Cable calc confirmation required!

[IAmSparkytus!]

Think I may have been oversizing cables for some three phase circuits...

For example, when ive calculated a 11.5V volt drop on a general circuit I've took this as being the 5% allowed but I've just come to realise that maybes this is actually a 2.875% volt drop at 400V

Can someone confirm please? Could save me money moving forward!

 

[pc1966]

The volt drop calculations are sometimes impractical. Working on industrial sites the final circuit might be connected to a sub-sub-sub-submain 400m away. The design data is normally lost or a water damaged scroll that only one operative knows of and the addition and alterations somewhere else or not recorded. I doubt theres any % left on a lot of these sites.

Very true.

If you measure the end of circuit PSCC and divide by 20 then you get the current for which it meets 5% VD. E.g. Measure 350A PSCC then 5% drop overall met for 17.5A.

Why? Well the PSCC is what flows at 100% voltage drop!

You don't have to meet overall 5% drop in the regs as such, as the DNO side is supposed to size the supply and voltage so under load you are within the limits, and on high current circuits it may not be practical at all as the DNO side drop could well exceed 5%, but the above is a quirk sanity check of the system when other aspects are hidden, undocumented, or incorrectly documented.

 

[IAmSparkytus!]

Just recalculated a recently installed cable and looks like I could have installed a 4mm 4core instead of a 10mm 4core to supply a 25amp TP fixed equipment!

4mm 4core
70m x 25A x 9.5mVAm = 16.625V
(16.625 ÷ 400) x 100 = 4.15%

but because I've calculated it at 230V I've ended up at the below...

10mm 4core
70m x 25A x 4.4mVAm = 7.7V
(7.7 ÷ 230) x 230 = 3.3%

Idiot.