Selecting The Correct Amp Fuse!

Thread Starter

TOWNWRENCH

Joined Dec 29, 2007
2
Cant Seem To Find The Answer On This One.when Adding A Circuit To A Truck Example:extra Lighting On A New Circuit,how Do Determine The Correct Amp Fuse.i Know There Is A Formula. Sources I've Tryed Seem Too Complicated.seems Most Mechanics I Know Say Just Throw A 20 Amp In There.and Then I Ask, How Did You Know That And They Say The Swag Method (some Wild _ss Guess)!thanks For The Help This Will Clear Up A Lot Of Other Unanswered Questions On Electrical Issues
Townwrench
 

techroomt

Joined May 19, 2004
198
a fuse rating should be chosen to protect the conductor (wire) it is serving. furthermore the wire should be chosen to adequately serve the load it is providing for. a smaller (less amperage) rated fuse can be used, as long as it is greater then the load requires. a common misconception is that the fuse solely protects the load or device. likely the device will be damaged long before the fuse blows.
 
I have a noobish question about fuses, and that is whether a fuse should be chosen specifically on Amperage. Let's say that I have a 5v DC supply rated at 1 amp. Is it OK to choose a .75 amp fuse which is rated for 125v?

Or does a fuse get rated according to Watts? I have some evidence here on the bench that leads me to think that it's all about flow (amps) rather than potential (volts). A short circuit on a 5v power supply melted the insulation on and admittedly small-gauge wire.

One part of my under-informed brain argues that circuit breakers would be rated in Watts if Ohm's law ruled in this case. Another part says "yes, it _IS_ rated in watts, in an oblique way, since glass fuses show a rating in Amps _and_ in volts."
 

rjenkins

Joined Nov 6, 2005
1,015
With small low-voltage fuses, current rating is the only important figure.

The fuse should be rated higher than the maximum load current, but significantly lower than the wiring feeding the circuit.

With higher voltage circuits (anything above eg. 50V), the voltage and surge or break current ratings of the fuse are important. If it's for use on 110V / 240V & upwards mains supplies, fuses should be 'HRC' type (High Rupture Current) and the ratings are ultra critical, but off topic for this board.

The voltage rating on a fuse is the maximum circuit voltage it can be used on to avoid hazardous situations.

Any time a circuit is broken (including by a fusewire breaking), an arc can form.
The arc can sustain current, but also drops considerable voltage; current x voltage is power - the fuse could be dissipating hundreds or thousands of watts for a short time, and if not properly rated, could explode or short out.

This is why HRC fuses are filled with silica or some other powder, to help quench the arc quickly. Low voltage fuses can be simpler as the 12V on a car circuit is just not high enough to maintain an arc over the length of the fuse.
 

Von

Joined Oct 29, 2008
65
Also,

Place the fuse as close to the power source as possible to protect ALL of the new/added wiring.

If you are adding auxillary auto lighting and tapping off of the battery for example, place the fuse within a foot or so or as close as is easily accessable.
 

Kamala

Joined Mar 20, 2010
4
The voltage rating of a fuse is more of a physical rating than an electrical one. It is a maximum value that should not be exceeded and has to do with the distance between the terminals and the dielectric properties of the materials used in its construction. If the magnitude of potential is too high, arcing may occur allowing current to flow around the fusible element, thereby compromising the efficacy of the device.

rjenkins said it better.

K
 
Thanks, everyone! Much clearer now.

... and I remember seeing a demonstration of "put the fuse as close to power as possible in order to protect the wiring". I was working in a steel mill during construction. Arc welding cables going everywhere. One day a crane set some heavy steel item down on a cable by accident. ZING! The insulation burned open along its entire length, easily 100 feet back to the welder power unit. Glad I was not there at the moment it happened, that must have generate Magic Smoke (and smell) in industrial quantities.
 

Ashoki

Joined Feb 12, 2016
1
I have a noobish question about fuses, and that is whether a fuse should be chosen specifically on Amperage. Let's say that I have a 5v DC supply rated at 1 amp. Is it OK to choose a .75 amp fuse which is rated for 125v?

Or does a fuse get rated according to Watts? I have some evidence here on the bench that leads me to think that it's all about flow (amps) rather than potential (volts). A short circuit on a 5v power supply melted the insulation on and admittedly small-gauge wire.

One part of my under-informed brain argues that circuit breakers would be rated in Watts if Ohm's law ruled in this case. Another part says "yes, it _IS_ rated in watts, in an oblique way, since glass fuses show a rating in Amps _and_ in volts."
Fuses are rated according to the operating current of the device which is the current that will activate the device. ie 13amps is the max current a plug top will carry with a 13A fuse inside.
 
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shteii01

Joined Feb 19, 2010
4,667
a fuse rating should be chosen to protect the conductor (wire) it is serving. furthermore the wire should be chosen to adequately serve the load it is providing for. a smaller (less amperage) rated fuse can be used, as long as it is greater then the load requires. a common misconception is that the fuse solely protects the load or device. likely the device will be damaged long before the fuse blows.
That is interesting, I have not seen it put this way. The shop I work at we normally use 18 AWG for most everything, it has 7 A ampacity. So I would use 7 A fuse even though the wire might carry only 1 or 2 A to the circuit?
 

dl324

Joined Mar 30, 2015
8,760
That is interesting, I have not seen it put this way. The shop I work at we normally use 18 AWG for most everything, it has 7 A ampacity. So I would use 7 A fuse even though the wire might carry only 1 or 2 A to the circuit?
He also said:
a smaller (less amperage) rated fuse can be used, as long as it is greater then the load requires.
In practice, the load always requires less current than the wires can handle, so the fuse should be sized for the circuit, not the supplying wire gauge.
 
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