Raptor electrocutions on overhead power lines occur globally. To evaluate a mitigation strategy designed for horizontal post insulators, a common configuration in the 25–69-kV range, we quantified perching by two Great Horned Owls (Bubo virginianus) and seven Red-tailed Hawks (Buteo jamaicensis) on non-energized 25-kV and 69-kV horizontal post insulators installed below vertically mounted perch deterrents. We quantified how often and how long raptors perched on horizontal post insulators beneath perch deterrents, and how often and how long raptors simultaneously contacted both ends of horizontal post insulators, to simulate phase-to-ground electrical contacts. Across 900.0 min of video recordings, raptors usually perched at the distal end of the insulator away from the perch deterrent, but 56 of 1095 perches (5.1%) included at least some time perched below the perch deterrent (Σ = 34.0 min). We never observed any Great Horned Owls simultaneously contact both ends of an insulator. We observed three occurrences of Red-tailed Hawks simultaneously contacting both ends of an insulator (twice on the 25-kV insulator; once on the 69-kV insulator). We speculate that in the absence of the perch deterrents, raptors would have perched toward the center of horizontal post insulators with higher frequency, resulting in a greater frequency of bridging both ends of horizontal post insulators. On energized poles, such behavior would increase electrocution risk. This suggests that perch deterrents may reduce raptor electrocution risk on vertically configured power poles. This study should be repeated with a design that evaluates perching on horizontal post insulators with and without perch deterrents, with larger sample sizes, and with additional raptor species, particularly Golden Eagles (Aquila chrysaetos).