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

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 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]