Converting single phase home to 3 phases

Discussion in 'Electricians' Talk' started by rogerk101, Nov 24, 2020.

  1. rogerk101

    rogerk101 Screwfix Select

    Apologies in advance for such a long write-up, but context is required for any replies to be worthwhile ...
    I have had a house in France for the last 30 years, which has been heated with the same oil boiler since I renovated the house 30 years ago. The boiler has finally packed up, and I can't get the spares needed to repair it.
    France will ban the installation of new oil boilers in 2021, so I'm looking into an air-to-water heat pump. The house is already heated with underfloor heating, which means that it should be a simple change of oil boiler to heat pump as the source of heat.
    Apparently the electricity supply in single phase is insufficient for the heat pump's requirements, so the heating engineer has suggested taking out a 3 phase contract. The electricity supplier will bring 3 phases to their meter box for me for free, and I am responsible for the rest on my side of their meter.
    The heating engineer got a quote for me for the conversion on my side of the meter and it comes to 2000 euros. I am shocked at the price, as I thought all that would be needed is to leave the house itself on it's current single phase and just have the motor of the heat pump on 3-phase.
    Am I wrong in thinking that I should need little more than a 3 phase RCD and a 3 phase MCB of the correct current ratings? MCBs in France switch both line and neutral so there should be no need for an additional 3 phase isolation switch between the 3 phase MCB and the 3 phase motor.
    I don't know if 3 phase RCBOs are that easy to come by nor whether they switch both line and neutral of each phase, so would appreciate any pointers if anyone is familiar with them.
    • Breakers
      • £100 for a 3 phase RCD + £100 for a 3 phase MCB
    My rough calc shows roughly £650 to £700 for the job, but the price is 3 times that.
    Any clues why my calcs are so different from the quote?
  2. quasar9

    quasar9 Screwfix Select

    More curiosity here than any actual experience.

    although your air source heating will use a 3phase motor, presumably at 415v, most of your domestic requirement will still be single phase 220v. Are they expecting you to balance (or more like split) the remaining domestic load among the three phases ?
    Ceebee likes this.
  3. rogerk101

    rogerk101 Screwfix Select

    I haven't had the heating engineer's quote yet ... only his nominated electrician's quote so far.
    I would assume it'll be 3 phase with 230V between each phase and neutral. I doubt there is a need for 415V.
    The house has been on a 6kVA single phase supply until now, so not a whole lot of power available, but there has never been a need for any more power until now, as heating has been with oil, cooking with bottled gas, and the most power hungry electrical devices to date have been the oven, the dishwasher and the washing machine, none of which is more than 2.5kW. We've never made a conscious effort to avoid using all three of them at the same time, so we've never tripped the main breaker.
    I'm assuming I'll be getting 6kVA per phase from the electricity supplier, although the three phase MCB the electrician specified was 16A per phase, so only 3680W per phase for the motor.
    As for balancing, I don't think there are any penalties for consumers consuming in an unbalanced way, so I don't plan to bother with it. I'll just keep the rest of the house on the single phase it's always been on.
  4. The Happy Builder

    The Happy Builder Screwfix Select

    Do you have or will you have a maximum demand trip?

    If so, will it trip if you overload one phase instead of balancing the load?

    Does your suppliers contract require you to balance the load?

    Will your meter record the highest loaded phase as your usage meaning you will overpay?
  5. The Happy Builder

    The Happy Builder Screwfix Select

    And will you overload the neutral?
  6. Paco de Lucia

    Paco de Lucia Member

    Getting 3 phase brought to the house for free is amazing, surely in the ok that itself would be 10k
  7. sparky steve

    sparky steve Screwfix Select

  8. Bob Rathbone

    Bob Rathbone Screwfix Select

    I would have thought that the house would stay single phase as their is a risk, when carrying out the phase balancing exercise, of introducing a 415v situation into an area when 230V would only be expected. Just run a SWA from the supply to the heat pump, leave the rest alone.
  9. rogerk101

    rogerk101 Screwfix Select

    Yes. I am told that the 5 core cable they will run will be with 16mm2 conductors so the supplier's fuses will be rated to protect them.
    The electrician's quote includes a three phase 40A main breaker which seems low for 16mm2 cable, but I'm sure it's low because the neutral is shared by all 3 phases. That's still potentially 120A through the neutral if any of the MCBs fail but that's when the supplier's fuse on their neutral should blow.

