Multiple sub mains, how would you do it ?

[vwes59]

This is a question with probably so many answers and solutions, but I wondered how people would do it.

I have a clients house which already has one submain on a 40A MCB to a Garage with a 20A RCBO radial and a 6A RCBO lighting circuit + a spare 32A RCBO. Selectivity not an issue.

They need a 2nd sub main in a new summerhouse which will be a gym (Well I say a gym, it'll have one treadmill), plus an outdoor inflatable hot tub (the type that can be plugged in, although it can draw 2.5kW).

It would be tricky (but not impossible) to run a second cable from the main board to the summerhouse, but the main board is in the middle of the house with no easy route to the outside, and I'm unable to follow the route of the first cable. (House has an isolator, so I could split off the mains too). This would be the best option granted, but the hardest to actually do given the location of the board.

I could use the spare 32A RCBO and run a cable from that to the new sub main, that route is far easier, but obviously I would need to take care with the loading. Luckily the garage has a negligible load.

Third option which I expect will be frowned at is to 'T' off the cable that goes to the garage, and route a cable from that to the summerhouse, thus the garage and the summerhouse come off the same 40A MCB in the main house, rather than via the 2nd consumer unit in the garage.

How would you do it ?

 

[Spoon]

They need a 2nd sub main in a new summerhouse which will be a gym (Well I say a gym, it'll have one treadmill), plus an outdoor inflatable hot tub (the type that can be plugged in, although it can draw 2.5kW).

So the only thing that will be in the summer house (ignoring lights) will be a treadmill & a plug in hot tub? No heater in there for winter or anything else?

 

[Electro-tech]

What size is the sub-main cable to the garage?

 

[vwes59]

So the only thing that will be in the summer house (ignoring lights) will be a treadmill & a plug in hot tub? No heater in there for winter or anything else?

Nope, no heater apparently.

 

[vwes59]

What size is the sub-main cable to the garage?

It looks like a 4mm Hi-Tuff cable (certainly isn't SWA), I measured it's approximate length from the board as about 10m, clipped direct mostly, just underground for about a metre or so under a path between the house and garage.

 

[davesparks]

It looks like a 4mm Hi-Tuff cable (certainly isn't SWA), I measured it's approximate length from the board as about 10m, clipped direct mostly, just underground for about a metre or so under a path between the house and garage.

I was going to say just extend the existing submain to feed another CU in the summer house.
But if its 4mm on a 40A MCB and Hi-Tuff underground then I'd be avoiding using that circuit and advising the customer about these issues.

Why does it need a submain in the first place? Could you not just run a socket circuit to the summerhouse?

 

[Electro-tech]

I personally would run a 2nd sub-main to the summerhouse allowing for additional loading, you know they are going to plug other items in at some point.

 

[brianmoooore]

Yet another submain where problems are occurring because a big enough cable with excess capacity wasn't fitted in the first place.
The increase in the cost or the original job would have been minimal.