Increasing automotive engine compartment temperatures have produced a need for more heat resistant insulations on primary wire. The heat problems with which insulations must cope are loss of resistance to cut-through and deformation, loss of elongation over the long term, and an increase in the destrictive effects of extended current overloads. A number of insulating materials are compared through standard laboratory tests. Deformation and cut-through resistance are shown through pinch test performance. Retention of elongation is demonstrated by both slab and wire tests after circulating air oven aging. Extreme current over load effects are compared by short circuit tests on mock harness assemblies.