Calculation of Connected Load

Yes, it is. If everything in the circuit is running at maximum load.

Single- and Three-Phase Power Measurements ( )t ( ) ( ) ( ) ( ) ( ) ( )

one wattmeter method of power measurement

single phase power measurement

Yes from the main incomers , amps * volts = watts...

"...amps * volts = watts...", only true for resistive loads (and DC circuits), otherwise power factor comes into play.

He only asked if it was possible, not how to do it... but it's true the meter type, waveform and load characteristics may or maynot come into play in the accuracy of the measurement....

May be question not clear.

It is like this one MDB feeding both single phase load and Three phase load by measuring the incoming current using clamp meter, is it possible to find out how much single-phase load and three phase load connected to MDB .

It's not getting any clearer....

No, you cannot determine how much current goes to the single/three phase circuits by simply measuring the three phase incomer current. The only way to do this is to measure the currents passing through each individual breaker pole, and this method will only work if there are no (hidden) single phase loads attached across any two phases of a three phase circuit once it leaves the MDB (Main Distribution Board).

If you suspect that there are single phase loads on the three phase circuits then you will have to disconnect/switch them off and take before and after readings on each phase.

Yes.

A pocket calculator might come in handy.

No. you can not measure single phase load from the SMDB. you can measure the Load in each phase. Switch off 3 phase loads, and then measure each phase so you will get single phase load

Yes, by adding all the single phase load togeather