Average cable length = (distance from the farthest floor distribution box + distance from the nearest floor distribution box)/2 Actual average cable length = average cable length × 1. 1 + (termination tolerance, usually 6)Calculate the required cable length for electrical installations accounting for straight-line distance, vertical rise, bends, and slack allowances. This calculator helps ensure you order the correct amount of cable with appropriate safety margin. This free-of-charge tool designed for the professional: electricians, installers, engineers, etc. Here's how to. After you have made your decisions on outlet locations and cable types, you need to determine how much cable you need for wiring the home. Complete the sections below to calculate your results.