Calculating 3ph voltage drop.
Posted: Sun Jun 26, 2022 5:41 pm
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:...
-----------
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?
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.
-----------
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:...
-----------
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?