which battery do i use?

[JBroome]

I'm going to run a power wire off one of my batterys for my amp to my subs and im not sure which one to use anyone know?



thanks

josh

 

[Tim]

It doesn't matter, they're wired together.

 

[JBroome]

thanks

 

[Tim]

Roll your eyes all you want... . every day at work I fix people's electrical screwups, if 12 v circuits were all I had to worry about, I'd be happy.



Make sure you use a fuse. Please, please make sure you use a fuse and put it as close to the battery as possible. 3" isn't too close.

 

[JBroome]

i was laughing at my self for over anyalizing it not at you sorry i didnt mean to offend you

 

[mberry]

Roll your eyes all you want... . every day at work I fix people's electrical screwups, if 12 v circuits were all I had to worry about, I'd be happy.



Make sure you use a fuse. Please, please make sure you use a fuse and put it as close to the battery as possible. 3" isn't too close.

Tim,



Why do you put the fuse close to the battery? Why is closer better? I hate wiring of any kind so I'm asking out of ignorance. Thanks.

 

[JHardwick]

Why is closer better?

Who says it is?

 

[nauidvr1]

The reason you put the fuse as close to the battery as possible, is in case of an electrical short the fuse will blow an you will only have power available on the short length of wire (from the battery to one side of the fuse). This will protect you from a possible fire.



If you put the fuse further away from the battery (like in the cab somewhere) there is a possibility that if you had a short between the battery and where you put the fuse, the wire now acts as a fuse an will become glowing red hot, melt the insulation, and cause a fire.



NAUIdvr1

 

[JBlock]

I rather wire the truck than the House any Day!!

 

[Tim]

Exactly. The wire between the battery and the fuse is unprotected.

 

[mberry]

Makes good sense. Thank you.

 

[GCross]

I never really thought of it in that context, but it sure makes sense when I do.



-George-

 

[rbattelle]

The reason you put the fuse as close to the battery as possible, is in case of an electrical short the fuse will blow an you will only have power available on the short length of wire (from the battery to one side of the fuse). This will protect you from a possible fire.



If you put the fuse further away from the battery (like in the cab somewhere) there is a possibility that if you had a short between the battery and where you put the fuse, the wire now acts as a fuse an will become glowing red hot, melt the insulation, and cause a fire.



NAUIdvr1

With all due respect, in my personal opinion this condition is impossible if the fuse current is properly chosen. By "properly chosen" I mean a 10 amp fuse on an 18 ga. wire (whose ampacity is at least 13 amp), for example.



The reason I believe the above condition is impossible is that the minute the fuse breaks under a load of >10 amp (in my example), the resistance goes to infinity (for all practical purposes), which causes the current to drop to 0 in the wire. The proof is Ohm's law (V=IR). The key is to always always always always match your fuse size to your minimum wire ampacity. Here's one of many sources for an ampacity chart. The charts may be assumed to be quite "conservative", but it's best to stay within their guidelines.



In other words, as long as you use common sense in selecting your fuse and wire sizes there is no difference in safety between a fuse mounted 20 ft. from the battery and one mounted 1 inch from the battery.



This is purely my opinion, based on my own research. I'm not trying to be mean or rude to anyone, and everyone is certainly entitled to their own opinion. I urge each reader to do their own independent research and decide for themselves whether I'm full of it or not. I am not an electrical engineer or electronics technician.



-Ryan

 

[Bertram65]

With all due respect, in my personal opinion this condition is impossible if the fuse current is properly chosen. By "properly chosen" I mean a 10 amp fuse on an 18 ga. wire (whose ampacity is at least 13 amp), for example.



The reason I believe the above condition is impossible is that the minute the fuse breaks under a load of >10 amp (in my example), the resistance goes to infinity (for all practical purposes), which causes the current to drop to 0 in the wire. The proof is Ohm's law (V=IR). The key is to always always always always match your fuse size to your minimum wire ampacity. Here's one of many sources for an ampacity chart. The charts may be assumed to be quite "conservative", but it's best to stay within their guidelines.



In other words, as long as you use common sense in selecting your fuse and wire sizes there is no difference in safety between a fuse mounted 20 ft. from the battery and one mounted 1 inch from the battery.



This is purely my opinion, based on my own research. I'm not trying to be mean or rude to anyone, and everyone is certainly entitled to their own opinion. I urge each reader to do their own independent research and decide for themselves whether I'm full of it or not. I am not an electrical engineer or electronics technician.



-Ryan

I think the point of having the fuse as close to the battery as possible would be so if the wire is shorted out the fuse will protect the entire length. If you had the fuse in the cab and somewhere between the battery and the fuse, several feet if in the cab for example, there is a short the piece between the short and the battery is what becomes red hot, melting insulation and starting a fire, the fuse inside the cab cannot prevent it.

 

[klenger]

Bertram65 is correct. Don't think of the fuse as protecting against an overload condition caused at the load end of the wire, but along the wire itself. Fueses or CB's are intended to protect the wire, not the load device. Any device on the circuit should have it's own internal over current protection, usually built into the unit.

 

[rbattelle]

If you had the fuse in the cab and somewhere between the battery and the fuse, several feet if in the cab for example, there is a short the piece between the short and the battery is what becomes red hot, melting insulation and starting a fire, the fuse inside the cab cannot prevent it.

This is true.



That's why properly routing your wiring through grommets and away from sharp components that can cut the insulation is critical.



I just thought others were implying that if you drew a large load on the "downstream" side of the fuse you could melt a wire despite the fuse being blown.



On edit: I need to learn to read more carefully. Re-reading naudivr1's post he makes it very clear exactly what he's talking about: a short on the hot-side of the fuse. I guess I didn't pay attention when I read it because I tend to always assume things are wired in such a way as to virtually eliminate the possibility of a hot-side short. Sorry about that.



-Ryan