Fuses. My brain is fried.

[SulLiveVan]

I've read so much about fuses, and I still can't figure out what size to use. One place says it depends on the wire gauge, another place doesn't mention wire gauge at all. I'm hoping someone can give me a definitive number based on what I'm trying to set up:

device: 12v fan, using 5amps max
wire: 16 awg
battery: 12v 100ah agm
no other variables, just the fan to the battery

I'd be grateful for any help anyone can offer. This site is awesome.
Thanks!
-Mike

 

[CityBear]

I would go with the lowest amp rating you have in the circuit. 16AWG wire is rated for 10 amps at 0-50 ft. Your fan has a max rating of 5 amps. I would go with the a 5 amp fuse.

 

[BradKW]

Rule of Thumb: You should always choose a fuse size to match the wiring, not the appliance... 16awg should have a max fuse of 7.5 amps.

 If you are using wire sizes smaller than the recommendations with a larger fuse, there can be problems. However, using a smaller fuse than what is recommended is the way to get an efficient system. Small fuse, big wire is always most safe.

 

[TucsonAZ]

The fuse is there to protect the wiring so that's what you would go based on not the things running off the wiring. For 16awg that could be 5-20amps depending on how many strands are in the wire. The more stands the lower the amp rating, So if it were only 3 strands you could do 10amps but if it were super fine like 40 strands it would be 5amp, I would go with the 7.5amp fuse personally.

 

[Weight]

Where to start. Strands do not effect the amp rating of copper wire. 16AWG is 16AWG is 16AWG, equaling 1 square millimeter across the wire. There is another variable; distance to and from the fan. If the fan is 15 feet from the battery, at 5 amps. 16 awg is usable, but will be above the recommended voltage drop at 5.21%. Fuse to the wire, at the battery end. 16 awg is safely fused at 20 amps. My advice is to use a heavier wire. At 15 feet 14 awg would be 3.24% voltage drop. 14 awg is safely fused at 25 or even 30 amps at the battery end. Using a larger fuse equals less voltage drop in the circuit. If you want to protect the fan, put a second fuse, 7 to 10 amp at the fan end of the wire. I don't use fuses at my lights or fans, only at the battery end of their feeds sized to match the wire AWG. Voltage drop is very important when considering efficiency of 12 volt circuits. Efficiency is important when draining storage battery capacity.

 

[Spaceman Spiff]

I've read so much about fuses, and I still can't figure out what size to use.

http://bdfuses.com/fusesnwires.php

 -- Spiff

 

[BradKW]

Sternwake, where are you getting the 20amp on 16awg? The chart I've used reads like the one Spiff linked above, 7.5 amps..?

 

[SulLiveVan]

Thanks for all the replies, I appreciate it. 
So, does everyone agree that any fuse under 7.5 would work for what I'm trying to set up? 
Sorry again...I'm an electrical moron.

 

[Trebor English]

Yes, 7.5 amp should work fine.  The 5 amp fan won't blow it, the fuse will blow before the wire catches fire if there is a short.

edit: While you may be an electric moton, I am the more common general purpose moron.

 

[Optimistic Paranoid]

You said you needed something to work on a 5 amp device.  Fuses, like everything else, always have a tolerance of + or - x%.  In short, you need something a little bigger than a 5 amp fuse because a 5 amp fuse might blow when the device was drawing maximum power.  It also needs to be smaller than the maximum safe current the wire can carry.  Fuses only come in certain standard sizes.  I believe it's 5 amps, then 7.5 amps, then 10 amps followed by 15 amps, etc. So a 7.5 amp fuse sounds like it would be perfect for what you are trying to do.  It's the next standard size above 5 amps, and it's less than the max current the wire can safely carry.

 

[SternWake]

Sternwake, where are you getting the 20amp on 16awg? The chart I've used reads like the one Spiff linked above, 7.5 amps..?

http://www.genuinedealz.com/voltage-drop-calculator

If i type 'G' in my address bar that address is a suggestion

 

[Trebor English]

Note the rated 105 C.  Most other charts are 60, 75, or 90.

