In my application the DC supply has rating of 1200V and 15A. I just want know the calculation for the trace clearance between power and ground.
To pick one safety standard- 61800-5-1 (ref IEC 60664-1), for 1200VDC pollution degree 2 and insulation material group III, requires you to provide 12.6mm minimum creepage for reinforced insulation. Look up the relevant standard(s) for the precise definitions of the terms used.
It's your responsibility to determine what standard(s) your product must conform to and to make sure your design meets all of the constraints simultaneously.
At 1200VDC consider milling (unplated) slots in the PCB.
Normal "air" can withstand about 1 kV per millimeter. That means at 1.2 mm spacing you are likely to get arcing. Obviously you want the spacing to be more than that, actually much more than that. I'd start with 5x more at least, but possibly even more depending on the usage and regulatory requirements.
Dirt coupled with humidity can greatly increase the leakage current across what would otherwise be a insulating surface. This is why there are often separate specs for creapage and clearance. Creapage is the minimum distance between two conductors along whatever insulating surfaces connect them. Clearance is the shortest direct-line path thru air between the two conductors.
Since the cost of failure can be very high (burn down house, kill people), there will be regulations you must follow, and they will be very conservative. These regulations will vary depending on intended use and how the overall circuit is protected. For example, something inside a grounded metal box that is not intended to be user-accessible will be more lax than a patient-touching medical device. Of course you shouldn't be anywhere near the latter if you have to ask here.
So the real answer is in whatever regulations you are required to design this device to in the jurisdictions you want to sell it, or in the requirements of the certifying agency your customers will require, whichever is more conservative.