Fuse box fried...

[CoronaRaptor]

I suppose I am having trouble grasping the 80% rule for a circuit.

Take a circuit fused for a certain amount, say 20 amps. If you put a load on it that exceeds the 20 amps will the fuse not blow and protect the circuit? That is assuming the fuse was appropriately selected for the gauge of wire and the length of wire.

Does the issue turn into the fact that it may not present a load high enough to blow the fuse but will generate too much heat to cause an issue? But then again isn’t heat a result of current?

I guess my question is that shouldn’t the fuse on the factory upfitters be sized appropriately to protect the circuit despite whatever load the circuit may see?

Is the 80% more so that you don’t blow a fuse during normal operation rather than for protecting the circuit?

What you are not grasping (you're not alone here), is that just because there is a 20 amp fuse present, most people think the circuit will protect up to 20 amps, but that is not how it operates, this is the MAXIMUM amperage this circuit can handle. What this means is, if you have a 5amp light on this circuit, you need to protect it with a 5-6 amp fuse. You can leave the 20 amp fuse in the factory location, but you need to protect your circuit with a 5-6 amp fuse as well. What will happen, if you don't add this additional fuse is: lets say your lights short out and would normally trip at 5 amps, but with a 20 amp circuit (fuse), the fuse won't blow and then the circuit overheats ( highly possible) and melts the wiring.

 

[Spectragod]

I ran into this crap at the dealer on my’13, they tried to blame my led’s for the instrument cluster failure. First it was they drew too much power, then it went to the lights were back feeding the circuit, when I explained I had diode protected relays on both the + and - side, and it was impossible. Then he put it all on the tech, saying he the service writer wasn’t a mechanic. I told him if his tech told him that, he wasn’t a mechanic either.

Then came the day my new cluster was installed, I was then told my “aftermarket “ wheels caused it to go bad, I said dude, those are OEM. His response, well parts just fail sometimes....... da fuq is wrong with these people?

OP, good luck, I hope you get your truck squared away.

 

[Spectragod]

What you are not grasping (you're not alone here), is that just because there is a 20 amp fuse present, most people think the circuit will protect up to 20 amps, but that is not how it operates, this is the MAXIMUM amperage this circuit can handle. What this means is, if you have a 5amp light on this circuit, you need to protect it with a 5-6 amp fuse. You can leave the 20 amp fuse in the factory location, but you need to protect your circuit with a 5-6 amp fuse as well. What will happen, if you don't add this additional fuse is: lets say your lights short out and would normally trip at 5 amps, but with a 20 amp circuit (fuse), the fuse won't blow and then the circuit overheats ( highly possible) and melts the wiring.

Which is why I always run a stand-alone relay that is fused separate from the factory feeds, more work, but fail proof.

 

[isis]

Yes, but 12volts isn't constant, it can fluctuate from 8-15volts and as such the amperage moves up and down. Sorry for not explaining properly. There is also inrush current, but don't want to confuse you more.

You’re not confusing me. I’m an electrical engineer. I’m confused about what you’re trying to say. Volts go down then amps go up. An LED bar is a constant load. A vehicle battery doesn’t dip to 8 volts. The local voltage might if your wires are inadequate. When the vehicle is running it is extremely stable voltage. Fuses are thermal devices and are made to handle inrush by nature.

 

[isis]

What you are not grasping (you're not alone here), is that just because there is a 20 amp fuse present, most people think the circuit will protect up to 20 amps, but that is not how it operates, this is the MAXIMUM amperage this circuit can handle. What this means is, if you have a 5amp light on this circuit, you need to protect it with a 5-6 amp fuse. You can leave the 20 amp fuse in the factory location, but you need to protect your circuit with a 5-6 amp fuse as well. What will happen, if you don't add this additional fuse is: lets say your lights short out and would normally trip at 5 amps, but with a 20 amp circuit (fuse), the fuse won't blow and then the circuit overheats ( highly possible) and melts the wiring.

This is 100% correct. You need the right fuse for the load, not for the switch.

 

[CoronaRaptor]

You’re not confusing me. I’m an electrical engineer. I’m confused about what you’re trying to say. Volts go down then amps go up. An LED bar is a constant load. A vehicle battery doesn’t dip to 8 volts. The local voltage might if your wires are inadequate. When the vehicle is running it is extremely stable voltage.

Put your meter on your circuit with the engine off and the key on, start up the vehicle and watched your voltage, let me know if the voltage changes when you rev up the engine too. Just saying. This isn't 115/230/460/575 where it stays constant on a circuit and the amperage/resistance change values.

 

[isis]

Put your meter on your circuit with the engine off and the key on, start up the vehicle and watched your voltage, let me know if the voltage changes when you rev up the engine too. Just saying. This isn't 115/230/460/575 where it stays constant on a circuit and the amperage/resistance change values.

This is also correct. And has nothing to do with the maximum power draw rating on an LED light bar. Or fuse ratings.

 

[Jakenbake]

What you are not grasping (you're not alone here), is that just because there is a 20 amp fuse present, most people think the circuit will protect up to 20 amps, but that is not how it operates, this is the MAXIMUM amperage this circuit can handle. What this means is, if you have a 5amp light on this circuit, you need to protect it with a 5-6 amp fuse. You can leave the 20 amp fuse in the factory location, but you need to protect your circuit with a 5-6 amp fuse as well. What will happen, if you don't add this additional fuse is: lets say your lights short out and would normally trip at 5 amps, but with a 20 amp circuit (fuse), the fuse won't blow and then the circuit overheats ( highly possible) and melts the wiring.

Ok, so in this situation say my wire running up to the light is of large enough gauge that it is more than safe at 20 amps. And the whole circuit is fused at 20 amps. Would the only part to fail be the actual light and not the wire or the switch, etc? Basically I need to protect the light and that is the 80% part?

 

[Jakenbake]

This is 100% correct. You need the right fuse for the load, not for the switch.

So in your example if someone were using an upfitter switch rated at 20 amps, but running an 8 amp load. This person should swap the 20 amp fuse for a 10amp fuse?

 

[CoronaRaptor]

Ok, so in this situation say my wire running up to the light is of large enough gauge that it is more than safe at 20 amps. And the whole circuit is fused at 20 amps. Would the only part to fail be the actual light and not the wire or the switch, etc? Basically I need to protect the light and that is the 80% part?

Kind of, it's very long winded, but basically don't rely on just your wiring to be sized correctly, it needs to be protected properly with a fuse, that's the most important thing to grasp from this conversation.