This isn't really a calculation. Presumably the operating voltage of the system is a given. Determine the largest voltage difference (whether phase-to-phase or phase-to-earth) that will be present, and select all components (motor starters, fuses, circuit breakers, instrumentation, and wire insulation) to have a greater rating.

It's not really a single calculation as it depends on the voltage, application, environment, pollution level, equipment containment/accessibility/type, particular local/international standards, etc.

Have you checked your local electrical wiring regulations, international electrical standards, etc as all the information on creepages and clearances is given in them for your particular application.

Additionally cables, wiring and equipment are generally marked with the maximum rated voltage (either on it or in the data sheets).

What exactly is the application?

The rated insulation voltage corresponding to different sytem voltage is mentioned in IS:2165.

Calculations have been elaborately done by experts and the required insulation voltage levels are given in the Standards. Better follow them in toto. However, some logic has to be there for the Insulation Withstand Voltage Values given in the Standards too. Isn't it? Let me explain.

Supposing that you have a Low Voltage, 3-Phase, 433 V, 50Hz. electric power supply system. This voltage (433V) is called the 'Rated Operational Voltagte (Ue)' meaning that this voltage will be present continuously between the phase terminals of the switchgear at all times. Also meaning that the insulation between the phase terminals of the switchgear has to be designed keeping this fact in mind. But, there is a problem. As per Indian Electricity Rules, there can be +/- 10% variation in the supply voltage. Negative side variation is no problem for the insulation, but higher side variation is a problem. So, the switchgear inter-phase insulation has to be designed NOT for the Normal System Voltage but for the Highest System Voltage, which could be present continuously too. So, 433 V now becomes 433V + 10% (i.e.) 476.3V.

But, that too wouldn't suffice. Imagine, your transformer is having a manual tap changer and the taps are provided for a percentage voltage variation of +/- 12.5% (i.e.) +/- 10% from the supply authority as above and +/- 2.5% as a safety cushion for load related voltage variations. Now, if your system voltage falls to, say, - 12.5%, you would keep the tap in the -12.5%. Isn't it? In your industry, after you do this tap changing, may be, you are called upon to attend to a breakdown call at the other corner of the plant and you leave the substation. As soon as you leave the SS, the system volatge jumps to, say, + 12.5%. Now, the LV Voltage from the TRF would be + 25%. Isn't it? So, teh inter-phase insulation of the switchgear would be subjected to 433V + 25% (i.e.) 541.25V. Add to this, a 10% plus side voltage variation may be due to load fluctuations, it becomes 595.375V. And, this magnitude of high voltage would sustain till corrective action is taken. The insulation has to withstand at least this much high voltage on a continuous basis, without causing any damage. So a 415V rated equipment has to have an insulation withtsand of at least 600V considering the worst case scenario. The standard Rated Insulation Voltage (Ui) for LV Switchgear is 690V.

Similar calculations are done for MV/HV/EHV too and respective Rated Insulation Withstand Voltages arrived at.

Hope it is understood.