Originally Posted by Ex-Calif
You double the positive run when calculating fuse size.
Because OP was asking if he should calculate based on round trip (24 feet) or 36 feet because he has an extra wire run. My question was whether the extra run was a positive cable because it was not clear from his post.
Small correction: you double the positive run when calculating WIRE size, to be within the desired V-drop (eg 3% or 10%) of the wiring
The point of fusing at the battery
is to protect the wiring
, in case of a short circuit resulting in current
- exceed the wire's ability to handle safely (fires, melting etc) and/or
- cause the battery to overheat or explode
... so it doesn't really matter whether the positive battery cable is 3 ft or 30 ft. The fuse current
rating should be higher than the largest expected draw, but the same or lower than the wire's safe maximum current. (which is what mitiempo already said above)
For a long run between two batteries on the same bank, you do need a fuse near each battery on the positive cable, because there are two sources of potentially dangerous currents for that cable. You don't need a fuse on the long negative run, because all of the metal bits it could short against are also at ground potential or are isolated, and also because all the sources of positive potential are already themselves fused.
All things considered, it's not a great idea to to have the batteries of the same bank separated by a large distance. If more bank capacity is desired, either locate the extra battery much closer to the bank, or put in a larger battery. If you only have that remote
location available for more batteries, consider making that a new (house) bank, that you can charge and draw from separately.