    Yes, it'll trip if the line of any of the phases exceeds 40A.

    I haven't seen their contract, but I've been told that there are no penalties if individual domestic customers don't consume in a balanced way.

    That is an EXCELLENT question ... thanks for raising it, and it's certainly something I'm going to be looking into. (If the whole house remains on one phase and then they just charge me three times whatever that phase consumed, then that would certainly change my attitude towards balancing.

    See comment above, but again, thanks for raising it.
  10. rogerk101

    rogerk101 Screwfix Select

    The French government is taking environmental issues far more seriously than the UK when it comes to grants for environmental improvements. No matter what your income bracket is, you are entitled to a grant for a heat pump installation as long as it's done by an approved installer.
    The electricity suppliers just have to put fuses in the other two phases, as there was already 3 phase brought to the ex-farm house. That said, they told me that it would still be free even if they had to drag a new cable to their meter box. My little house here in Oxford has three phases in my meter box, so if I wanted to move to 3 phase I'd just get them to put fuses in the other phases.
  11. rogerk101

    rogerk101 Screwfix Select

    Thanks for those pointers. I'm very familiar with the tariffs and have on different occasions oscillated between the 3kVA and 6kVA subscriptions. The advantage of the 3kVA is that it is a low subscription rate so great for a holiday home used only a few months each year, but the units are quite expensive. I also used to use their split tariffs where I would do my washing and dishwashing using the half price units between 11pm and 7am, but that was when I lived there full time. I plan to move back there full time next year, so will probably reinstate that.
    sparky steve likes this.
  12. rogerk101

    rogerk101 Screwfix Select

    That's exactly what I was thinking originally which is why I was so shocked at the 2000 euro quote. However, if as @The Happy Builder suggested, they charge you for the highest consumption phase and then just multiply that by 3, then balancing may be useful and will certainly need pretty much everything in the sparkie's quote.
    I only got the quote yesterday, so I'm on a really steep learning curve here; so thanks again to all of you who have provided input and asked challenging questions.
    sparky steve likes this.
  13. Bobby Dazzler

    Bobby Dazzler Active Member

    Sorry roger, I can't help with your query, but I can add my experience.
    We've been on 3 phase and neutral ever since we bought our place. I replaced the main consumer unit to accommodate 3 phases.
    As it comprises several buildings, I just use a different phase and neutral for different buildings, with a new single phase supply to each building. No need for any balancing.

    Prices for electricity have gone up a fair whack.
    About 12 years ago, I was paying about 30€ a month. Now I'm paying about 60€ a month, but we are resident permanently now, whereas in the past we've been back and fore frequently. As the current overall cost is far less than our last residency in UK, many years ago, I don't get too concerned.
    Only large loads are immersion heater (only in summer due to wood burner in winter), and oven. Lighting and ventilation (MVHR) has increased over the years. Very, very occasionally use an electric heater, but only on rare cold evenings or mornings, when the wood burner is not going.

    Apart from the frequent power cuts, there's never been a problem. The meter was replaced very recently for a smart meter, not the smart meter used in UK, just a remote readable meter.

    As ours are stone built buildings, with insulation floor and roof only, I have discounted the idea of air source heat pumps for the time being. I would seriously consider ground source heat pumps as we have the available land, but just don't have the time at the moment to give it any thought.
    Similarly, we have south facing roofs, so solar panels (photo-voltaic and solar heating) would be a great help, but just don't have the time or inclination just now.

    Friends installed reversible air source heat pumps for their restaurant, a couple of years ago, cost about 15,000€.

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