How Big of Fuse?

   I am writing this to help determine the size of a fuse to use in the alternator’s main battery output cable, i.e. the cable from the alternator to the battery. To date I have never used a fuse or circuit breaker in this line. But since many would like to use one, I would like to explain what they will and will not protect against.

   First using too small of a fuse or circuit breaker can cause many problems. In a 130amp 3G the fuse should be a minimum 160amps, preferable 180 or 200amps. As when the stock alternator is loaded way down, it can produce 160amps at about 12.8 volts. This varies from alternator to alternator and is much higher for high output alternators. Better to be safe than sorry, so using a 200amp breaker will keep the margins or error wider. But does a 200amp breaker offer the same protection as a 130amp breaker. The answer is yes when you consider what it is actually protecting you against. Lets look at possible causes of shorts in the alternator charge circuit.

Regulator Stays On: This causes the field to be continually on. Which puts about 6amps directly through the rotor to ground. This causes great heat, but the battery usually drains to zero before a fire. Regardless it is only 6amps, so neither a 130 or 200amp breaker will open.

Rotor Shorts Internally: This causes a directly positive to ground short. The 24-20gauge brush lead will melt instantly. Kind of acting like a fuse of about 10amps. Then the brush spring will try and carry the load, and melt as well. So neither the 130 or 200 breaker will open.

Diode Shorts: This causes a direct positive to ground short. The diode’s lead is only designed to carry 50amps (stock), 70amps (HD aftermarket). So it will melt almost instantly. So neither the 130 or 200 breaker will open.

Stator Shorts: This causes a heavy positive to negative short. This can cause a fire as all the amperage created by the alternator is going to ground. But the alternator is the source of the power, so the breakers will not be able to prevent this. Once the alternator has fried the stator, it then becomes possible for the power to come from the battery to the stator. But this is only if the diodes have failed by becoming shorted. If so see above.

Main Battery Post Becomes Loose: This can cause a fire, in fact 1000’s of fires happen on Ford products equipped with their 2G alternators, because of this. Since the fire is started by heat in the loose connection, it is the alternator that is producing the power that creates the fire. So breakers and fusible links will not and did not prevent this.

Main Battery Cable Shorts: This causes a direct to ground surge, usually large enough to burn/blow the wire in half instantly. If you are using a 4ga cable, it is possible several hundred amps will go to ground. So either a 160/200/300 amp breaker will open in this situation.

Side Notes: A couple of things you should know. The next gas deliver truck you pull up beside, likely has no fuse between the alternator and batteries. If your breaker pops while the alternator is charging, there is a good chance it will blow the diodes. Sometimes they blow with enough force to send shrapnel through rads, A/C condensers, and people if your hood is up.

   In the end a breaker only protects your system from a cable short between it and the alternator. It will not help with most other problems, including a cable short between it and the battery. So going a little bigger with the breaker, if you use one, is actually safer both physically and financially.