Seaside Pete Posted March 3, 2015 Report Share Posted March 3, 2015 (edited) Hi Whilst rewiring my P15 I installed 2 fuse boxes (1 from ign and 1 live from Batt). I ran a wire from the ign side down to the stop light switch and from the other side of the switch to the 3 stop lamps (not rocket science). Using the formula watts devide by voltage equals amps 3 x 21 watt bulbs = 63 watts devide by 12v = 5.25 amps. I installed a 10 amp fuse which should be a bit heavy, however the fuse blows. If I install a 15 amp fuse no problem. I have tested and there doesn't appear to be any short in the circuit. Any ideas? If there was a short that I cannot find would the 15 amp fuse also blow? Thanks for any help in advance. Pete Edited March 3, 2015 by Seaside Pete Quote Link to comment Share on other sites More sharing options...
Merle Coggins Posted March 3, 2015 Report Share Posted March 3, 2015 Are you sure of your wattage? A 12v 1156 or 1157 has 27 watts. but a 10A fuse should still be good enough. Maybe you have a bad ground with high resistance that is adding load to your circuit. Merle Quote Link to comment Share on other sites More sharing options...
Seaside Pete Posted March 3, 2015 Author Report Share Posted March 3, 2015 Hi Merie I am using UK bulbs which list as being 21 watt, However as you say this should not make any difference. I will double check the grounds. Thanks Pete Quote Link to comment Share on other sites More sharing options...
Solution Chester Brzostowski Posted March 3, 2015 Solution Report Share Posted March 3, 2015 The bulb wattage is only one factor. Each connection in the circuit has the potential for resistance which will add to the total power drain. If the lights are bright and the 15A fuse works I would use it. Also if you are using duel filament bulbs each filament will add to the total power picture. Parking lights with break lights could double your power drain. Chet... Quote Link to comment Share on other sites More sharing options...
Seaside Pete Posted March 3, 2015 Author Report Share Posted March 3, 2015 (edited) Thanks Merie/Chet With the 15 amp fuse everything works fine. I was just a bit worried that there was a short somewhere that I had not spotted but I would have thought that the 15 amp fuse would have blown as well. Pete Edited March 3, 2015 by Seaside Pete Quote Link to comment Share on other sites More sharing options...
greg g Posted March 3, 2015 Report Share Posted March 3, 2015 Chet, the way he has wired it, power, fuse, stop switch, brake lights, the running lights are out of the equation. I would suggest the stop light switch itself is the culprit in adding uncalculated resistance, especially if its original, or was not addressed with the rewire. Quote Link to comment Share on other sites More sharing options...
Oldguy48 Posted March 3, 2015 Report Share Posted March 3, 2015 I would try removing the three bulbs, and checking for any current draw back at the fuse. If there is no current drain at that point, the bulbs apparently are the only loads on that circuit. It would be a quick and easy check. Then re-install them one at a time, and continue to measure the current. That might answer your question. Wayne Quote Link to comment Share on other sites More sharing options...
TodFitch Posted March 3, 2015 Report Share Posted March 3, 2015 Don't know if this is a factor, but light bulbs have variable resistance. When cold (off) they have a very low resistance and as the heat up the resistance rises. The rated wattage is when the bulb is hot, the current draw will be higher when they are just turned on. If you have a fast blow fuse it might need to be a higher rating than if you have a slow blow fuse. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted March 3, 2015 Report Share Posted March 3, 2015 A fuse only has one job and that is to prevent the smoke from escaping from the wire. What size wire are you using? Is it rated for the maximum amp draw on the circuit? Quote Link to comment Share on other sites More sharing options...
Chester Brzostowski Posted March 3, 2015 Report Share Posted March 3, 2015 Good point Don, wiring should be 16 or no smaller than 18 gauge if using 12vdc as source. Of course this is a 12 vdc P15 because 6 vdc would explain everything. Greg I agree with your point about the switch. Also as suggested checking for additional unintended ckts is also a good thing to pursue. Chet... Quote Link to comment Share on other sites More sharing options...
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