Three phase voltage drop

The supply transformer will normally be set at 433V at the terminals to counteract this very problem.

The volt drop won't be 20+20 = 40 volts because the voltages aren't in phase......

Are we talking a STAR balanced load or a DELTA balance load?

If this is a standard 3 phase 4 wire system, and is TRULY balanced, without third harmonics or other complications,

Then I believe that a load connected between phases will see 360 volts, this being the nominal 400 volts less 20 volts in each phase conductor.

A load connected between one phase and neutral will see 210 volts, this being the the nominal 230 volts, less 20 volts drop in the phase conductor. No voltage drop in the neutral conductor due to no neutral current in a perfectly balanced system.

In practice there will be some imbalance and therefore some voltage drop in the neutral.

Also as has been said, standard DNO substation transformers are 433 volts rather than 400. A privately owned transformer might be either 433 volts or 415 volts.

Err what  ???

20v in each phase conductor certainly turns 230V to 210V, but the phase to phase voltage is only affected pro-rata -  so the 400V also  loses 20/230ths. 

i.e that  is 8% voltage droop

So more like 33V lost, so the 400V becomes  367v to 370V within rounding errors.

This probably means your cable is undersized or you should look at a higher distribution voltage if the load is constant.

The transformer is 200 kVA. I have been told these transformers come with tappings, as you say to counteract this very problem.

I was also told the tappings are usually set at 10% i.e. 440v.

The tappings are usually on the primary side (HV) so it probably won't be the 10% tap that you are thinking of (and it will a -ve tap - less windings on the primary gives a higher voltage on the secondary). 

The site is a large waste water works and needs both 400v and 230v at various points around the site, so it is STAR.

I am a bit confused, I would agree that each phase voltage will drop by 20v i.e. down to 210v.

Not sure about how the line voltages are affected if at all i.e. if each line voltage drops by 20v, then the difference between them

should still bt 400v ?

Using Vl = sqrt(3)* Vp, I also get Vl = 367v.

What happens when the load is imbalanced i.e. first phase loses 20v and second 10v ?

ok thank you