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I do a lot of technology work for a large country house in the UK. We're currently looking at getting electricity 150m up the garden.

Despite being a residential property, the electrical supply appears to be 3-phase (the main supply cable looks to be about 50mm diameter and there are Danger 400V warning signs around it.) Given that we might want to run large events in marquees at the site, it seems sensible to try to dispense of the need for generators and get 3-phase up there if possible. We would prefer to bury it but can run it above ground if necessary. It would terminate in a well-constructed, waterproof timber building I'm hesitant to refer to as a shed.

So my questions:

  • Compared to solving this problem with a high-current single-phase installation, are there likely to be any extra costs or regulatory hoops to jump through to do it with 3-phase?
  • Can we legally terminate it in our timber structure and distribute both 3-phase and single-phase sockets from there?
  • Do we need to get permission from the power company or any regulators?

And for the record—no, I'm definitely not going to attempt any of this myself; I just want to know what I'm talking about before I start calling electricians.

  • 3-phase to a large country house is not unusual. How do you plan to route the wires, pole line, or underground? How much load (in amps or watts) are we talking about? Can you say for sure that all loads would be 240V single-phase, or would there be call for 400V or 3-phase? Given the distances involved, it may be worthwhile to use transformers to step up the voltage for long-distance transmission. That would be a function of how much voltage your burial-grade wires can carry. (Pole lines have no limit that would affect you). – Harper Jul 4 '17 at 13:36
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Compared to solving this problem with a high-current single-phase installation, are there likely to be any extra costs or regulatory hoops to jump through to do it with 3-phase?

No, there will not be any extra regulatory hoops.

As to materials cost a three phase DB will cost more than a single phase one but the cable is likely to work out cheaper. Overall I would expect for a given power level a three phase feed to work out cheaper than a single phase one.

Don't expect this to be a cheap job, mostly because you are going to need a long peice of fat cable. Using aluminium cable is an option to lower the cable costs but it can be difficult to find electricians who know how to work with it safely.

Any electrican who regually does commercial work will know how to deal with three phase.

Can we legally terminate it in our timber structure and distribute both 3-phase and single-phase sockets from there?

As long as the structure is sturdy, weathertight and reasonablly permanent the fact it's timber shouldn't be a problem.

Do we need to get permission from the power company or any regulators?

The job will be notifiable under "Part P" because new circuits are involved but nearly all electricans will be part of self-certification schemes so that should be a non-issue.

  • I'm a bit...shocked that you'd have trouble finding electricians who can work with fat aluminum safely where you're at... – ThreePhaseEel Jul 19 '17 at 22:17

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