Supply cable to a new dwelling

[xr4x4]

That doesn't look too different to a STD house with a 100a main fuse!

 

[ricicle]

I'm a bit worried that the circuits required for a 4 bedroom cottage are going to total much more than 76A

Put an ammeter on your main tails at various times during the day (even when the power hungry appliances are all in use together) I doubt you will see anything over 76A

 

[1john]

Diversity is what you need there.

 

[westie101]

Diversity is what you need there.

I wish more folk would learn about that, the number of additional load applications I see that are simply all the maximum demands added together is getting beyond a joke!

(though it would be interesting to see what size the main service is and what existing load there is)

 

[ajrobb]

Diversity has already been applied to each circuit. A normal 4 bedroom house supply would have 25mm² meter tails rated at over 100A for the main fuse. The voltage drop on typical meter tails is negligible. At 100A, the voltage drop over 50m of 25mm² would be 1.8 * 50 * 100 / 1000 = 9V. This is more than the 6.9V drop limit for lighting circuits.

If the supply circuit is to be rated at 100A (main fuse), a 35mm² cable would have a voltage drop of 1.25 * 50 * 100 / 1000 = 6.25A and would be suitable for lighting.

Would another option be to de-rate the supply fuse to less than 76A?

 

[rippa700]

Maybe we are a bit more eco than average - for a start it is only one floor, all lights will be dedicated ecobulb sockets etc. I think we will be at or under 76A - I know my house is and that's bigger. I'll get the electrician to figure it all out as I now at least have the duct and as the above comment stated I can backfill the trench which is my main concern at this stage.
Thanks all for the help

At 1.8 mV/A/m voltage drop for 25mm² cable, you should be able to supply 76A over 50m within the 3% voltage drop for lighting. I may be ignorant, but I'm a bit worried that the circuits required for a 4 bedroom cottage are going to total much more than 76A.

• 32A cooker appliances
  32A downstairs sockets (100m² floor area)
  32A upstairs sockets
  16A immersion heater
  6A downstairs lights
  6A upstairs lights

124A total
intrigued

 

[securespark]

While I am not arguing the fact that even with diversity applied, the figure is not close to 76A, I can't see diversity applied to those figures.

Unless the cooker is around 16kW, you will come in under 32A.

Sockets: allow 100% for first, then 40%.

Immersion: OP wants 2x 3kw.

Lighting: 66%.

That's less than 124A, despite adding another IH element.

 

[securespark]

I now at least have the duct and as the above comment stated I can backfill the trench which is my main concern at this stage.

Draw a poly rope through it first!!

 

[ajrobb]

I'd like to know why lighting circuits are limited to 3% voltage drop. It can't be to guarantee supply voltage as UK households are typically 4% percent above 230V and 9% above some Continental voltages. My guess is that it is to limit flicker as other devices are turned on and off. Given the long time it takes CFLs to reach full output, are CFL fittings in this cottage subject to the same flicker as tungsten filament bulbs?

Also, if the 3% is there to limit flicker, surely this depends on the largest appliance rather than the total load. For the same calculated voltage drop, a large 25mm² supply to a whole house is going to exhibit much less voltage fluctuation as individual devices are turned on and off than say a 2.5mm² supply to an outhouse with a washing machine. (An exception to that could be a house with multiple storage heaters controlled by a single timer.)

 

[rippa700]

Yep - I did that and it was not that easy either!

I now at least have the duct and as the above comment stated I can backfill the trench which is my main concern at this stage.

Draw a poly rope through it first!!

 

[westie101]

UK households are typically 4% percent above 230V and 9% above some Continental voltages.

Declared UK supply voltage is 230V + 6% & -10% (253 Max, 217min)

In reality we did not adjust anything when it changed from 240V so most supplies are on the higher side.

 

[JohnW2]

Declared UK supply voltage is 230V + 6% & -10% (253 Max, 217min)

I take it that was a 'deliberate error' and you meant to type -6% & +10%

Kind Regards, John.

 

[westie101]

Yep, just checking!!

At the time of introduction figures of + or - 10% for the future were banded around

 

[JohnW2]

Yep, just checking!!
At the time of introduction figures of + or - 10% for the future were banded around

From what I've seen, it's still be banded around as a possibility for the future, but with most people thinking that it will probably never happen.

Kind Regards, John.