Fuses, Electrical System

[Bob Bidonde]

This is my analysis of fusing the electrical power supplys in the Model A's electrical system, and I am posting it for a fact finding check.

Many Model A's have a fuse in the battery power wire connected to the terminal box as shown in the picture. If a short circuit occurs, this fuse will open and disconnect the battery from the terminal box. This fuse protects the electrical system providing the generator or alternator is not running when the short circuit occurs. However, if the engine is running and the generator or alternator is supplying current to the terminal box when a short circuit blows the fuse in the battery power wire, there is going to be smoke because the generator or alternator will have a current output to the short circuit!


To protect the electrical system while the engine is running, it is also necessary to put a fuse in the generator or alternator wire going to the terminal box in addition to the fuse in battery wire. The maximum current output of a stock 6V generator is 22 Amps. An alternator will spike to 30+ Amps just after starting the engine. So the fuse in the generator or alternator wire to the terminal box needs to be 2.5 times the current rating.