Incomer sizing calculation for a 3phase 4 wire 415V 50Hz switchboard

The diversity of the board is what, please?

If all those items could be running at the same time, you need to add up the total FLA (full-load amps) and multiply by 1.15 (for combinations of general loads). There are some specific situations that may call for a different multlplier; for instance, if they were all motor loads, you would add up all the FLAs, plus 0.25 times the largest motor FLA. This gives the feeder ampacity; the main breaker would be ΣFLA + 1.25 (largest motor FLA), if I remember correctly. See local codes. [Caveat: it has been a while since I designed UL-508 panels, so these numbers may be a bit rusty. But they should convey the idea.]

If you can be sure that not everything will be running at once, then you can research the aforementioned diversity factors, which may allow for a smaller feeder and main breaker.

In addition to all the points mentioned you will need to the fault level at the bus bar.

Dear Guest,

As per AC fundamentals, The total current should be calculated in vector form, so you should know the power factor of each load approximately. once the total current is calculated, check what transformer capacity you need to use, considering additional margin(in KVA) for spare feeders in future. in case you have motor load which take heavy current during starting, the transformer must be able to withstand those heavy currents.

if you could provide additional details about your loads and regarding their continuity of usage, the members in this forum will be able to help you in a better way.

Thanks for the reply. Actually this switchboard will be fed from another one switchgear. Also there is no motor feeders in this switchboard. All are lighting and welding supply feeders. Could you please confirm that 400A incomer rating will be sufficient?

Absolutely NOT, on the information given. Even if we assume your loads are only 0.80 times the CB ratings listed, they still add up to WAY OVER 400 A. Only if you can assure that half or so of them are idle at any given time....