Balancing Loads Between 3 Phases

[Atticus]

I have a 415V, 3ph supply feeding 10 seperate accommodation cabins via 2 ditribution boards (5 cabins per board) and 10 x 25kVA transformers.

Each cabin has more or less the same design and hence the same installation guidelines. Therefore the water heaters are all on the blue phase and the 2 x 1ph air conditioning units per cabin are on the red and yellow phases respectively - those are the biggest loads

The current distribution per cabin is approx: L1 = 22.76A, L2 = 29.86A, L3 = 24.4A.

So there is a slight imbalance, BUT, by the time I multiply that by 10 the total phase loads are: L1 = 228A, L2 = 299A, L3 = 244A.

Is this acceptable?

Any advice would be gratefully appreciated.

 

[james 29]

What are the transformers for you havent made it clear?

 

[Steve]

sorry, whats this about blue red and yellow phases?

 

[JohnD]

you must be young

 

[PompeySparks]

Only certain types of load (such as motors) give a perfectly balanced three-phase loading. With seperate loads on seperate phases you could never hope to balance the phases and this imbalance is not a problem. The closer to balanced they are, the less current returns via the neutral (which is why most three phase motors do not need a neutral!) - just make sure you have a neutral sized the same as each of the phases!

 

[jondiy]

Although you multiply 10 and assume that it will be 10 times the current flowing in total , you have to bear in mind that each cabin will use different amounts of current depending on who is in there, some guy may use 20 amps, but another may use 40 amps and mr economical may only use 10 amps , so your phase loads are going to be unbalanced , whatever you try and do. All I know is, you must have a pretty hefty incomer to power all that. What rating are the main tails and fuses btw??

Did some work a while back on a 3 phase system (100A fuses) that had 2 phases running at 30 amps on average, and the other phase was drawing 90 amps!!! Reason for checking this was that one of the phases was causing an rcd to trip when the neighbouring premises closed down for the day (powered down). This had been happening every day for about 6 months and the board that was tripping was electrically sound.

On inspection I found it to be the phase that was drawing the 90 amps.
The loads were shifted around on the distribution board , and I managed to achieve averages of 50 amps per phase. Since doing this, that rcd had never , as far as I am aware , tripped.

So I wouldnt be worried with your installation.
Incidentally, have you tried a clamp meter on the meter tails , to see what totals you have??

Obviously it depends on the accomodation level , but try and do it when the cabins are full , and not at night time when they are sleeping.

 

[Atticus]

Oops! Begging your pardon. Brown, black and grey that should read.

Thanks for the advice everyone.

The transformers will give the desired voltage levels at each cabin i.e input can be, in this case: 380, 415, 440, or 480 Vac and the output will be 415Vac, 240Vac and 110Vac if required.

Ah, so I didn't need to get so worked up about phase imbalance after all. Of course, I should have considered the fact that there was never going to be equal usage and hence it was always going to be impossible to get near perfect balance. It's all been done before but I'm new to this and had a bit of a minor panic attack.

The supply cable and circuit protection is to be taken care of by the client. I suspect there will be more than one, as it is going to be a substantial load for one supply.