What is the rule for box fill when using conductors insulated with different materials?

The rule for box fill regarding conductors insulated with different materials dictates that the largest conductor size determines the volume calculation. This principle is rooted in the NEC's provision to ensure adequate space for conductors within junction boxes, which prevents overheating and potential failure.

When calculating box fill, the NEC requires that the cross-sectional areas of all conductors be considered. However, since different insulation materials can have varying thicknesses and impact how much space the conductors actually occupy within a box, the largest conductor’s size becomes the benchmark for the calculation. This approach simplifies the assessment of required box volume to accommodate the varying physical sizes of conductors, providing a safety factor in managing heat and preventing overcrowding.

The other choices do not align with this standard. For instance, specifying that all conductors must be the same size ignores the diversity of conductor types and their respective applications. Stating that smaller conductors do not count toward box fill would compromise safety by neglecting the volume they occupy. Lastly, using the average size of conductors is not a practical approach, as it could lead to inadequate space if the largest conductor is significantly larger than the others. Thus, anchoring calculations to the largest conductor size simplifies and enhances safety strategies in electrical installations.