Dry air can support a maximum electric field strength of about 3 X 10⁶ V/m. Above that value, the field creates enough ionization in the air to make the air a conductor. This allows a discharge or spark that reduces the field. What is the maximum voltage (approximately) between two parallel conducting plates separated by 2.5 cm of dry air?

75 kV

V = Ed as electric field is equal to potential gradient