Online assistance for electrical trade people in New Zealand and Australia Login  |  Register  |   Forgot Password
Assistance for electrical trade people




Click here to send Forum Admin a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: Feed for submain

May 25 2012 19:51

Hi all.

I am feeding a large rumpos room , remote from house.....Aprox 60mtrs away.

Have sorted all the sub ccts for it arround 20 sw skts , and lighting.

All sub ccts were used on the 2.5% VD calc.

Now , for the mains , i will alow another 2.5% making for a total of 5% from the point of supply ( the house ). Now how do i get on calculating for the VD in the mains of the house??? obversly there will be some VD from the mains from the pole to the main switchboard , witch we take as 2.5 percent correct ,

My question is , how do u guys calculate it? in therory i should have used 1.25 percent for the new submain , and another 1.25 for the sub ccts in the rumpos room ,

but by doing this will increase the cable sizes signifacantly. i will prob need to use 4mm2 for sockets in the Rumpos room.

Is this the standard? are people using 1.25 for submain , and 1.25 for sub ccts within the submain , or does everyone use 2.5 for submain , and 2.5 for sub cct , and class the house as a \"point of supply\"???

Your thorghts appreciated!


May 25 2012 20:18

It varies with me. On a new build I\'ll work it all out from point of supply to furthest point.

On existing installations I just make a best guess / practical approach. I assume that there is already a 2.5% drop in the mains and get as close as is practical with the sub and associated circuits from there.

May 25 2012 20:23

I think ive answerd my own qtn , ive found in the bible that the outbuilding shall be regarded as a seperate electrical installation , therefore , the point of supply is the house , and you would alow 2.5 in the submain , and 2.5 in the sub ccts and this is OK.


Brian H
May 25 2012 20:29

Rules..Rules..Rules..If correct all calcs pertaining to that Installation would be Followed To the Rule of the Day.Bonus points to the Fact Power Company alterted calcs to 5%.End of the Day,your Calcs Via Part 2 differed from actual calculations,then take it further...

Brian H
May 25 2012 20:33

Rule of Thumb,16mm squared.2 Plus E

May 25 2012 21:13

Craig May 25 2012 20:23

Your comment
I think ive answered my own qtn , ive found in the bible that the outbuilding shall be regarded as a seperate electrical installation , therefore , the point of supply is the house , and you would alow 2.5 in the submain , and 2.5 in the sub ccts and this is OK.

My comment
Incorrect, the 5% volt drop requirement is from the point of supply at the connection to the electricity distributors network to the most remote part of the electrical installation.

The use of the out building provisions does NOT effect the above statement, so the total volt drop is the sum of the volt drop of the electrical installation mains, sub mains to the out building and the final sub circuits in the outbuilding.

The main reason for the 5% volt drop requirement is to ensure that the actual voltage at normal full load conditions at the socket outlet or other supply connection to lighting fittings, is not less than 218.5 volts, assuming a nominal 230 volts at the point of supply. If the volt drop is greater than 5%, some appliances may not work safely, or, the light bulbs will have reduced light output.


Brian H
May 25 2012 21:19

excuse my ignorance Pluto,2.5% calcs not relevant. I could do the Maths,but its late,and im enjoying political freedom.

May 25 2012 23:10

Ok pluto , how do you do it?

You have a house feeding an outbuilding.

that outbulding contains several powerpoints and 2kws worth of lighting.

how do you calculate the mains????

May 26 2012 16:51

Check out the worked example

May 28 2012 09:30

step 1
calculate max demand as per App C2
Step 2
size submain cable accordingly, allowing for V drop. RoT use 2.5% from MSB, but if mains old / undersized / whatever, calculate them as well, at current = supply fuse rating.
It\'s not hard, the tables give you mV per A per m ; and you know the A and the m. So at 2.5% you have 5750 mV to play with.
Step 4
If you run out of mV, increase either submain size or subcircuit sizes.

BTW, that reference you noted in about outbuildings does NOT mean the outbuilding is a seaparate installation. You\'ve taken it badly out of context, perhaps by failing to read it completely. Yes it does say \"the installation in the outbulding shall be regarded as a separate installation\", but it\'s in the EARTHIN section, not the control & protection section; and it\'s one of two options for EARTHING of the outbuilding. Therefore the EARTHING shall be treated as a separate installation. It goes on to say \"and shall be EARTHED in accordance with the relevant clauses of this Standard and with the following requirements.
The only way to miss the concept that this clause is about EARTHING is to be woefully (or perhaps wilfully) thick.