In-line fuse amperage calculation?

[alexhiguera]

How does one calculate the apms of a fuse to be used for a particular circuit?

Ej. nav lights, acc, instruments etc.

If one switch/circuit connects three navigation lights of 3 amps each bulb, would the amperage for that circuit be 9 amps in the in-line fuse?

Is there any othe way to calculate this?

Can the in-line fuse be installed before his particular switch (from distribution block to switch) or does it have to be after the switch (the lead from switch to the lights)?

[gonzo]

Most devices have an amperage rating. Add the rating of all the devices in the circuit. If you have watt rating divide the watts by 12 and you'll have the amperage.

[alexhiguera]

so then like I said example. three bulbs of 3 amps each witll use a 9amp in-line fuse for that circuit?

fuses don't come in 9 amps, should I use the next closest rating say 10amps or 15amps?

can it be installed before the switch or after the switch?

[gonzo]

10 Amps would be the correct fuse. It goes before the switch. That way if the switch shorts out it will be protected.