use a 30A circuit breaker between battery and relay? Also is 10 awg gauge enough for 24A or should I go with 8 for more capacity?

Thank you!

Fuses and Circuit breakers protect the wiring if there is short causing too much current to flow. So you only need the circuit breaker, if you want to prevent the wiring to the relay from being damaged in the case of a short (not likely). I would probably leave out the circuit breaker, but I don't know your exact wiring details, so there may be a reason to keep it.

10 awg can support 24 amps just fine as long as the length is less than 20 ft or so.

Thank you! This is the diagram I’m intending to use- the circuit breaker is only protecting the wire from relay since fuse block have each fuse for each accessory- would I be fine if I leave the circuit breaker out just rely on the fuses on fuse block?

I don't see a reason to keep it.

Thank you sir! One more question... why do all relays with connector (40A) that's for sale all come with 12 awg wire- I thought 12 awg wires only support roughly 20A- can I connect the 12 awg wire directly to my car's battery or should I swap out the wire for 10 awg- my circuit draws 24A max- thanks!

Cost. With a little searching you can find charts listing the current capacity of various wire sizes. By observing these limits you will be fine. Often the actual needs are quite a bit less, so "usually" thinner wire works just fine.

Here is an example of a few I found right now:

https://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/

Fuses are a type of circuit breaker, so while you have them for the individual circuits and the main breaker may seem redundant, it will protect the wiring between it and the relay box and, perhaps more important, if all the circuits are running at maximum it protects against an over-draw. This is basically what those pesky fuseable links do. It will depend on the length and route of this main wire, but if there's a long run I'd probably use a reset-able C/B there.

Wiring gauge will also depend on what is being supplied with the power - in some instances one may use a size or two (or three) larger than might be acceptable, from a current rating viewpoint, in order to maximise the voltage across the component. If it's a simple on/off wire one may find a lighter gauge works well, such as switching a relay or a warning light, but that'll depend on the case in point. As a general rule, power is proportional to the square of the voltage, so a wire that has a 10% voltage drop will only be supplying 81% of the power, and sometimes it can be as high as 20%, which would be only 64% of the power. For fuel pumps, cooling fans, etc. this can be significant.

If using something like halogen headlights, IIRC the light output is ~proportional to the cube of the voltage, so 90% voltage would be 73% light output and it'd be yellower, 20% (and I have seen this) is only 51% and one could see SFA on a wet road.

Seems I may be under estimating the lighting loss, especially as what is still being produced may not be useful for vision - http://www.lamptech.co.uk/Documents/IN%20Voltage.htm

Thank you both for clearing things up!!!