This is a short article on fusing solar or other charging sources. It can also be helpful for other voltage sensitive equipment.
When fusing devices such as solar controllers, alternator output, wind or battery chargers they need to be fused close to the battery (source), or within a few inches of a change in wire gauge. It can be helpful, from a voltage drop perspective, to use the largest size fuse the wire can handle. Doing so will minimize any added voltage drop across the fuse.
I often hear "My solar panel can only put out 4.5A so I will use a 5A fuse.".
Unfortunately this is a flawed mind set to be in. You should ideally eb thinking; "My 10GA 105C rated wire has a max ampacity of 60A outside of an engine compartment so I should use a 50A - 60A fuse."..
This photo shows a 5A fuse at a 5A load. It is adding .1V of voltage drop to the losses already there in the wiring and terminations.
10A Fuse - 5A Load
Here is the same exact scenario except this time a 10A fuse with a 5A load. It is dropping just 0.051V vs. 0.111V...
30A Fuse - 5A Load
This is the same set up now with a 30A fuse and just a 0.019V drop....
The smallest wire in this case is the short section of the 12GA fuse holder. It has a max ampacity of 45A...
The key take away here is that using the largest fuse that is well within the safe range for the wire results in the least voltage drop in your system wiring.
Max Ampacity Chart - Non-Bundled Wire
Here's the wire sizing chart for max ampacity. This is the chart to use when sizing for fusing or over current protection, when the wires are not bundled.
Find your wires gauge in the left column then move over and find the jacket temp rating. Most "marine" UL wire has a jacket temp rating of 105C.
When sizing for voltage sensitive sources, or when ever voltage drops can make a difference, it is a good idea to size to the max ampacity of the wire rather than what your charge source can supply..
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