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  #1  
Old 21st May 2010
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Default fuse amperage?

Recently installed a set of 35w driving lights. I wired directly to battery with an inline fuse (using 5 amp), to HD auxilliary switch to lights. The fuse blew at some point in the ride today. Is 5 amps too low a rating? Thanks for the input.
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  #2  
Old 21st May 2010
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They should be wired into the accessories system, not direct to the battery. The battery is at least 19ah. This is why the fuse blew. If you insist on having them direct to the battery, you're going to need a higher rated fuse, an inline resistor of some sort and possibly a new battery after a while. Still, they really need to be into the system 'after' the voltage regulator.
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Old 21st May 2010
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Quote:
Originally Posted by williamv1203 View Post
They should be wired into the accessories system, not direct to the battery. The battery is at least 19ah. This is why the fuse blew. If you insist on having them direct to the battery, you're going to need a higher rated fuse, an inline resistor of some sort and possibly a new battery after a while. Still, they really need to be into the system 'after' the voltage regulator.
Ampacity of the battery has no bearing at all on fuse rating and where it connects has nothing to do either. (except if it should be switched with the key). Only the load. If there are two 35 watt driving lights, that is a total of 70 watts. At 12 volts, that is close to 6 amps. A 5 amp fuse is too small for that load, not to mention that the bulbs will draw more than the rated load for a few milliseconds as they come up to temperature, which means either a larger fuse yet, or a slow blow version.

As an aside, the fuse is only to protect the wiring and not the load in general. The rating of the fuse depends on the size wire in the system and is sized such that if the load shorts out, the fuse will blow before the wiring burns up.

Just for info, I am a EE with 30 years of automotive electrical/electronic design experience with a large OEM automaker. (now retired and enjoying the ride )
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Old 21st May 2010
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Question

Then why isn't everything as far as accessories on the bikes not direct wired to the battery if the amperage and voltage output is of no importance?

If a power adapter that is rated at 9v and 2a is replaced with one rated at 12v and 5a, you're saying that the device it is plugged into won't burn out?

BTW, not arguing with your 30+ years electrical experience...
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Old 21st May 2010
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Quote:
Originally Posted by williamv1203 View Post
Then why isn't everything as far as accessories on the bikes not direct wired to the battery if the amperage and voltage output is of no importance?

If a power adapter that is rated at 9v and 2a is replaced with one rated at 12v and 5a, you're saying that the device it is plugged into won't burn out?

BTW, not arguing with your 30+ years electrical experience...
I didn't say that the voltage is unimportant, just that the ampacity (amp hours etc.) didn't matter. The adapter rating is the output. You would not want to replace with a 12 volt as that would probably burn out the device, but you could replace one rated at 9 volts, 2 amps with one rated 9 volts and 10 amps without a problem.


It is similar to your house. The fact that the transformer on the pole can deliver up to 1000 amps or so does not matter. You don't connect directly to the transformer, but rather to the panel, which has a main breaker (the newer bikes have the equivalent (the maxi-fuse) and then the various breakers in the panel. Note that the breaker rating in a house is based on the wire size (#14 gets 15 amp, #12 gets 20 amp , etc). The particular load does not matter. You plug a TV that only draws a couple of amps into a circuit that is fused at 20 amps all the time.

Automotive fusing is a little more dependent on the load because many of the wires are larger than you would "technically" use due to load in order to have strength when they get pulled on, but it still holds that the fuse is to prevent things from catching fire and not really to keep the load from getting damaged.

Most things are not wired directly to the battery because the normal feed goes from the battery through the main fuse to a fuse block and is then split among the loads through the individual fuses. This helps keep the wiring evenly distributed and is easier to build and to work on. The accessory line is also generally switched so that it goes off with the key to keep from drawing current when the bike is off.

Hopefully this helps.

Last edited by dondad; 21st May 2010 at 14:35.. Reason: added comment
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Old 21st May 2010
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It is a simple calculation...

Amperage draw is the only real factor...

Ohm's Law

watts / volts = amps

Where you draw the power from is irrelevant, save for the existing load on any single circuit....

If you wire directly to the battery with an inline fuse, the fuse must be rated at or just above the nominal draw, as dondad has indicated....(obviously, the load cannot exceed the ampacity of the wire used to complete the circuit....if it did, the wire itself would become a fuse, of a sort... <ie: fusible link> )

If you wire into the accessory (or any other circuit) there must be sufficient "overhead" in both the ampacity of the wire, and the fuse rating on that circuit to support the additional load...

Dondad nailed it...I'm just clarifying...

BTW, those lights USUALLY come with a relay, so that they can be switched without the full load of the lamps passing thru the switch itself....if you bypass this relay and wire directly thru a switch, you're gonna burn up the switch (on any small accessory switch, anyway...) and likely blow a fuse, too, as it may short to ground, thereby exceeding the ampacity of the fuse...
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Last edited by cantolina; 21st May 2010 at 15:01..
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Old 23rd May 2010
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thanks for the info and education. I'll get a relay and wire off the accessory circuit.
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Old 24th May 2010
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Quote:
Originally Posted by Sporty Dave View Post
thanks for the info and education. I'll get a relay and wire off the accessory circuit.
Maybe I'm thinking of a typical car, but isn't the voltage of a running
bike around the 14 volt point. Your 70 watt load divided by 14 volts
is 5 amps. As has been said before, the fuse needs to be just
above the 5 amp value to survive.

If something goes short or draws too much current above the normal
5 amp draw, you want the fuse to blow, not the wiring.

The use of a relay transfers the current switching away from the switch.
Generally this is more important with inductive loads, such as horns,
especially air horns where the inductive load current tries to jump
the switch gap as the switch is opened resulting in
arcing across the switching contacts. That's a bad thing

Beware how you wire in the relay. Some relays have a built in diode
across the coil that will result in a dead short if the relay coil
is not connected with the correct DC polarity.


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Old 24th May 2010
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When I wire a circuit, I take the load and add 30% to 35% and that is the figure I use for the fuse.
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Old 24th May 2010
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Quote:
Originally Posted by MadMax25 View Post
Maybe I'm thinking of a typical car, but isn't the voltage of a running
bike around the 14 volt point. Your 70 watt load divided by 14 volts
is 5 amps. As has been said before, the fuse needs to be just
above the 5 amp value to survive.

If something goes short or draws too much current above the normal
5 amp draw, you want the fuse to blow, not the wiring.

The use of a relay transfers the current switching away from the switch.
Generally this is more important with inductive loads, such as horns,
especially air horns where the inductive load current tries to jump
the switch gap as the switch is opened resulting in
arcing across the switching contacts. That's a bad thing

Beware how you wire in the relay. Some relays have a built in diode
across the coil that will result in a dead short if the relay coil
is not connected with the correct DC polarity.


.
It is true that the voltage is usually around 14 volts, but a 5 amp fuse for a 5 amp load is too small. In this case, it really makes little difference. You still need a larger than 5 amp fuse. If it were me, I would probably use around a 7.5 amp slo blow. Incandescent bulbs draw about 13 times the rated load for a short time as they warm up when power is first applied to them. I don't know the wire size on the lamps, but I would guess it is at least 14 gauge, which could be fused up to 15 amps. (btw, not necessarily recommending this ) A relay would be better for the switch, but the fusing for the load side of the relay is still the same. The diode note is a good reminder. Many relays use this to "catch" the inductive kick that comes when the coil is de-energized. Hooking it up backwards is definitely not a good idea as noted.
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