How much spacing is needed in high voltage circuits and setups? The general guideline in common use is to allow 7,500 to 10,000 volts, dc per inch in air. Those who ask are frequently surprised by the answer: None. Between live parts of opposite polarity, 251-600V, Through air gap is 1", Over surface is 2". However, there are. Each assembly type is to be subjected to an impulse voltage test in accordance with its constructional Standard or, alternatively, the minimum distances for bare conductive parts in switchgear and controlgear assemblies given in Table 2. 1 Minimum clearance distances are to be used. Kg/m2 Annexure-1) 4" EH IPS Al. 5 Indal Aluminium busbars book.