Hi all I'm looking at adding an accesories fuse box into my car to run a CD player, a cigarette lighter and tacho, and heater.I was think of running it off the ignition through a 30 amp fuse to a 30 amp relay, then to the fuse box. I was thinking of running a 15 amp fuse for cig lighter, a 15 amp for cdplayer, a 5 amp for tacho and 15 amp for heater. Would this work or should I up the relay size?

I think the fuse sizes you are quoting there are on the big size. The tacho will draw less than an amp, and the heater will be very close to one amp as well. The stereo may take a few as they charge large caps on startup and heavy use, but will you use the cigarette lighter for cigarettes or charging phones?

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I'm just going to use the cig lighter for charging phones running a gps speedo until I get used to miles. What size fuses should you use?

USB chargers for phones usually are no larger than 2.4 amps, I would use just a little larger than this, maybe 5 Amps as they use a little power regulating down from 12 to 5 volts.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Thanks what about the others?

Perhaps just match the 5 amp fuses throughout except for the radio, easy to keep spares.

With the radio, it will most likely have a blade fuse on the back already.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

You can calculate it out as long as you know the power of the item.

I=P/V
Current(Amp)=Power(watts)/voltage

E.g single 60W headlight globe, Volt=12V(battery) I=60/12=5 Amps, two head lights so 10 Amp under normal conditions so I'd use a 15 Amp fuse as they normally running they are 10 Amps. Keeping in mine tail light/parkers my be on this circuit too. (it's just an example)

Really the battery is some where between 12-12.6V, you just need to look at everything that will be on that circuit.

Another way would be to look at what fuses are being used on similar circuits in say a newer car

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So 5 amp fuses for all you think mick?

That is what I would do myself.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.