3 phase supply to single phases only 2 phases used

[catdog1121]

I went to some holiday cottages today, the main supply in was 3 phase however only 2 of the phases were used. The L1, L2 and Neutral cables were wired into a three phase meter, then coming out was L1 and Neutral, L2 and Neutral to create two single phase supplies which were divided off using 60amp isolator fuses to each of the cottages with the load roughly balanced over the two phases.

My understanding of 3 phase has only come from large datacenters where although the equipment mainly just used single phase it was always important to try to balance the load over the 3 phases as you got charged based on the peak phase.

Given L3 is not being used does the three phase meter just charge over the 2 phases used or would it be better to have the third phase connected and the load balanced over the three phases.

 

[PEG]

Hi,are there any plans to add a third cottage?

 

[catdog1121]

Nope there are about 7 cottages in total plus owners cottage, seemed to be basically 4 cottages on each of the two phases and has been like this for many years.. Each cottage has a 60amp fused isolator in the supply cupboard split using henley blocks from the meter, this then goes to each cottage which each have a consumer unit. No real issue there but was just surprised that given it had a 3 phase meter it only used 2 of the phases, I guess I was expecting all 3 phases to be wired into meter, then from meter into a box to convert to 3 single phases then fed to each cottage, was surprised to see two neutrals coming from the meter.

My main concern was if they were being charged peak phase then obviously having one phase doing nothing means the peak on the other phases would be higher so the bills would be higher. I could be totally wrong.

No plans for any more cottages.

 

[Toneyz]

I would have thought that it would have been better to have two cottages on each phase and the other (7th) on one of the phases to balance out more.

 

[Charlie_]

Perhaps the supply cable is only an old 3 core DC cable and so only 2 phases are available

 

[catdog1121]

I would have thought that it would have been better to have two cottages on each phase and the other (7th) on one of the phases to balance out more.

There are 7 plus owners cottage so 8. But yes I would have thought owners cottage plus largest cottage on one phase and then the other 6 cottages split over the other two phases. There seems to be enough power from the 200amps available for the cottages as no issues ever, all gas heating and no electric showers.

I think it will all be down to how the meter reads the power, if it reads it from power used then does not really make any difference, if it reads it peak power from the 2 phases then better to only have 2 phases as more likely to be a higher difference with 3 phases but if it reads peak power from 3 phases but only using 2 then it would be better to utilise the unused phase.

 

[catdog1121]

Perhaps the supply cable is only an old 3 core DC cable and so only 2 phases are available

Cottages are only about 15-20yrs old, was the old stables of a farm house, there are three 100amp fuses in the cut outs, (obviously dont know if the fuse is actually in L3) But would suspect all 3 phases would be present.

 

[dmxtothemax]

it really doesn't matter a great deal the 3 phase load is not balanced,
it only matters a little if running delta whuch you are not
you are running star configuration.
so if the cables used are correct for the load
then I would not worry at all.
urban myth !
as they say it's a technical nicety not nessicitty.

 

[catdog1121]

it really doesn't matter a great deal the 3 phase load is not balanced,
it only matters a little if running delta whuch you are not
you are running star configuration.
so if the cables used are correct for the load
then I would not worry at all.
urban myth !
as they say it's a technical nicety not nessicitty.

Thanks, I agree with what you say it was more I was not sure how the meter calculated usage, all other 3 phases I have known get metered on the peak phase rather than actual usage. Thats they only thing that would make a difference.

 

[remedial]

Given the nature of the business wouldn't it be almost impossible to balance the laods anyway? e.g. Not all cottages would be occupied at the same time.

 

[R-fur]

Did you check if it was three phases coming in? Some farms were fed from a split phase transformer. Look and see if the HV line is two wire or three wire. Check the voltage between L1 and L2 around 500v indicates split phase.

 

[Deleted member 26818]

Is it the same L1 and L2 used for each pair of cottages, or is it L2 and L3 for the next pair and L1 and L3 for the next, and so on?

 

[catdog1121]

Did you check if it was three phases coming in? Some farms were fed from a split phase transformer. Look and see if the HV line is two wire or three wire. Check the voltage between L1 and L2 around 500v indicates split phase.

It has 415V warning labels over the cut out... my suspicion is that L1 and L2 are used for the cottages and then next to the cottages is the old farm house that maybe uses L3. I would guess then the meter would be based on Kwh rather than peak phase. The three phase head was completely sealed and I didnt want to take it apart and break the seals.

 

[catdog1121]

Is it the same L1 and L2 used for each pair of cottages, or is it L2 and L3 for the next pair and L1 and L3 for the next, and so on?

There are 8 cottages, 4 on L1 and 4 on L2.. L3 not used.

 

[Lucien Nunes]

all other 3 phases I have known get metered on the peak phase rather than actual usage

Please correct me if I am wrong but I suspect this to be an oft-repeated myth that has become accepted as fact by many electricians. Ordinary 3-phase meters are just energy meters that register the total energy consumption across all the circuit(s) / phases(s) they are supplying, which is then charged at the prevailing price per kWh at the time it was used according to the tariff. I do not believe that the supplier is permitted to charge for (nor that the meter is permitted to register) energy that was not used.

On large supplies (usually more than your standard 100A 3-phase LV service) the supplier is entitled to vary the tariff according to the nature of the load. So, on a peak demand tariff, if you exceed a certain load for more than a moment, the rate for the billing period changes to a higher price per kWh, on the basis that if your load is peaky it costs them more to generate and transmit. But you still get charged only for the kWh you actually used.

For optimum efficiency of the supply network it is certainly desirable to balance load just as it is desirable to achieve unity power factor and reduce peaks. A few percent of extra unnecessary loss can eat away at the profit margin for the generator and DNO, so their tariffs for large users favour loading patterns that are easiest for them to supply, much like economy 7 for the domestic user. I think it is either this, or badly remembered fragments of theory regarding the 'two wattmeter method' of measuring 3-phase supplies, that have given rise to the belief that a conventional 3-phase meter does something other than just measure total kWh.

True / false??