3 Phase Volt Drop

[councillor]

When working out a Volt Drop for a 3 phase system what it the max allowed. 4% as in single phase?

Say if you have a VD of 8v this is ok for 230 single phase (just). But what about a 3 phase system i.e. is the % worked out as a % of 230 or as a % of 400v

Pretty sure its a % of 230V as your working out the VD per phase

My name is Stu Dent thanks for any help

 

[Adam_151]

if there is 230v between conductors then this allowed to drop by 4% of 230v = 9.2v,

If there is 400v between, then its allowed to drop by 4% of 400v = 16v

If its a submain or other unbalenced circuit, do it via phase voltage, work out the voltage drop between a phase and neutral by considering whats dropped on the phase and neutral, that'll give your worst case, in reality at least some of the neutral current should cancel and reduce the volatage dropped on the neutral.

If you have a motor though, work it from 400v, allow each conductor to drop 8v max, if its a balenced load but there is a neutral for a 230v cotactor, then just ignore the neutral for perposes of VD

Well thats what I'd do anyway

 

[councillor]

Thanks Adam in simple terms here goes:

If I have to power a 3 phase circuit 100m away from supply and the piece of equipment is rated at 13A I would be ok to run it in 4mm sq

(mV/A/m) x I x L/1000

from table 4D2B

9.5 (4mm) x 13 x 100 / 1000 = 12.35 V (this is under the 16V you stipulated)

does this sound correct

 

[bernardgreen]

When rating cable for use with motors you need to consider the voltage drop when the motor is taking its start up current. This current may be several times the running current. A high voltage drop may mean the motor takes longer to reach full speed and normal current leading to delayed action current overload devices tripping out.

 

[Adam_151]

Bernard is right, forgot to mention that

And I'd be worried about r1+r2 on a 100m circuit in 4mm²!

 

[Jaymack]

It's normal to allow 2% VD for feeder circuits, and 4% for lighting and small power - measured at the farthest point in the circuits.
Motors are usually designed for 3 or 4% in a running condition, the VD on starting can be 6/7 times this figure for D.O.L. starting, but much less than this for reduced starting voltage - star/delta or soft start etc. Each situation here requires assessment i.e. motor duty whether high starting torque is required for say fan duty.

For VD calculations here, convert the 3 phase loads to single phase and calculate the cable sizes as the standard method.

Jaymack