What is the required size relationship between a conduit and the wire bundle inside it?

• A. The conduit must be 25% larger than the wire bundle.
• B. The conduit must be 10% larger than the wire bundle.
• C. The conduit must be the same size as the wire bundle.
• D. The conduit must be 50% larger than the wire bundle.

Answer: (A)

In aircraft wiring, you size conduit to leave room for the wire bundle to fit and to be pulled through bends without damage. The standard rule you’ll see is that the conduit should be about 25% larger than the outer size of the wire bundle inside it. That extra space makes pulling easier, prevents abrasion on insulation as the bundle bends, and provides some cushion for future adjustments or maintenance. A conduit only 10% larger would be too tight for installation and could pinch and wear the conductors. Using the same size leaves no clearance at all, which can trap heat and make installation difficult. A conduit 50% larger is more than needed for typical installations and adds unnecessary weight and bulk.