The basic formula is: 3-phase VD=(1.732×K×I×D)÷CM. Here is a website that explains various ways to determine line load and volatage drop, including the formula method and some examples:

Reply to MsWest's Post: I RECIEVED YOURE RESPONSE.
THANK YOU
I DONT KNOW IF I'AM ASKING THE QUESTION CORRECTLY TO YOU.
YOU HAVE SUPPLIED ME WITH THE VOLTAGE DROP FOURMULA.
WHAT I WAS LOOKIN FOR FOR TO SUBMITT ON MY PLANS FOR BLD PLAN CHECK WAS AS FELLOEWS.
I HAVE A GIVE TOTAL GIVEN WATTAGE
I JAVE A GIVEN VOLTAGE

IF I DEVIDE THE WATTS BY THE VOLTS
AS IN A SINGLE PHASE PANEL
IS THE SAME FORMULA USE IN THREE PHASE AS WELL
I THOUGHT IT HAD A MULTIPLER IN THE FORMULAS AS WELL
DAVE

Okay, good. Now I understand what you're looking for.

The formula for 3-phase is the same as what you mentioned for single phase, except with 3-phase you multiply by 1.73. It's the 1.73 multiplier that you're looking for.

For 3-phase, the formulas look like this:

For amps: I = P / E X 1.73 (watts divided by volts times 1.73)

For watts: P = E X I X 1.73 (volts times amps times 1.73)

P=watts

E=volts

I=amps

Once again, yes, the formula is the same as for single phase, except for the 1.73 part of the formulas.

I hope this is what you need. Please let me know if it isn't.

Thanks!

EDITED TO ADD: Sorry, I posted the formula incorrectly a few minutes ago, but I have corrected it. Thanks again!