Proper fuse size

[bugeyed_99]

Hope this is the right thread for this, couldn't find an electrical thread. I am trying to figure out the proper fuse size for my push button starter and lights I have installed and any future electrical I will add to the machine. I have used 18g wire but am thinking I might have to go heavier.

Here is the data on the lights:
6. Voltage:9-18V 55W
7. Amp Draw: 3.2A
8. Power: 55W 4500lm

Ive read two different things on two different sites, one said fuse the wire not the item and listed an 18g wire as needing approx a 2.5A fuse with a max current of 4.6A. The other listed 18g wire as being able to handle a max current of 15A thus a 7.5A fuse would be recommended.

Can anyone clarify this for me and give me an idea on how to properly select the correct size fuse.
Thanks

[JoeMeg]

"one said fuse the wire not the item and listed an 18g wire as needing approx a 2.5A" This is correct however,you will get many opinions as to the specific ampacity of a piece of #18 AWG depending on the insulation. I think the 2.5/4.6 max statement is rather conservative but also think 15A comment is pushing it. I think the 7.5A fuse is "about right" for #18 and as mentioned earlier,don't worry about the load,just get the current to it safely. As far as the starter switch,if your just energizing the coil of a solenoid,I think #18 w/ a 5A fuse would suffice. If you really want a lot to think about just google "Allowable ampacities of insulated conductors".

[ARGOJIM]

Never seen a green/start wire fused as it is a momentary wire, though safety can't hurt as long as you carry a spare.

[ken white]

Fuses are sized to protect the wiring insulation from melting.

Wiring is sized for allowable voltage drop which depends on point-point length.

Normally a 3% voltage drop is used for most calculations.

18 AWG is 6.39 ohm/1000 ft, so assuming about a 3% loss for 12 VDC, you can run a single wire about 17.6 ft in one direction, or about 8.8 ft when running positive and negative wires which most installation require - I don't like to use the frame for a current carrying conductor.

18 AWG ampacity is about 5 amps but may be lower or higher depending on insulation temperature rating, where its located, and how its bundled - make sure its automotive wire since it will be exposed to oils and such.

I would probably run 16 or 14 AWG and fuse at about 7.5-10 amps since the lamps have a very low cold resistance which creates higher surge currents.

[bugeyed_99]

Thanks for the replies guys, looks like some re-wiring going on this weekend. Thanks again