Fuses, Current, Watts, What What What!

Submitted: Tuesday, Jun 26, 2018 at 20:27
ThreadID: 136903 Views:3358 Replies:6 FollowUps:3
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Dear All

Firstly. I have no electrical qualifications at all, however I do understand the relationships between Volts, Amps, Watts and Resistance.

We are going to UK soon and hiring a car. So I thought it would be useful to make a double adaptor for the 12V outlet, to be able to run the Garmin and charge the IPad and IPhone at the same time. See photo. total current draw at 12VDC about 8A so I thought a 10A fuse would be about right.



When putting the components together I noticed that the fuse in the male plug was marked 250VAC 3A. So I initially thought that’s 750Watts .. why so for a 12V plug? That is equivalent to 60 A in a 12V circuit. Didn’t make any sense to me at all. So off I went on a web search. I didn’t find anything scientific, from a reputable organisation to explain the relationship between V,I,P and fuses.

What I did find was a lot of comments on various forums. This one made some sense to me.

“ If you think of electricity as a flow of electrons through a conductor (eg. copper) the voltage (V) can be thought of as a measure of how much energy each electron is carrying (its potential). The current is the number of electrons passing through the circuit and is measured in Amps (1 Amp = approx 6.25X10 to the power 18 electrons per second!). The power of an electrical devise is measured in Watts (W) and is quite simply calculated by W = VxA which is easy to understand if you consider that each electron gives up the amount of energy it is carrying to the device as it flows through it. The ability of a given conductor (eg. copper) to carry a current is dependent on its dimensions, the thin wire in a fuse will heat up, due to the resistance (another story!), as the amount of current (No. of electrons per second) increases until it reaches its melting point. It is the number of electrons flowing, not the amount of energy each is carrying, that results in the heating effect. So it is easy to see that the rating of a fuse has everything to do with current, but is unrelated to voltage.”

So the question is: Will a 10A fuse rated at 250VAC blow at a current exceeding 10A in a 12VDC circuit?

Looking forward to hearing from those of you that know.

Cheers John





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