flats supply volt drop

[J.C.E]

afternoon chaps

estimating for a job for a communal area and 3 flats (3stories)
only been emailed the drawings by the contractor so far so no site visit yet

-assume there is a 3ph supply

was thinking to have 1x single phase DB local to the income- to cover the communal lighting and small power.

Then also local to the income to have 3x 100amp switched isolators (flats are to have 1x electric towel rail, 3x panel heaters+ immersion)
......local to the main income and then have my sub main SWA's go off to the flats

I am basing my estimate on 1x 100amp switched isolator+ 25mm SWA + consumer unit inside flat.


for voltdrops calcs- do I just need to calc for the swa meterage between the isolator and the flat c/u


1.9 X 100amp X 25meters /1000 = 4.75volts -which is within 3% and 5%

-leading nicely to my 2nd question!!- what does a sub-main have to be within 3 or 5% as it will of course be doing lighting and power with flat.

cheers for any advise.

 

[Dave the spark]

afternoon chaps

estimating for a job for a communal area and 3 flats (3stories)
only been emailed the drawings by the contractor so far so no site visit yet

-assume there is a 3ph supply

was thinking to have 1x single phase DB local to the income- to cover the communal lighting and small power.

Then also local to the income to have 3x 100amp switched isolators (flats are to have 1x electric towel rail, 3x panel heaters+ immersion)
......local to the main income and then have my sub main SWA's go off to the flats

I am basing my estimate on 1x 100amp switched isolator+ 25mm SWA + consumer unit inside flat.


for voltdrops calcs- do I just need to calc for the swa meterage between the isolator and the flat c/u


1.9 X 100amp X 25meters /1000 = 4.75volts -which is within 3% and 5%

-leading nicely to my 2nd question!!- what does a sub-main have to be within 3 or 5% as it will of course be doing lighting and power with flat.

cheers for any advise.

Volts drop for sub main will be 5% allowable. It's your final circuit for lighting that will be subject to 3%.

Dunno about your first question I'm afraid

 

[davesparks]

afternoon chaps

estimating for a job for a communal area and 3 flats (3stories)
only been emailed the drawings by the contractor so far so no site visit yet

-assume there is a 3ph supply

was thinking to have 1x single phase DB local to the income- to cover the communal lighting and small power.

Then also local to the income to have 3x 100amp switched isolators (flats are to have 1x electric towel rail, 3x panel heaters+ immersion)
......local to the main income and then have my sub main SWA's go off to the flats

I am basing my estimate on 1x 100amp switched isolator+ 25mm SWA + consumer unit inside flat.


for voltdrops calcs- do I just need to calc for the swa meterage between the isolator and the flat c/u


1.9 X 100amp X 25meters /1000 = 4.75volts -which is within 3% and 5%

-leading nicely to my 2nd question!!- what does a sub-main have to be within 3 or 5% as it will of course be doing lighting and power with flat.

cheers for any advise.

You need to include the final circuits in your volt drop calculations for the sub main. So you need to achieve less than 3% drop from the intake to the furthest part of each lighting circuit.

 

[J.C.E]

can't get my head around how your meant to be able to work that out from drawings though!? how are you meant to work out the exact meterage from the consumer unit to the furthest point- before its even installed? or is it a bit of estimating involved?

so say the furthest light/switch from the flat c/u was 15meters away- wired in 1.5mm t/e

29 x 6a x 15m /1000 =2.6v

then add that to the swa sub main voltdrop (so for 25mm....)

1.75 X 100a X 25m /1000 = 4.4v

2.6 + 4.4 =7 volts

so I would have to use 35mm SWA to supply flat?

right idea?

 

[Wilko]

Hi - thats a good worst case method, but maybe a realistic max demand for domestic use is half that? So I'd say 25mm is ok for voltage drop on that shortish run (?) .

