Proper Wire Sizing... How Not to Make Fuses...

Okay Folks,


Back to electrical stuff...


I mentioned below about how the fundamental job of a fuse is to protect a wire from fire. You want the fuse to be the weakest link in the car's electrical system. It should open long before the wire that it is protecting gets hot. This means that builders should take care to really understand how much current a load draws when they size the wire connecting it and the fuse protecting it. Most manufacturers of automotive accessories will publish the current draw.


Just to make things a little more complicated, there are really two current draws that you need to know about. The easiest is the steady-state current draw. For example, a light bulb may have a steady-state current of 7 amps. This is the current that flows through the filament after any transients, in-rushes or start-up events go away.


The current draw that is a little more complicated is the in-rush. This is the initial surge of current that some loads draw. There are basically two types of loads that have in-rushes: inductive loads and incandescent loads.


Inductive loads include any load that has an inductance, or coils. Think about things like the windings in a motor or a solenoid or a relay coil. The physical turns of the coils in these devices create inductors that generate magnetic fields when the current flowing through them changes. Changing current flowing through an inductor drives a change in the voltage which cause surges in the current when the voltage changes suddenly. When you turn on a motor, the current surge can be 2 to 10 times higher than the steady state current.


Light bulbs work by passing current through a filament that has resistance. In all cases, the resistance of the filaments is a function of its temperature. It has a relatively low resistance when it is cold but the resistance goes up significantly when it heats up. Since current flow is inversely proportional to the resistance, the current starts high when the bulb is first turned on and drops as the filament heats up. This first surge of current through he filament can be 2 to 5 times its steady state current.

This means that when you choose a wire and fuse combination, you need to consider the steady-state current and any in-rush current that the load may draw.

We do some of this heavy lifting for you in our standard ISIS 3-Cell Kit. All of the output wires from the POWERCELL are sized with 14 AWG wires. Each output is rated to a maximum of 25-amps. Feel free to contact our technical support team if you have questions on sizing wires or fuses for your ISIS Intelligent Multiplex System.

Jay