Supply Calculations help?

[energizedelec]

iv been asked to install a supply to a house and im a bit rusty. can someone please walk me through the cable calculations?

supply is a protected by a 100a bs1361 fuse, cable will be swa installed direct in ground, cable length around 40 mtr.

thanks

 

[Amp David]

Firstly you'll be required to not rely on that 1361 cut out fuse for protection of the supply, so decide on what size you will require.

 

[energizedelec]

100a circuit beaker, 25mm SWA ?

 

[malcolmsanford]

In ≥ Ib then Iz ≥ Ib ...............................

 

[Amp David]

100a circuit beaker, 25mm SWA ?

Wish it was that simple mate.

 

[VoltzElectrical]

I thought supplying houses was the DNO's job?

 

[ackbarthestar]

100a circuit beaker, 25mm SWA ?

Discrimination?
Usually as a rule of thumb 2:1 but you would need to compare BS1361 aka BS88-3 with 60898 data sheets.
Additionally,
You may even get away with something like a 80A BS88 KMF switch for a sub-mains.

 

[energizedelec]

ok so after applying diversity Ib = 91.5A and In=100A
so from table 4d4a multi-core armored 70'c refrence method D you get 99A for 25mm cable.

then volts drop mv/a/m = 1.75

so...
1.75 x 91.5A x 40 mtr /1000 to convert to volts = 6.405v

now if it was a final circuit not for lighting this would be ok (as it doesnt exceed 11.5v 5%). but its for a distribution circuit so if i was to then have other circuits running of it the volts drop would increase how do i account for this??

 

[Richard Burns]

Increase the cable size