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My Battery Always Dies - Part 4 Checking the Sizes of Fuses, Overcurrent Protection
Previous : My Battery Always Dies - Part 3 Sizing your Battery for Your Accessories
Next: My Battery Always Dies - Part 5 Checking the Sizes of Wires, Why and How
Fuses
 Fuses are designed to sense something using more power than it should. In other words a fuse catches short circuits, failed equipment, and provides a safety catch in the event of catastrophe (lightning, etc). If a fuse senses too much current (amperage) it simply blows out.
Fuses are rated at roughly 80% sustained load, if you run the fuse at 100% load for a while, it will blow, sensing an overload situation. A 40 amp fuse run at 40 amps will blow shortly. Therefore to get the sizing right, multiply by something between 1.2 and 1.5 to compensate. The bottom line is you want to have a fuse very close to the load, well under the wire, inverter, and battery capability specifications.
Depending on the sizing of the system you are playing with, increasing a circuit by a few amps (1 amps to 5 amps), a bit (10+ amps - 120+ watts), or a bunch (50+ amps - 500+ watts), you should concern yourself with a varied few things. We broke it up accordingly below.
Installation:
Large Fuse holders are bolted or screwed into a flat spot in either the engine compartment, shielded area in the undercarriage, or in the cockpit of the vehicle. Many small fuses can be installed on the wire to the accessory, as long as they are shielded from general impacts. The positive cable from the battery connects to one side of the fuse holder, the positive cable to the appliance connects to the other.
*Installing the fuse will power up the accessory if the battery is connected.
1 amp - 10 amp Fuses, 12 volts
Many car circuits are designed for small loads (under 15 amps). Generally these are fitted with fuses ranging from 1 - 10 amps. When upgrading fuses between these circuits, generally there isn't much to worry about. Simply pull the 2 amp fuse, replace it with a 5. Not a big deal most of the time. Just watch the temperature of the wire, if it gets hot, swap it out with a bigger one.
10 amp - 40 amp Fuses, 12 volts
Fuses in these ranges power systems from 80 watts all the way up to 400 watts. At this point you should generally start to concern yourself with wire sizes, voltage drops, and subsequent fire dangers. A practical example; Many stereos come with power cables and fuses inline designed to carry the loads all the way to the battery. The manufacturer provides you with wires that are sufficient to keep the voltage up across the length. The engineers know how much amperage the system draws, and it's voltage needs. Notice the wires are thicker than the old ones you have for a stock radio. CB radios, car stereos, light kits, and other similar units are routinely supplied in this manner, more on wires below.
60 amp - 400 amp Fuses, 12 volts
High voltage DC loads need to be run on very thick wires, have installed fuse holders bolted down, and should be worked on without the batteries hooked up to the circuit, and the vehicle OFF. Battery disconnect switches in addition to fuses are routinely used for this reason.
To size wires for these installations, run calculations on your wires, no matter the line length. Remember safety first. Amplifiers, Inverters, Heaters, Coolers, and other Power HOGS are routinely supplied with 2/0 awg to 6 awg wires to compensate for these higher amperage loads. Please consult the wire drop calculator here for the calculations if you don't use the wires that came with the unit.
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BD Batteries
Previous : My Battery Always Dies - Part 3 Sizing your Battery for Your Accessories
Next: My Battery Always Dies - Part 5 Checking the Sizes of Wires, Why and How
Legally: REMEMBER, ELECTRICITY HURTS, IT CAN KILL YOU. BD BATTERIES TAKES NO RESPONSIBILITY FOR YOUR ACTIONS WITH THIS INFORMATION. IF YOU AREN'T TRAINED TO WORK WITH ELECTRICITY, CALL SOMEONE WHO IS. IF YOU CHOOSE TO MEDDLE, YOU AGREE TO TAKE RESPONSIBILITY FOR YOURSELF! THIS GUIDE IS IN NO WAY A COMPLETE ELECTRICAL GUIDE, AND SHALL NOT BE PRESENTED AS SUCH IN ANY FORMAT OR FORUM. PERFORMANCE BATTERIES ARE CAPABLE OF ARC WELDING STEEL AND OTHER VERY INTERESTING THINGS INCLUDING YOUR HEART. BE CAREFUL, DON'T HURT YOURSELVES.
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