Breakdown strength of N-2 under standard lightning impulse voltages in non-uniform field was experimentally investigated. The experiments were carried out with sphere-plane electrode system with protrusion and without protrusion. The electrode gap spacing and the relative gas pressure were varied within the range of 5 mm to 25 mm and of 0 bar to 4 bars, respectively. At each pressure and gap spacing, impulse breakdown voltages for both negative and positive polarities were measured. The 50% breakdown values were obtained using the least squares. Breakdown voltage of N-2 increases either with an increasing gap spacing and/or with an increasing gas pressure.