Volt drop question

luke12345 · Jul 29, 2015

Hi I've got a question regarding volt drop.

When calculating volt drop to a 3 phase distribution board do I use 230v (11.5v) or 400v (20v)

The board I'm attempting to put a cable to is 75 meters away.

Design current 100A
From TABLE 4D4A Cable selected = 25 mm²
For 25 mm²: mV/A/m 1.75 for 230v
Voltdrop = (mV/A/m x Length x Design current) / 1000Voltdrop = ( 1.75 x 75 x 100 ) / 1000
Voltdrop = 13.13 Volts

Which is too high so:
From TABLE 4D4A Cable selected = 35 mm²
For 35 mm²: mV/A/m 1.25 for 230v
Voltdrop = (mV/A/m x Length x Design current) / 1000Voltdrop = ( 1.25 x 75 x 100 ) / 1000
Voltdrop = 9.38 Volts

Which is fine for volt drop.

But if I use the value for 400v in TABLE 4D4A Cable selected = 25 mm²
For 25 mm²: mV/A/m 1.75 for 400v
Voltdrop = (mV/A/m x Length x Design current) / 1000Voltdrop = ( 1.5 x 75 x 100 ) / 1000
Voltdrop = 11.25 Volts

which is well below the 20v limit for 400v.

Now I know it wont break the bank putting a 35mm in. I just wan to know how to do it correctly.
Hopefully I'm getting my calculation correct and my question makes sense?

davesparks · Jul 29, 2015

You need to ensure that the VD on the final circuits fed by that DB will comply, so if you have any 230V final circuits you'll need to work with 230V for definate.

Dropping 9.38 V on the submain leaves you with precious little VD allowable on the final circuits!

luke12345 · Jul 29, 2015

Thats a fair point.

So if I went for a 50mm:
mV/A/m 0.93 for 230v
Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 0.93 x 75 x 100 ) / 1000
Voltdrop = 6.98 Volts

Which gives some breathing space.

So is this correct to use the 230V values?
Are the 400v values only used on 400v systems such as motors/pumps?

GEOFF S · Jul 29, 2015

luke12345 said:
Thats a fair point.

So if I went for a 50mm:
mV/A/m 0.93 for 230v
Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 0.93 x 75 x 100 ) / 1000
Voltdrop = 6.98 Volts

Which gives some breathing space.

So is this correct to use the 230V values?
Are the 400v values only used on 400v systems such as motors/pumps?

Hi

Use 230 Volt, but use three phase impedance values.

Cheers

D Skelton · Jul 30, 2015

Remember that the 5% and 3% max volt drop values are advisory only. You really need to be looking at the voltage tolerance levels of the equipment you will be installing at the sub-main end. Without these values, you can only so much as guess, and in the case of guessing, it's far better to work out what voltage you are prepared to drop on the submain in order to ensure that you're not dropping more than the advised amount by the end of each final circuit.

If following the advice given in appendix 4, you only have 6.9V to spare for lighting circuits from the point of supply to the end of the final circuit. I'd suggest that if there are to be lighting circuits supplied by the final circuits at the end of this submain you don't want to drop more than say 3V, maybe 4V on the submain itself.

Transpose the equation so you have (3V x 1000)/(Length x Design Current) and that will give you the mV/A/m value of the cable size you should install.