House supply TT and loadings

Question

Is there a limit to what an overhead supply can handle, loading wise? 
 Sorry if that sounds daft, I haven't worked with TT much in the past. 
 Existing house has an oven supply, no electric shower, and just your average size place with normal amount of circuits for a 3 bedroom house. 
 Fuse is 60/80A 
 A double garage is being built for a car enthusiast, he is requesting the following. 
 An electric garage door. ( no details as of yet ) 
 Car Stacker Lift ( no details as of yet ) 
 2 HP compressor 
 AC unit ( external condenser with one cassette inside ) ( around 4 amps ) 
 General sockets and LED lighting. 
 Submain length is 45 meters. 
 My initial thoughts was to install a 10mm 2 core to a new DB which will be staked SPD and RCBO's 
 I can't see that the equipment will be used at the same time, at the most maybe the AC will be on while he is lifting a car in the air with a few general things plugged in and the lights on. 
 Compressor won't be used at the same time the door is being opened or a car is being lifted in the air for example. 
 I thought the 10mm will be ok on a 32A MCB protected by the 100ma main switch. 
 Now is is asking for a car charger. I have suggested a Zappy which is 7kw 
 So now I'm thinking of a 16mm 2 core on a 40A MCB 
 Your thoughts would be apreciated.

broadgage · Accepted Answer

Relatively small and short term loads such as power operated garage doors or vehicle lifts can generally be ignored when considering if the incoming supply is sufficient. 
 An air compressor is more of a concern due to inrush or starting currents resulting in lamp flicker. A small air conditioing unit is unlikely to much matter as will be used only in hot weather when electric space heating is not used. 
 A 7kw EV charger is more of a concern as it is a long hour load. I would be very doubtful about a 7kw charger on a service of less than 100 amps.

AJJewsbury · Answer

Is there a limit to what an overhead supply can handle, loading wise? 
 
 In principle the earthing system (or cables being overhead rather than underground) makes no difference to the power availability - entire villages can be supplied by overheads. In practice there will be limits though - due to the size of the cables that have been previously installed and the capacity of the local transformer - which may well have less spare capacity in a small rural setting. Typically the limit for ordinary supplies is 100A/phase, but some DNOs prefer not to go above 80A these days and some even default to 60A. Upgrading to 3-phase (or possibly split phase) might be a possibility depending on what the local infrastructure is like, but probably isn't going to be cheap. 
 - Andy.

mapj1 · Answer

There is nothing magic about overhead lines - the maximum supply is determined by the cable size and the transformer load. Some underground cables are pretty thin too . Properly only the DNO can advise on what loads can be connected, and should be advised if large loads are being added. Strictly this need to notify applies to showers, cookers etc as well, but in practice no-one does. you may be able to follow the lines to the transformer with a pair of binoculars, and you can estimate what the Trx size is and how may properties it serves but that may not be the full story 
 Find your DNO here https://www.energybrokers.co.uk/electricity/pes-areas/search

House supply TT and loadings

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