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# DomesticSub-main calculation help!

Discuss Sub-main calculation help! in the Electrical Forum area at ElectriciansForums.net

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#### energizedelec

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

iv been asked to install a sub-main to supply a house. The supply is quite far away from the house (40mtr) the cable will be installed direct in ground (SWA)
im abit rusty on my cable calcs and could do with some help and advice. hear is where i am at the moment...

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??

S

larger CSA....

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#### energizedelec

yeh i understand it would be a larger CSA but would i need to calculate the volts drop on the circuits that will be connected to the supply and add to the supply cable??

#### Guitarist

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Basically, if you are going to be connecting lighting to this supply, then your VD allowance should take this into account (3%).

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#### Amp David

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Mentor
Arms
I would say so. If your lighting circuit is another 20m, then its at the end of this where your volt drop needs to be within limits.

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#### Engineer54

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

I would say you'll be lucky to see 91.5A being drawn from any standard domestic supply!! 99% of the time you would still be lucky to see 60A, and then only for a matter of a few minutes...

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