Calculating voltage drop for a 3ph installation or submain.
The tables in 3008 are for 3ph, and there's a conversion for single phase.
No problem with all of that.
But, the 3ph calculations are for a balanced load. There's further calculations in section 4.6 about calculating for a un-balanced load. But the wording in the clause doesn't make sense.
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A conservative solution to the voltage drop assessment in these situations would be to
assume balanced three-phase load conditions and perform calculations using the current
flowing in the heaviest-loaded phase. In many cases this will still be necessary if the
out-of-balance conditions are inconsistent or intermittent.
However, where the currents in each phase can be shown to be of different magnitudes for
consistent periods, voltage drop calculations can be performed on a single-phase basis by
geometrically summing the voltage drop in the heaviest loaded phase and the voltage drop
in the neutral, as follows:...
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A "conservative solution" would be to calculate as single phase, which would assume the highest phase load, plus the load on the neutral.
I think the wording should say " to assume UNbalanced three-phase load conditions.."
Presumably, the calculation for the heaviest load (full calculated or limitation load) as un-balanced is exactly the same as using the single phase VD calculation?
Or is it the 3ph calculation, but using 230V instead of 400V to work out the %VD?
Should installations like houses or small commercial without any 3 phase loads, be calculated as completely "unbalanced" because that is a possibility?
Calculating 3ph voltage drop.
Re: Calculating 3ph voltage drop.
I believe "assume balanced", as published, is the correct the wording.
By taking the current of the heaviest-load phase, and doing the (3-phase) volt-drop calculation on the basis that the other 2 phases also have that same loading (ie balanced); we are being conservative.
It's not a worst-case scenario, but it's a reasonable compromise that is easy to apply and will keep us on the safe side.
The other option involves calculating the out-of balance (i.e. neutral) current; and then calculating volt drop on N and heaviest-loaded active separately using single-phase calculations; then adding the results together.
Still conservative, but more work; and we seldom have adequate date on which to make these calculations. The underlying basis here is predictable and consistent out-of-balance loadings - but in most cases out-of-balance loadings vary in ways that can't be adequately predicted.
By taking the current of the heaviest-load phase, and doing the (3-phase) volt-drop calculation on the basis that the other 2 phases also have that same loading (ie balanced); we are being conservative.
It's not a worst-case scenario, but it's a reasonable compromise that is easy to apply and will keep us on the safe side.
The other option involves calculating the out-of balance (i.e. neutral) current; and then calculating volt drop on N and heaviest-loaded active separately using single-phase calculations; then adding the results together.
Still conservative, but more work; and we seldom have adequate date on which to make these calculations. The underlying basis here is predictable and consistent out-of-balance loadings - but in most cases out-of-balance loadings vary in ways that can't be adequately predicted.