UK Outbuilding Supply from house maximum demand

[daavvee]

A couple of questions for you.

The electrician that rewired our house ran a 10mm2 SWA to our outbuilding. The total length of the cable is about 45m. It is in ducting underground at a depth of approximately 1.2m.

Now, my understanding is that the max load for 10mm2 underground over 45m would be about 8.8kW.

So, for my questions...

When installing a consumer unit in the outbuilding:

Would I treat the incoming SWA cable to the outbuilding as if this were the main supply in any other home?
I only ask as the main cable from the DNO looks thinner than a 10mm2 SWA. Though the tails are 25mm2

When installing a consumer unit do I have to consider the maximum load of 8.8kW from the outbuilding consumer unit to my house or can I focus on the cable selection and demand on what is effectively the ‘consumer’ side of the consumer unit?

My proposed setup for the outbuilding would be:

6A RCBO - Outside Lighting - 56W
6A RCBO - Downstairs Lighting - 98W
6A RCBO - Upstairs Lighting - 112W
6A RCBO - Smoke Detectors - 20W
16A RCBO - Electric Radiators - 3.4kW
16A RCBO - Outside Sockets - <1.8kW
32A RCBO - Downstairs Sockets - <2.4kW
32A RCBO - Upstairs Sockets - <500W
32A RCBO - Kitchen Appliances - 5.3kW
45A RCBO - Water Heater - 10.0kW

 

[loz2754]

The mains incoming cable looks smaller likely because it's a concentric cable.

Your 10mm2 SWA is not adequate for the proposed loading. Even just the 10kW water heater alone takes up most of the cable capacity.

What is the rating of the device protecting the SWA?

 

[daavvee]

The mains incoming cable looks smaller likely because it's a concentric cable.

Your 10mm2 SWA is not adequate for the proposed loading. Even just the 10kW water heater alone takes up most of the cable capacity.

What is the rating of the device protecting the SWA?

Where you say the device protecting the SWA, I assume you’re referring to the MCB in the consumer unit inside the house, if so, it’s on a 32A MCB.

 

[loz2754]

Where you say the device protecting the SWA, I assume you’re referring to the MCB in the consumer unit inside the house, if so, it’s on a 32A MCB.

Yes that's what I was referring to. So that's your limiting factor. 32 amps at 230V.
32 X 230= 7.36kW maximum.
That's not taking into account the volt drop either.

 

[DPG]

Did the electrician who installed it know you were going to use a 10KW heater?

How easy would it be to replace the SWA? It sounds like it's buried pretty deep.

 

[daavvee]

Where you say the device protecting the SWA, I assume you’re referring to the MCB in the consumer unit inside the house, if so, it’s on a 32A MCB.

Yes that's what I was referring to. So that's your limiting factor. 32 amps at 230V.
32 X 230= 7.36kW maximum.
That's not taking into account the volt drop either.

Hmmm, that’s a shame. I had mentioned the potential use to the electrician before any cables were run.

taking into account the current carrying capacity of a 10mm2 SWA over the 45m, can the MCB be changed to get increase the maximum load.
I will accept that without changing the SWA cable to a larger CSA, then I’m now limited by the MCB in the main house.

Did the electrician who installed it know you were going to use a 10KW heater?

How easy would it be to replace the SWA? It sounds like it's buried pretty deep.

The water heater is something that has been considered after the fact, so not the fault of the electrician.

The cable is in ducting and could be changed, until I realised that the electrician ran the cable in a safe zone in our ground floor bathroom, which would mean removing tiles and chasing walls of a newly renovated house!

I’ll have to do some more calculations. Taking into account maximum demand and diversity, I’ll see what I can do.

thanks for your help. I’m very grateful.

 

[Mike Johnson]

Sounds like the electrician that installed the SWA did not know the outbuilding was going to turn into a separate dwelling.

 

[daavvee]

Sounds like the electrician that installed the SWA did not know the outbuilding was going to turn into a separate dwelling.

They were told it would be a separate home office with the need for sockets and lighting. It would have been a kettle, microwave and toaster type setup. It’s only now that I was wondering if we could make the most of the building.

mad the current carrying capacity of the cable according to the wiring regs is approx 70amps. Could I not just swap out the 32A MCB in the main board to something with a greater breaking point such as 40A or 50A?
Alternatively, reduce the size/rating of the water heater and kitchen appliances.

 

[telectrix]

a 40A or 50A MCB should be OK, except volt drop gives a rough in head guess of 9V @ 50A. bear in mind tht the water heater and the heaters will be thermostatically controlled so not all drawing max current simultaneously. best to see if youcan reduce the load to get within 40A.

 

[daavvee]

a 40A or 50A MCB should be OK, except volt drop gives a rough in head guess of 9V @ 50A. bear in mind tht the water heater and the heaters will be thermostatically controlled so not all drawing max current simultaneously. best to see if youcan reduce the load to get within 40A.

Thanks for this.
I’ll go back to the drawing board and see what I can do.