A simple gas blow switch consisting of an axisymmetric tulip/pin contact and a cylindrical polytetrafluoroethylene (PTFE) nozzle has been used for empirical studies of medium voltage load current interruption in air. Only the thermal phase of the interruption is considered. Based on the results of more than 3 000 interruption tests, a logistic regression model has been developed to describe the relationships between the switch design and test circuit parameters, and the current interruption capability. The most important design recommendation is that the inner diameter of the nozzle should be only slightly larger than the pin contact. The upstream over-pressure needed for successful interruption increases as a polynomial function of degree 0.5 – 1 with the rate of rise of recovery voltage and of degree 2 – 3 with increasing current.