Excel input for 7671 calculation

If you run through the formula it gives the voltage drop in volts along one straight length of single core cable. Thus the coefficient b that they refer to needs to be multiplied by 2 for single phase. Now they say it only needs to be multiplied by 1 to get the three phase voltage drop. That would only be the voltage drop along the single core and the line voltage would be root 3 that value.

I can see that. I suppose it then depends on what you're doing with the result. Is it to see what the load actually experiences or is it just to verify some limit? If the latter, if the limit is defined in terms of 230V - e.g. 5% or 230V or 11.5V for example, then 1x would still give the same answer for balanced three phase as √3 times the voltage drop compared with 5% of 400V say (effectively both sides are multiplied by √3, so you can validly divide through by √3 and get the same result). Sometimes it's just simpler to think of 3-phase as just three single-phase loads (and neglect the N current).

On the other hand I can see a possible flaw in that they seem to assume that a 3-phase circuit is completely balanced - which perhaps isn't always the case in reality (probably not in the majority of cases even). 3-phase distribution circuits are almost certain to feed some single phase loads. There may be some swings and roundabouts in terms of maximum current can sometimes only occur when at least some of the 3-phase loads are running, in which cases N currents may be relatively small, but in other cases having 100% load on just one phase might be entirely plausible.

   - Andy.

If you run through the formula it gives the voltage drop in volts along one straight length of single core cable. Thus the coefficient b that they refer to needs to be multiplied by 2 for single phase. Now they say it only needs to be multiplied by 1 to get the three phase voltage drop. That would only be the voltage drop along the single core and the line voltage would be root 3 that value.

I can see that. I suppose it then depends on what you're doing with the result. Is it to see what the load actually experiences or is it just to verify some limit? If the latter, if the limit is defined in terms of 230V - e.g. 5% or 230V or 11.5V for example, then 1x would still give the same answer for balanced three phase as √3 times the voltage drop compared with 5% of 400V say (effectively both sides are multiplied by √3, so you can validly divide through by √3 and get the same result). Sometimes it's just simpler to think of 3-phase as just three single-phase loads (and neglect the N current).

On the other hand I can see a possible flaw in that they seem to assume that a 3-phase circuit is completely balanced - which perhaps isn't always the case in reality (probably not in the majority of cases even). 3-phase distribution circuits are almost certain to feed some single phase loads. There may be some swings and roundabouts in terms of maximum current can sometimes only occur when at least some of the 3-phase loads are running, in which cases N currents may be relatively small, but in other cases having 100% load on just one phase might be entirely plausible.

   - Andy.