Extend new circuit from outhouse into another property with an existing supply, which is bad idea or not allowed?

A house of this size should definitely have at least two ring mains. Particularly since it sounds like it is being used as a small business premises rather than just a home. I would agree with Andy's suggestion - split the ring into two.


My present house, a four-bedroomed bungalow used as a dwelling only, when I moved in had a single ring main serving back and front but not the middle which was served by a haphazard number of spurs. I reorganised this into two ring mains. I felt satisfied that I had tidied up the installation.


The boiler sounds like a combination job, so electric power consumption is not an issue, though ideally it should be on its own subcircuit.


The safety issue here is that the main switch of a building should isolate everything in the building, and everyone should be confident of that, including non-electrical-savvy people. This will not be the case if part of the building is supplied from an outbuilding.


It sounds as though your reason for wanting to do it this way is partially to simplify the wiring and partially to get Building A to support part of the load of Building B, which is at its limits. I hope you have your metering and billing arrangements agreed!


Who are "they", that are keen to deploy a 3-phase supply? If "they" are the electricity network supply company, then I am not surprised. Should they bear some of the cost? Worth some enquiries I think.


I have seen, over the years, electrical installations that have become confused and even hazardous, simply because successive contractors want to do a quick and cheap fix instead of sorting things out properly. I would be unable to give you a professional recommendation to go ahead with this. One day, things are likely to go disastrously wrong, and someone will need to dig deep to sort it.

At first I thought this was  DIYr seeking advice

​​​​for there own home.

Obviously not. If you were to adopt your economical method, it would be near nigh impossible to justify it " officially". If you showed this for an "assessment" for a scheme, perhaps, I would like a front row seat and a bag of popcorn please.


So the house is just 60amp single phase and seems to be a lot going on.


You have 3 phase elsewhere.  


Can you not ditch the 60 amp house supply and run new supply direct from 3 phase to suit the needs of the house?

I certainly agree with others that it's messy in any case and desirable to avoid. Much more so if the same room will have parts supplied from the two sources. Would the room in question, with the new domestic appliances, have any other electrical equipment on the existing supply (lights, other sockets)?  If not, treating it as a separate installation is certainly easier.       

 

What earthing systems do the two supplies (buildings A,B) have?  Will there be bonding that couples the systems: both sources to a building part, or both sources main-bonded due to nearby earthed equipment on the different sources?


Perhaps more important than clearly marking an "emergency socket power off" switch in the room, would be a warning at building B's main supply (60 A 1ph) that this is not the only source of supply in the building.


I didn't notice the wiring-matters article that was mentioned here. But I agree from having watched dishwasher and washing machine cycles that the ~2 kW heater of normal domestic models is only on for a few minutes at a time (except for a big 90 degC washing-machine cycle) and the remaining load is only up to a few hundred watts. Things like kitchen mixers and the control circuits for a boiler can probably be ignored in the total loading. A single server doesn't draw 3 kW except as a very brief starting current or because it's a very special sort that I'd be interested to hear more of. Some servers have multiple power-supply units, for redundancy in case of unit failure or failure of the supply to the unit. Each of these units might be rated 1 kW in order to cover the whole load easily, but the total actually used by both/all the input units together is unlikely to be even half that much in operation. You can guesstimate by thinking of how much heat the server puts into the room: it's basically the same as the electrical input. Compare to a 1 kW or 3 kW fan heater.


Still, with all the other things you describe, and clearly no desire for servers or cooks to wait for the DNO to attend to a blown 60A fuse, I appreciate the desire not to push luck. Can't the DNO bump building B's main fuse up to 80A or 100A? The size may be more a historic default than an indication of the rating of the service. You mention that it's not a nice prospect getting a cable from building B's source to the room .. could it be run around an outside wall?  


I'm not particularly proud of it (it came at a cost), but I managed a domestic consumption rather over 30 MWh a year ago, for a detached house and greenhouse. Fully electric space-heating, after removal of an old oil-boiler. Fully electric water-heating, by instantaneous 13.5 kW heater. Plenty of cooking and kettle load, besides the wash/dry/dish. But the peaks weren't beyond 30 kW and on most days were well below 25 kW. They were only brief. Diversity does good things. Removing the instantaneous water heater, the peaks are of course much lower so that even a 60 A single phase rating would be happy, and yet the total energy consumption is the same. Your case doesn't suggest any really big single load like that one.


