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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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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?
 
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.
 
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??
 
Last edited:
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.
Hi I have a customer on an old farm supply. Many other electricians have assumed it was just 2 of 3 phases supply but it’s not. I learned about farm supplies from my tutor at technology college 50+ years ago who worked for the local supply company.
They used the 2 phases for farm machinery which required a high load so used a 480 v supply but then had a normal 240v supply for the house. The 2 phase supply is just 2 x 240v supples working in opposition to each other. i.e. 180 deg out of phase.
my customer wanted to run some new 3 ph plant but the cost of installing a new 3 phase supply was extortionate so I installed an inverter for him and put in a standard 3 phase dist board. Work a treat and now have a customer for life ! !
 
2 phase supply is just 2 x 240v supples working in opposition to each other. i.e. 180 deg out of phase.

We should really call it 'split-phase' not 2-phase, because it's just a single-phase 480V supply with a grounded centre-tap as the neutral. There was such a thing as 2-phase, with the phases 90° apart, used from the very early days of AC. Like 3-phase it has an inherent sense of rotation that makes induction motors self-starting, and an absence of power zeroes hence lower torque ripple, but 3-phase offers yet more versatility and economy and became the polyphase system of choice. Split-phase does not offer these advantages, only the ability to deliver 240V and 480V from a 3-wire supply with less copper than would be needed with 2-wire circuits.
 
We should really call it 'split-phase' not 2-phase, because it's just a single-phase 480V supply with a grounded centre-tap as the neutral. There was such a thing as 2-phase, with the phases 90° apart, used from the very early days of AC. Like 3-phase it has an inherent sense of rotation that makes induction motors self-starting, and an absence of power zeroes hence lower torque ripple, but 3-phase offers yet more versatility and economy and became the polyphase system of choice. Split-phase does not offer these advantages, only the ability to deliver 240V and 480V from a 3-wire supply with less copper than would be needed with 2-wire circuits.
One problem I got an emergency call out on a strong windy night where lights were going up and down and dist board smoking. On investigation I found we had lost the neutral supply and 480v was being applied to the household single phase circuits. Had to pull the main cut out fuses and inform the electrical supply company. Next day they found a branch from a tree Had fallen across the overhead neutral cable from the pole mounted transformer and severed it The supply company paid for a electrical contracting company to replace the house dist board and buy all the appliances which had blown up! They didnt‘t like 480v up them.
 
I don't know which is more damaging, 400V from a broken neutral on a 3-phase system with highly unbalanced loads or 480V under the same conditions on split-phase. 480V might be more spectacular in some cases, but the higher the voltage, the more likely it is to blow equipment fuses etc. A longer, lower overvoltage might cook transformers that would otherwise just pop their fuses.

I have seen (and been obliged to repair) lots of studio and electronic kit that has seen 400V due to 'user error' normally on temporary supplies via single-pole connectors such as Cam-Lok or BAC. Either neutral loss or neutral crossed with a phase, which at least assures that things will actually get smoked rather than just go wibble.

I am not blameless in this regard, having once plugged up a set of 400A BACs in semi-darkness and got the blue and black in the wrong holes. I put the main breaker in with the intention of checking the voltages as one always does, but overlooked a 10kW tungsten studio light connected to red phase that was already switched on. It lasted one second, then took 5 minutes to sweep up.
 

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