Right. I presume that the 3-phase ones assume a balanced load of 70A per phase (hence zero neutral current).
As I said at the very start of the thread, those are roughly the results I would expect IF the VD they are quoting for 3-phase is the drop in the phase-neutral voltage (for each phase). As you demonstrated in your manual calculation, if one uses the BGB's figure of 1.5 mV/A/m (for combined drop of L and N in a single-phase circuit), then the figures one gets are 15.75V and 7.875V (rather than 16.9V and 8.45V that your software is producing). It looks as if your software may, for some reason, be using ~1.6 mV/A/m, in which case manual calculations would give results almost the same as the software.
As has been discussed, it depends crucially on the balance of the loading. If you have perfectly balanced 3-phase (hence zero neutral current, even if there is a neutral), then the voltage drop (in terms of neutral-phase voltage at load end) for a current of xA per phase will be half of that for a current of xA in single phase. At the other extreme, if you put all xA down one phase of a 3-phase cable (so that xA also flows through neutral), the VD will be the same as for a single phase situation (indeed, it effectively is single-shase). Your design calculations therefore presumably have to be on the basis of the estimated worst possible degree of phase imbalance,given knowledge of what the circuit will be supplying. If it is only supplying 3-phase loads (machines, motors etc.), then I imagine that the load will never be significantly unbalanced, so you could presumably reasonably use the 'low' VD figure. However, if you have single-phase loads/circuits connected to individual phases, then you move closer to the situation of potential serious unbalance, hence the higher (double) VD figure.I have always sized three phase sub-mains as if they were single phase. I was always taught this, and can recall doing some calcs on site during an NIC inspection to determine volt drop too. Now not too sure what I will do when I need to calc another
The suggestion that the BGB allows a 20V VD for 3-phase, whilst true in one sense, is (probably, dependent on the intent of the BGB!) potentially misleading, since that relates to the between-phases voltage (which we often don't think in terms of, nor calculate in terms of). In a balanced situation, an 11.5V drop along each of the phase conductors represents a VD of 5% (11.5/230), hence at the limit of what the BGB allows. Another way of looking at that (balanced load) phase-neutral drop of 11.5V is to call it a ~19.9V (11.5 * √3) drop in phase-phase voltage - again ~5% (19.9/400) [the only reason those figures are not perfect is because 230V P-N actually corresponds to 398.37V P-P, not exactly the 400V we usually assume)] So, the 5% VD limit can be looked at as 11.5V P-N or 20V P-P
Does any of that help?
Kind Regards, John
