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.

 

[Wilko]

Hi - as the 10kW water heater will draw over 40A, I’d suggest a directly heated tank with a 3kW element. Yes, the new Outbuilding load would need to be considered at the origin where the installation’s fuse may be 60A, 80A or 100A.

 

[Deleted account]

Have you considered the maximum demand of the property it is run from. Remember the maximum demand for the whole instalation Needs to be considered. You have exceeded more than half your alowance on the new shed. Ho2 much do you need in you4 main property.

 

[telectrix]

as a digression, when are the IET going to drop the ridiculous 3% for lighting VD , bearing in mind that a VD of >3% will have not detrimental effect on LED lighting, and the parameters for the supply allow 6% drop anyway.

 

[daavvee]

Hi - as the 10kW water heater will draw over 40A, I’d suggest a directly heated tank with a 3kW element. Yes, the new Outbuilding load would need to be considered at the origin where the installation’s fuse may be 60A, 80A or 100A.

Smart, I hadn’t thought about that. I should have the space once the stairs go in.

 

[Gurnatronics]

How much of a kitchen are you talking? Is There copper pipework entering in the ground?

 

[pc1966]

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?

A quick check for 10mm armoured wire with 70C max has "method D" (buried in duct) at 60A limit. Also for 45m and 5% drop it is 58A, but for 3% drop for lighting supplies (contentious point - as above) it is 35A.

Before changing the supply protection you need to know the Zs value at your board to know if it will allow a fault on this sub-main to disconnect in under 5s as required by the wiring regs. Here the coice of 2-core SWA or 3-core also makes a difference as if you have the copper core in parallel with the armour you have much lower fault impedance at the end so an easier time disconnecting on a short, etc.

Assuming it is 3-core, then your sub-main is about 0.2 ohms and if your CU is 0.35 ohms (guess, assuming max for a typical TN-C-S supply, which might have been the reason your electrician originally went for 10mm 3-core due to bond requirements) then your end of cable Zs is about 0.55 ohm. You would be looking then at limits of:

• 32A C-curve or D-curve MCB (on Zs for 5s or less, or 3% VD)
• 50A B-curve MCB (on 5% VD & CCC limits above)
• 50A BS88 fuse (on 5% VD & CCC limits above)

But more fundamentally if you have a MCB feeding the out-building then any fault is likely to take out this supply MCB as well as the MCB/RCBO on the faulted circuit. It is very difficult to get reasonable selectivity between them.

Looking at the Hager commercial catalogue tables of selectivity (just because I have it to hand) a 50A B-curve MCB is selective with other B-curve to 32A rating but only to around 230A fault current - about half of what you might see.

The 32A C/D-curve upstream is better, you get fault selectivity to around 610A / 1kA so it should not trip (more if you are lucky in the C-curve case), but only to 25A load MCBs. And really 32A is a bit limiting for your planned use!

So really it leaves the 50A fuse option, that should be selective to 700A with a 40A load MCB, and to 800A with a 32A B-curve for typical sockets circut (and more fundamentally, total selectivity with 13A appliance fuses). So it looks like a tolerable combination. But that means getting in a professional electrician to split your supply tails and install a separate fused-switch to feed the SWA sub-main, and not using a MCB slot in your existing CU.

Finally, as mentioned above, you need to look at the overall house supply limit. Generally BS88 style fuses are selective at 1.6 ratios, so if you have a 50A fuse here then you need your DNO fuse to be 80A or more to avoid a major fault in the out building taking out the supply to your main home.

But equally you need to consider your total typical load so you don't overload the supply with, say, two approx 50A loads on a 80A capable supply.

 

[pc1966]

Obviously the above is all guess work - your electrician should repeat that sort of calculation / selection process with measured values for your CU Zs (and maybe measured from the out building, assuming it is energised and safe to do so).

 

[Mike Johnson]

Looking at the Hager commercial catalogue tables of selectivity (just because I have it to hand) a 50A B-curve MCB is selective with other B-curve to 32A rating but only to around 230A fault current - about half of what you might see.

Some light Sunday morning reading with breakfast. ?