Have my 16" fan, positive leed, hooked directly to the alternator with a 30 amp fuss for safety. The negative leed, I have going to a switch under the dash for control. When the motor is not running I can turn on the fan and it does not blow a 30 amp fuss. When the motor is running it will blow the 30 amp everytime. I can jump up to a 40 amp and all is fine but seems a bit much considering the fan is rated at 17 amp draw.

Any advise on why this would be happening?

 

Ttt

 

Perhaps the long wire run is building up too much resistance and blowing the fan. Is the wire getting hot? Do you have a large enough wire? I would use the ground to trip a relay. That way all the big wires have short runs.

 

I agree with markdtn. The longer the distance of a wire, the greater the resistence will be. The greater the resistence, the higher the draw. Your fan may draw 17 amps, but the wire in combination with the fan draws higher. When the motor is running, the alternator has a draw by other things drawing power(distribution of power). Unlike your turned off motor, which only has your fan drawing power, hence the 30 amp fuse works fine, the motor running creates a new picture. Electricity, like many other things take the course of least resistence. The power goes to the least restrictive areas before it will go to you fan. Because of this fact, the draw of the fan IS actually higher with the motor running because it has to fight everything else(less resistence) to get electricity for your fan, hence the higher draw, and the blown fuses. Hope this helps ya out

 

On a load device like a fan its a better idea to use a relay and put it on the Hot side. Make the ground permanant. Check out MAD vetts site, they have a good simple solution. Wire size is the likely problem. With the engine running the applied voltage will be near 14v as aposed to 12 with the engine off. Hugher voltage = faster running fan = more amps.

 

Sorry but this is 100% wrong.

The most important law in electricity is Ohm's law which states:

Current (amps) = Voltage (volts) / Resistance (ohms)

You can see that as resistance rises current falls (if voltage stays constant).

As for your fan blowing fuses I would question your return path, does it go to a proper ground? SIXFOOTER is right with the relay switching the hot. I doubt your switch is rated for that much current. 30amps is a high enough rating for your 17amp fan even when the engine is running. If you are blowing 30amp fuses there is somthing wrong and by putting in 40amp fuses you are not fixing anything but rather will be making things worse.

One other possibility is you have a 'fast blow' style fuse. You may want to try a slow blowing fuse. When a fan first starts up it will draw a large amount of current for a short period of time. It may be 30 amps but only for like 0.1seconds. This current surge may blow a 'fast blow' style fuse but a slow blow fuse would not be bothered by a quick surge like that.

 

[QUOTE=Boofers]Sorry but this is 100% wrong.

The most important law in electricity is Ohm's law which states:

Current (amps) = Voltage (volts) / Resistance (ohms)

You can see that as resistance rises current falls (if voltage stays constant).

As for your fan blowing fuses I would question your return path, does it go to a proper ground? SIXFOOTER is right with the relay switching the hot. I doubt your switch is rated for that much current. 30amps is a high enough rating for your 17amp fan even when the engine is running. If you are blowing 30amp fuses there is somthing wrong and by putting in 40amp fuses you are not fixing anything but rather will be making things worse.

One other possibility is you have a 'fast blow' style fuse. You may want to try a slow blowing fuse. When a fan first starts up it will draw a large amount of current for a short period of time. It may be 30 amps but only for like 0.1seconds. This current surge may blow a 'fast blow' style fuse but a slow blow fuse would not be bothered by a quick surge like that.

That is correct, but the problem with long wire runs is a voltage drop.

 

All electric motors have an in rush load which can be much more than the amp draw at normal operation. a 20 amp slow blow fuse should do the trick. If it doesnt, then there is another problem such as something else operating of that circuit. If the wire size is too small that will not cause the fuse to blow. The wire would burn up first. Fuses work of heat but an undersize wire would probably burn in two before the element in the fuse melts.

 

[QUOTE=kalister1]
Voltage drop in a car should not be a problem unless the wire being used is way to small. Voltage drop can be solved by oversizing a wire or conductor. However, I do not think you can run enough wire in a car to have voltage drop.