Panel Sizing Query

What voltage? (Or what amperage for the kw-designated items?)

3 phase 415 V

There's more to sizing than just looking at the total load per feeder.

Somewhere there may exist a design document that may detail criteria that does not show up on the SLD, things like allowance for future load growth, critical loads with tight voltage drop criteria, high and/or simultaneous motor starts with high torque/inertia mechanical loads, etc., etc. Or it could be something as simple as a company-wide philosophy such as "...always choose the next size up from what the calculations dictate..." or "...we've always done it that way..."

Welcome to the real world of engineering where not every answer makes sense or is found in the back of the book.

Thank you for the answer

But why for the same load ratings different incomer ratings.?

The loads on both panels are of same type and power rating .

The only difference is the no of spare feeders.

Can you please tell me what is the normal philosophy you will use when you include spares along with the maximum demand in calculating the incomer size as well as the busbar..?

Busbar sizing and design also has to take into account prospective short circuit current, which may mean a different busbar sizing or arrangement/support for two panels that are supplying the same load.

Are these two panels on two different sites or different sections of the site where short circuit currents could be quite different?

Thanks,

So if the panel is located near the source (ie the main distribution panel which is fed from the transformer) the short circuit current available will be more (approximately) because of the lesser cable length. And the panel which is located far from the source will have less. (I am neglecting the types of loads because in this case loads are same)

Please correct me if my understanding is wrong..

Generally yes.

Consider going back to real simple basics and imagining the cables as different sized resistors and the transformer as the source if this helps you picture what's happening in a real world system (which is obviously more complicated than this).

But in the SLD along with the busbar amperage they have mentioned the SC value as 50 kA for both the panels..

Well then perhaps the second panel has some additional expansion built it to its design, in that additional or larger sized circuit breakers could be added or replace the existing ones later. There may not be enough information on the SLD alone to ever know for sure.

It could also be a mistake (consultants aren't perfect).

Since you don't bother to inform us of your location, there's no way to know what codes and regulations are in play.

Ask the consultant to enlighten you.

Or consult the code books.

For busbar sizing you should consider SC level and tripping time of CB too.

While sizing the panel, especially for motor loads following two things are important:-

(a) starting sequence of all these motors

(b) number of restarts per hour

In the second case (where 400 A Incoming feeder is provided), we can only assume that the very few motors start simultaneously.

The exact answer can be arrived by knowing the correct starting sequence.

Loads involving motor have high diversity. Diversity factor can be taken as 1.4

Wherever two motors of equal ratings are provided; one is usually stand-by.

Hence check for these factors to arrive at the answer.

I am 100% sure that you are from India.

Prashanth

I forgot to add one more point :-

Sizing of incoming breaker for a panel (supplying motor loads) also depends on co-ordination with upstream breaker.

if the selected breaker size is too less (improper co-ordination) & in case of fault, upstream breaker may trip.

Hence this may be one of the reasons for selecting (630A feeder) in your case.

Prashanth