Postline ballasts are used in lantern, post-top fixtures that are mounted on poles require ballasts to fit inside the pole. Postline ballasts have an elongated core & coil encased and potted in high temperature resin in cylindrical housings. The housing is designed to fit within standard 3” or 4” diameter poles. If the ballast requires a capacitor and ignitor they are included in the housing. Ballastshop carries several brands of postline ballasts including Philips Advance and Robertson.