Proper relay power fuse size

[RBRONKEMA GHTFD]

Hey guys, got a question on what size fuse to use for a relay power wire. My question is when you run the constant power wire to a relay what size fuse do you use? Do you base the fuse size off what the relay is going to be running? I have always been curious about this and when I have use them I am sure I go overboard with fuse sizes. Thanks.

 

[Bikinjohn]

Normally you base it off the size of the wire, however you can use what you're powering as a secondary consideration. What are you powering, what guage wire are you using?


If you're running a light that draws 3a but you want to run 12ga wire, you could use a 5a fuse since the wire will never carry more than the 3a the light is drawing. You could also use a fuse that will allow for the maximum amps to flow, so you could use between a 9 and 40a for 12ga wire depending on what amp chart you check and the distance of the run. Here's an example of a chart from Crutchfield:





So a little more info like item being powered, distance from power source, planned wire guage would help get you a more clear answer. If you are using a 30a relay and you use the proper guage wire you could use up to a 30a fuse. I usually base it off the item being powered and make sure my wire size is more than enough.

 

[dovy6]

If I'm not mistaken fuses are rated to blow at up to 135% of their value after one hour. Meaning a 10 amp fuse will run at 14 amps for 1 hour before it blows. All load wires should therefore be rated at 135% the amp rating of the fuse. Also, as per Ford: "Use 80% of the fuse rating to determine maximum steady state load to reduce nuisance fuse failures."


So if your load is 8 amps use a 10 amp fuse.