 

[SulLiveVan]

You said you needed something to work on a 5 amp device.  Fuses, like everything else, always have a tolerance of + or - x%.  In short, you need something a little bigger than a 5 amp fuse because a 5 amp fuse might blow when the device was drawing maximum power.  It also needs to be smaller than the maximum safe current the wire can carry.  Fuses only come in certain standard sizes.  I believe it's 5 amps, then 7.5 amps, then 10 amps followed by 15 amps, etc. So a 7.5 amp fuse sounds like it would be perfect for what you are trying to do.  It's the next standard size above 5 amps, and it's less than the max current the wire can safely carry.

Thanks John. That spells it out perfectly, and makes sense to someone as clueless as me. 
I really appreciate the help.

 

[DannyB1954]

Another explanation:
You never want to use a fuse that is larger than the wire can carry. You can use a fuse that is smaller. If it is a single item in the circuit like a fan, there is no reason to have a larger fuse than what the fan needs. Having only one item per circuit is the safest way to go, but often it is desirable to have more than one item. Lets say a year from now you want a dome light or 12V power outlet near the original fan. Your 16 ga wire will likely be too small to add the items to. Putting in a larger fuse with the smaller wire is a no no.
Many will build with wire larger than what they need for this reason. 10 ga will handle 30 amps, 12 Ga will do 20 amps, 14 Ga will do 15 amps.

 

[Weight]

The whole question of fuse size. Fuses must be as close to the source as they can. Fuses are sized to the size of the wire. Not the device. We are protecting the wire from shorts. A fuse is really a resister. It reduces power available, expressed as voltage drop. A 7 amp fuse will effect a higher voltage drop than a 15 amp fuse. There are wire tables that list the safe amperes a wire can carry. That does not mean it carries that much amperage. That means the wire will not burn if under that amperage. So we protect the wire with a 20 amp fuse at the battery. Even if the device only draws 5 amps. The objective is to reduce voltage drop in the circuit. Every connection, every fuse, every inch of copper wire, all work together to reduce the voltage available at the other end of the circuit. Even charging circuits. Some devices will shut down if under voltage. But yes you can use a 7.5 amp fuse and your fan will work. Put a fuse at the battery end of your wire.

 

[DannyB1954]

So if you wire with 10 ga and use a 30 amp fuse for a 5 amp fan, that is acceptable?
What happens if the fan shorts out and only consumes 25 amps? That is still enough to start a fire.

 

[Weight]

How will the fan's short be limited to 25 amps? The 10 gauge wire can carry up to 50 amps with out getting to 100 F. No fire there. It is aceptible to use smaller fuses. What ever makes you sad.

 

[DannyB1954]

How will the fan's short be limited to 25 amps? The 10 gauge wire can carry up to 50 amps with out getting to 100 F. No fire there. It is aceptible to use smaller fuses. What ever makes you sad.

If the short is after some resistance in the circuit, the amperage can be under 30 amps. If the short happens in the motor after it has gone through some of the windings it could generate enough current to get things very hot.  

A cigar lighter uses less than 20 amps, so things can get plenty warm with a short that has some resistance. The heat generated by a short will be much higher at the point of the short rather than in the wire carrying the current. This is the point of highest resistance. 

A 50 amp load requires a 8 ga wire to be safe.

 

[Trebor English]

In regular house wiring all the outlets are connected with 12 guage wire to 20 amp breakers.  12 inch oscillating fans and Mr Coffee coffee makers contain devices that work like fuses.  If the current is too high they blow.  If the appliance gets too hot they blow just due to the heat even if the current is low.  If the fan blade cannot turn the motor gets hot.  If the coffee maker thermostat gets stuck on the hotplate overheats with no excess current.

Cigarette lighter sockets, not 12 volt power points but actual cigarette lighter sockets, contain thermal protection in the socket.  If a lighter gets pushed in and held it gets too hot without enough current to blow the fuse feeding it.  The socket itself protects.  Some short blowing the fuse, some blow like a fuse.

 

[Weight]

I'm done with this argument. It is a no win. I don't know what chart you use, but mine are presumably different.