Sounds like a fair estimate as you will be switching supply current directly across very low resistance static coiled copper conductors

My woodworking machines specify 6 x the on load current, but of course they are off load when started.

If you want to get more technical, flick a meter on ohms across the windings and measure the resistance. This sets the highest the current can ever be, however, there is also some inductance, which is largely dominated by the air gap in the magnetic path, that takes the edge off that current peak a bit Then there is the back EMF that rises generator-like as the motor picks  up speed so the current that flows is the voltage difference between supply and generated divided by the winding impedance. The ramp up time can be estimated by listening to a start up, and is however long it takes the motor note to stabilize - it is a strong function of load - hence load lifting (line unloading) valves on large compressors to get the thing started before the back pressure and the torque rises to the final value.

The figures you quote will be about right for a motor of a few HP starting under constant load, but be aware such things can be very variable indeed. 

Mike.

If the motor was installed this would be an option but we are at design stage and of course nobody has any details of what motor will be used. We are going to have 4 motors installed on one switchboard with staggered start so only one more is starting at a time but I want to be sure the upstream MCCB won't trip. In all likelihood it will be fine as we will specify the MCCB with adjustable overload, short circuit and instantaneous trip. My main concern was the overload trip time being too short so it sees the starting current.

Well if you do not even know the motor type at this stage, that changes things rather. Actually it makes a close fitting design all but impossible!

In your place, as well as agitating for some design authority decisions to allow better data to design from, I'd want to have provision for a board for which C and D type breakers are available, maybe even W or Z, and avoiding to share with things that will care if the volts droop a bit - so perhaps put any lights or computer controls or IT kit on a separate sub-main if possible, certainly if the feed to the motors is any length. You have not yet indicated the motor power, but I presume it is not more than something like single figure KVA, or the additional expense of a variable speed drive, as well as reducing the inrush problem, becomes worth considering because it will allow fine tuning of the power consumption once in operation.

Whatever you do, any fusing or breaker provision will have to be a pretty loose fit, set really to be good for clearing dead shorts in the supply cables, while  the protection for the motor (overload) will need to be handled in the control gear for motor itself. In turn depending on supply impedance you may need to use an earth leakage relay or large delay RCD or similar if the Zs is not low enough to clear the relatively slow blowing breakers or fuses promptly against an earth fault.

Please accept my apologies if this is either not relevant or stating stuff that you already know - I do not really know your situation.

Mike.