 

[J.C.E]

Hi - thats a good worst case method, but maybe a realistic max demand for domestic use is half that? So I'd say 25mm is ok for voltage drop on that shortish run (?) .

ha I know it wouldnt be full 100amp mark- but there seems to be no gas as it has electrics heaters and immersion etc

 

[davesparks]

can't get my head around how your meant to be able to work that out from drawings though!? how are you meant to work out the exact meterage from the consumer unit to the furthest point- before its even installed? or is it a bit of estimating involved?

so say the furthest light/switch from the flat c/u was 15meters away- wired in 1.5mm t/e

29 x 6a x 15m /1000 =2.6v

then add that to the swa sub main voltdrop (so for 25mm....)

1.75 X 100a X 25m /1000 = 4.4v

2.6 + 4.4 =7 volts

so I would have to use 35mm SWA to supply flat?

right idea?

You work it out from the drawings and an estimate of the lengths, the same way you work out how much cable to allow for in the quote.

That is a very odd lighting circuit which has 6A being used at its furthest point!
Is 100A really necessary to supply a flat? Have you calculated maximum demand?

 

[J.C.E]

You work it out from the drawings and an estimate of the lengths, the same way you work out how much cable to allow for in the quote.

That is a very odd lighting circuit which has 6A being used at its furthest point!
Is 100A really necessary to supply a flat? Have you calculated maximum demand?

40+32+32+20+6+6 x 0.4= 54.4amps- I know it would only need 60amps- thought i would play safe as will be all electric heating+imm

 

[westward10]

You work it out from the drawings and an estimate of the lengths, the same way you work out how much cable to allow for in the quote.

That is a very odd lighting circuit which has 6A being used at its furthest point!
Is 100A really necessary to supply a flat? Have you calculated maximum demand?

Thought exactly the same, most flats I have worked on have 63A supplies, smaller cable easier to install.

 

[Dave OCD]

To be competitive on the price you may well be able to use a 16mm submain to the ground and 1st floor flats and 25mm to the top floor for example, at most I'd fit an 80amp fuse in the switch fuses, although a 63a would be most unlikely to fail.

 

[Leesparkykent]

Remember @J.C.E you will probably have to install a ryefield board or similar as it will be classed as a BNO job.

 

[J.C.E]

Remember @J.C.E you will probably have to install a ryefield board or similar as it will be classed as a BNO job.

BNO?

head>>ryefield>meter>80a isolator then SWA to flat c/u?

ryefield needed just for 3flats and 1x L/L db???

 

[Leesparkykent]

BNO?

head>>ryefield>meter>80a isolator then SWA to flat c/u?

ryefield needed just for 3flats and 1x L/L db???

Yeah mate they changed the rules a few years ago so they have to do minimal work.

BNO is building network operator.

http://www.ukpowernetworks.co.uk/internet/en/help-and-advice/documents/UKPN BNO Guide.pdf

What they like to see is an isolator then ryefield. The isolator is obviously the single point of isolation for the whole building.

Head>isolator>ryefield>meter>switch fuse>distribution circuit to flat.

 

[Leesparkykent]

RY6100 - 6 Way Ryefield Board taking 100A Fuse Units, TPN 6 Way Ryefield Distribution Board - https://www.sparksdirect.co.uk/6-way-ryefield-board-taking-100a-fuse-units-tpn-6-way-ryefield-distribution-board#.WJOSDuvfWK1


Glasgow 100A TP+N Metalclad Switch Disconnector Grey | City Electrical Factors - http://www.cef.co.uk/catalogue/products/393310-100a-tp-n-metalclad-switch-disconnector-grey

 

[J.C.E]

Yeah mate they changed the rules a few years ago so they have to do minimal work.

BNO is building network operator.

http://www.ukpowernetworks.co.uk/internet/en/help-and-advice/documents/UKPN BNO Guide.pdf

What they like to see is an isolator then ryefield. The isolator is obviously the single point of isolation for the whole building.

Head>isolator>ryefield>meter>switch fuse>distribution circuit to flat.

so is the BNO company responsibly for installing from the head to a 3ph isolator (do they usually supply?)

I install the ryefield I guess- but won't they DNO/BNO have to install meters- so surely they would have to wire between
head>isl>ryefield>meter and me the contractor wires from meter>switch fuse>flat