how to calculate sub breakers from the main breaker

how do I calculate exert amount of sub breakers to match a specific rated main breaker. e.g 200A main breaker to sub breakers

The only "calculation" is "how many circuits are expected to draw how much, at the same time" - and if it's more than 200 amps, you'll need a bigger main, or two 200A panels, or something like that.

It's quite normal to have far more than 200A of breakers attached to a panel with a 200A main. Whether that ever becomes a problem of tripping the main is a matter of use patterns. As an example, it's rare for an electric range to have all burners and the oven on full power, at the same time.

Likewise, many 15 and 20 amp lighting and outlet circuits are rarely fully loaded.

Breakers (and wire) for "continuous" circuits are derated, so they only draw (at most) 80% of the rated breaker amps (breaker is sized for 125% of load - the math works out) if correctly sized.

Most appliances that get a dedicated circuit call for a breaker considerably larger than the actual running current. Starting currents may be much higher, but are rarely if ever applied to all circuits at the same time.

On the other hand, if you had 10 20-ampere electric resistance heating circuits (drawing 16 amps per derating) that would be all you could put on one 200 amp main (as the entire load is continuous) other than some load that would not turn on at the same time (say a big honking air conditioner, if the control systems are sane and don't run heat and A/C at the same time).

• Lighting circuits are even less likely to be fully loaded if you've moved to CFL or LED bulbs, of course! (I have a fixture that was designed to draw about 300W, but is now drawing 75W at full brightness... and I find I often dim it down to half of that.) Oct 13 '14 at 0:56

The numbers on the breakers are of no significance at all. It's the calculated load that tells you what load can be put on a panel.