heater mcb

Your are correct a 125A CB will suffice:

Assuming unity power factor, the heater will draw 10 Amps

I heater = 72*1000/(sqrt(3) * 415) = 100A

The CB should be rated to 125% of the continuous load:

100A *1.25 = 125A

All of the above is perfectly correct in theory....however....

Welcome to the Outer Limits!

You mention both MCB (miniature circuit breaker) and MCCB (moulded case CB). 125amps is the absolute upper limit for MCBs. This load sounds suspiciously like a water heater, and these can offer big inrush currents (+50% is typical) when the water is heated fom cold. The MCB will almost certainly be OK, particularly if you use a Type D MCB (motor rated) BUT leaves you with nowhere to go if problems do start to occur. (Note that water heaters can have lower power factor also, and voltages can vary by +/- 10%).

If finances permit, I would prefer to use a 125amp MCCB with an adjustable trip unit. This can be set higher if the water heater starts to misbehave (assuming you also give yourself some leeway with the cable rating).

If possible, always try to design yourself a way out.

Best of luck.

The MCB rating also should be in accordance with the Local ELectricity Authority of that particular region . Different countries have different electricity rules and regulations .

For eg in UAE we follow a thumb rule

1 MCB Rating = TCL * 2 ( for normal loads )

2 For Chillers ,Motors and power facotrs we use MCB rating = TCL * 2.5

In this case 160 A MCCB is recommended.

thanks for the help..........can anybody suggest me any book or site regarding selection of switchgears and design of panels.

Heater elements are considered resistive loads with unity power factor, the actual load current could be calculated from the following equation for a three-phase active power:

KW=SQRT.3 * V * I

"I" therefore equals to KW/(SQRT.3 * V)

72KW at 415V translates to 100Amps or thereabouts.

A 125A MCCB will suffice. I doubt if there is any Mcb rated above 100A.

MCB are there fro 1 A - 125 A , but in this case it is secure to use 125 A MCCB