motoman Posted June 5, 2012 Share Posted June 5, 2012 I am rewiring my car and am a little confused on how to properly install a fuse into a circuit. From what i have been reading is it the gauge of the wire that is limiting factor on what size fuse to use. For example, I am wiring in a walbro 255 fuel pump using a relay and 10awg wire. According to Auto performance Engineering at 50psi the pump draws 9.8 amps at 13.5v. The rail pressure for my car is 43psi so i figure 50psi should be high enough. They guys over at oznium.com say 10awg wire should not exceed 30amps. Should i run a 10 or 15amp fuse on this circuit? My guess is 15amps because motors tend to have an amp spike in them during start up yet 15amps is still under the maximum current that a 10awg wire can safely carry (30amps). Thank you for your help. Motoman Quote Link to comment Share on other sites More sharing options...
Miles Posted June 5, 2012 Share Posted June 5, 2012 Fuse selection: Use a fuse 125% of the current flow. So: 9.8 amps x 1.25 = 12.25 amps round up to 15 amp fuse. Yes, a 10 AWG wire can handle 30 amps. You may want to Google and save a wire size vs amperage table. For wire lengths over 20 feet or wires in a confined bundle, go to one larger size. Quote Link to comment Share on other sites More sharing options...
Pharaohabq Posted June 5, 2012 Share Posted June 5, 2012 I'm glad you're still working on your car. Is the Engine in yet? Yes as Miles said, 125% is a good rule. The Fuse is to protect the wires and what's connected to it. A 15 amp is great. The fuse is always in line of the wire. It's best before the device you're running power to. So Power, Fuse, Device, ground. If you went power, Device, Fuse, ground, they it would work, but if the device shorted to ground, then your fuse would not help at all, and the wire and device could burn. So go, Power, Fuse, Device, Ground. Quote Link to comment Share on other sites More sharing options...
motoman Posted June 5, 2012 Author Share Posted June 5, 2012 Yeah i have been working on it slowly. Just haven't been on the forums in awhile. The engine is in i just need to wire it up. Is there anyway i can look up why its 125%? I would like to know alittle more on that subject. I have a general understanding of it, but it never hurts to know more. Quote Link to comment Share on other sites More sharing options...
beermanpete Posted June 5, 2012 Share Posted June 5, 2012 Most fuses will blow after about 1 hour if operated at rated current. Here is the Littlefuse specification for the blade style automotive fuses. http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_BladeFuse_ATO32V.pdf You also need to allow for surge currents that occur at start-up. Motors, lights, and many other loads draw many times the running current for a moment when power is applied. Install the fuses as close to the power source (battery) as practical. You need to protect the wire run as well as the load. Quote Link to comment Share on other sites More sharing options...
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