Nuclear plant reactor pressure vessel heat-up and cool-down pressure temperature (P-T) limit curves are determined using ASME Section XI, Appendix G. ASME has adopted into ASME Section XI Appendix G the allowable use of the ASTM E1921 master curve fracture toughness based reference temperature (T0) to index the KIc curve. ASME Section XI Code Case N-830 allows the use of the KJc 95% lower bound master curve indexed using T0 directly. In ASME Section XI Appendix G, the equation RTT0 = T0 + 19.4°C, as an alternate RTNDT, shifts the KIc curve to approximate the KJc 95% lower bound master curve, however, the KIc exponential curve parameters are different. Thus, this paper evaluates the impact on plant heatup and cooldown pressure-temperature limit curves between the two ASME approved methods for typical pressurized water reactors (PWR). Different degrees of embrittlement are assessed to determine differences in the two approaches on reactor pressure vessel (RPV) beltline operating curves. Furthermore, in the proposed Revision 19 of Regulatory Guide 1.147, the US NRC has included a condition on the use of Code Case N-830 that prohibits the use of the current KIc equation in ASME Section XI Appendix G when these values are above the KJc lower bound 95% curve at temperatures below T0 − 64°C. This paper briefly discusses this NRC condition on the P-T limit curves.