Power distribution blocks are the devices used in cars to distribute electrical power to the various parts of the car that require power to function. A power distribution block is effectively the same as a fuse box, as installed in buildings, but the former term is used to describe such an electrical device when found in a car or other motor vehicle.

All power in a car, truck, or any motor vehicle comes from the car battery. The internal combustion engine, of course, is what powers a motor vehicle in the sense of making it capable of movement, but the engine first requires electrical power to start up. Cars also have indicators and other lights, which need electricity. Since the earliest days of motoring, the starter motor and lights have been the two basic elements of a car requiring electrical power. However, in more recent times, a range of additional devices have been increasingly commonly fitted to cars, such as car radios, high performance car audio systems, air conditioners, automatic locks, sat navs, and even laptop computers. The power distribution block has therefore become a very necessary part of the modern car, feeding power from the battery to the various parts of the car that require electrical power.

The last few years have seen a growing number of gadgets and devices forming part of the modern day car and this has increased the likelihood of cars experiencing electrical and technical faults. While cars are built to deal with the new demands, it does increase the chances of breakdowns and car breakdown cover is seen as essential. Such faults can be hugely inconvenient for drivers, who are likely to have invested a great deal in such additional features.

As already mentioned, the car battery is the basic power source in any car. The battery feeds power to the alternator, which first converts the DC current output from the battery to AC current, then feeds it to the power distribution block. Housed in the same area of the car as the engine, the power distribution block contains a number of different fuses, which regulate the power passing through them. Each fuse is wired to a different element of the car requiring electrical power. If for some reason a fuse blows, the particular item connected to that circuit will cease working and it will be necessary to replace the blown fuse with a new one.

One important thing to note with power distribution blocks is that the power ratings of the fuses must be appropriate for the requirements of the components being powered from them. Modern car audio systems can demand a lot of power when the volume is turned up, especially if the system incorporates a subwoofer. The power demand varies with peaks in the music being played. If the power required is momentarily more than the rating of the fuse, the fuse may blow.