Full Load Current

here is a calculator.

http://www.sengpielaudio.com/calculator-ohm.htm

it's a piece of cake, just focus on:

I=P/ V.cosө = 400/420=0.95 A , we here assumed that the load is purely resistive.

thanks..........

CORRECTION

I admit I was wrong yesterday when I worked out the consumed amperage (as the OP'd been asking) on the basis of single phase system, and this is irrational to find a single phase voltage of 420V(at least I didn't hear of)...so the equation will be:

I=P/ √3.V.cosө = 400/√3.420= 0.55A

As I think the power of 400w mostly represents 4 lamps of 100w connected in series.

Here, I hope I'd get benefited from my mistake which is to answer without applying proper thinking to the given requirements, to make light of the questions to be answered or worked out……

Thanks for all ….