You seem to have done a good job of measurements, but if they're only energy over periods of hours or days they won't help so much to show the risk to the main fuse. Please excuse if this attempt to persuade you to a choice you've already examined and found impractical is annoying. If the room can be made to be entirely on the separate supply, and there's not an earthing/bonding difficulty, and there's good labelling, then it doesn't sound too bad. A further matter is whether all buildings are owned by the same party or might get sold separately. 


[and now I see new messages while I was writing .. if it's all one property, then I also wonder why everything doesn't use the 3ph supply, to reduce standing charges]

I think we have a problem that all this leads to multiple supplies from unknown sources in the same installation which is not permitted by the ESQCR regulations. You are also not permitted to have different Earthing systems, which could be the case with two supplies. The obvious solution is to feed the whole installation from the 3 phase supply (which is surely adequate), and ignore the second single phase supply. This should not need too much work, to rewire bits of the supply in building B. There is no objection to any routing of the submains or the construction of any of the buildings provided the IP rating of equipment is suitable for the environment. Main bonding may be required in building B depending on the external pipework etc. but the 3 phase supply should at least provide sufficient power from a single DNO supply. If you do not do this you will be in a maze of regulation from many sources and will need a long time to correctly document what you have done, assuming you can avoid the problems thereby raised.


You need to be aware that using the Earthing system from a different supply is potentially dangerous, and probably never done in correctly designed installations, however you do have the option of TT assuming some way can be found to separate the Earth electrode from any supply earthing.

There were several overlaps in posting there, and I will go a bit further. Alcomax is right, we need simplicity. It sounds to me as if you need to get the 3 phase supply to the main house even if you need a trench which may not be as difficult as you assume. The options are to get the DNO to supply 3 phases to the house in place of the existing single phase (not ruinously expensive if you get someone with a digger to do the trenches) or to run this other cable which you say may be difficult but still only digging and careful threading under the tree roots. The new cable length needs to be considered too, and the new DNO supply may be easier than other options. I assume that the supply cable is underground, if overhead it is quite simple and cheap. You will find that most contractors will give you high prices for this kind of job, and probably suggest a lot of additional works are necessary to do the job at all. Most will run away from the two supplies problem I outlined earlier. I would expect the 10mm cable would run your new laundry fine, with satisfactory volt drop unless very long, in which case it is not the best place to put the laundry! The NICEIC is not the only contractor body, and not in any way any kind of requirement, so you can probably find a local electrician who may well be better and more reasonable to properly help you. Any questions may be asked here for solution from him or you.


Regards

David CEng etc.

If building B is the house, I am curious to know why building A has a 3-phase supply.

As others have said, in any one building you need a single 'off' point.


you can use contactors to cut off one supply when the other is removed. It is not elegant, but it does acheive single point isolation, or you can co-locate switch gear - in effect two consumer units side by side, as you may find in a block of flats. You need to   be very careful about the earthing arrangements in such cases.

As others have alluded there are ugly but safe solutions - cable running on the outside of the building and maybe overhead for example


Long term I'd suggest to consolidate on the 3 phase if it can take the full load,and the metering for these various users can be separated..

A while ago there was I think a wiring matters article about ring circuits that demonstrated the load from a typical kitchen appliances over some hours and how the 20A cable wasn't overloaded for long periods.

I've identified the article - it was from the Spring 2002 (gulp!) Wiring Matters - which unfortunately I can't seem to find on-line at the moment .


I have copied the graphs though:




The general gist of the article was that this was satisfactory even on a moderately unbalanced ring (so one leg may be carrying a fair bit more than 20A) as, even under the unlikely worst case of everything starting together, the duration of the overload was relatively short (well under an hour) - so was satisfactory from both a cable and overload protective device point of view.


So basically diversity can give you a lot more than just adding up ratings plates or even measuring instantaneous currents might suggest.


   - Andy.