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The dissipation factor of polyethylene terephthalate film increases exponentially between ~70 and ~105°C, then decreases to about 160°C, then increases monotonically with temperature. It is shown that this peculiar dissipation factor behavior of PETP with temperature can lead to thermal runaway of alternating current capacitors if they are too large or the applied voltage is too high. NumericaI results for the critical condition have been calculated so that engineering designs for these capacitors can be checked for thermal stability. The calculations have been checked experimentally. The net result is that the center of large ac PETP film capacitors will operate at above 120°C. Above about 145°C at the center, a small increase in voltage or ambient temperature could force the capacitor into catastrophic thermal failure.