# Supply Calculations help?

Discuss Supply Calculations help? in the Electrical Forum area at ElectriciansForums.net

E

#### energizedelec

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

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

This official sponsor may provide discounts for members

#### Amp David

##### -
Mentor
Arms
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.

E

#### energizedelec

100a circuit beaker, 25mm SWA ?

#### malcolmsanford

##### -
Arms
In ≥ Ib then Iz ≥ Ib ...............................

Mentor
Arms

#### VoltzElectrical

##### -
I thought supplying houses was the DNO's job?

#### ackbarthestar

##### -
Mentor
Arms
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.
You may even get away with something like a 80A BS88 KMF switch for a sub-mains.

E

#### 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

##### -
Mentor
Arms
Esteemed
Increase the cable size

This official sponsor may provide discounts for members

Reply to Supply Calculations help? in the Electrical Forum area at ElectriciansForums.net

This official sponsor may provide discounts for members