Calculating total demand and diversity in single phase+three phase circuits.

[Powermanagement]

Good evening all,
I just joined the community and I really hope I can contribute as much as I could be helped. Really.
I'm completing my 2365 lvl3 course however I just got my 305 B assessment paper back. Apparently I didn't calculate well the max. Demand and diversity of a given circuit having single and three phase final circuits.
For the single phase part I don't seem to have any problem understanding and calculating Max. Demand and diversity.
However I seem to have messed up the whole three phase calcs.
For instance, for a 20A tp-n lift supply I multiply (Sq. Root of 3 × 20A × 400v) ÷1000 ..giving me 13.8 A or something. Apparently this is quite ridiculous and I really genuinely don't know how/understand how to do it correctly.
I only use that because I saw it somewhere in a forum..and I just applied it to my assessment.
My question is.. Please guys, how do I calculate max. Demand and diversity for the 3 phase circuits and more importantly, how do I add up the whole single phase lot to it?. Do I just simply sum it to one of the phases?
Many thanks indeed for your time. Any help will be very much appreciated.
David

 

[Spoon]

Hi mate and welcome to the forum. The formula you have used will give you kVA. If it's Watts you are after then you will need to know the PF (Power factor) and use Sq. Root of 3 x PF x 20A x 400V

 

[telectrix]

your lift is 20A per phase, so you add the single phase loads to each phase that they are on. ( a perfectly balanced 3 phase will be equal ). then to get the KVA, work it out for each phase then add the 3 up.