DescriptionSecondary Insulation Bushing: All types except NS. Must be used when strips are mounted for air heating only or when connected in series on line voltages above 480V. Note: To accommodate bushings, a 17 /32 " x 11 /16 " dia. mounting hole in tabs should be specified for heaters. Delivery from stock. To order secondary insulation bushings, add suffix “/PCN 255716” to model no. and add $10 to price.