How to Calculate the Wire Harness Bundle Size | Hunker How to Calculate the Wire Harness Bundle Size By C. Taylor. SAVE; When calculating the size of a bundle of wires, you are looking to calculate the diameter of the bundle more than anything else. This calculation will help you decide where the cable can fit or what kind of hole needs to be drilled to accommodate the wires. Once assembled MegaSquirt Wiring Harnesses & Wire Bundles Wiring Harnesses & Wire Bundles. All MegaSquirt Wiring Harnesses use TXL high temperature automotive grade wiring/ wires are pre-printed with the ECU pin number and wire functions.