Calculation submain cables

[2piesFC]

Hi. Im looking for some advice when calculating submain cables.
Basically there is a new connection of 80Amps that is to be used for a dwelling and a small workshop. I had thought taking to a feeder box with one submain and then two feeds away into the buildings.
A direct run from dno to feeder box is 70m a preferred run would be 100m
25m into dwelling.
50m into small workshop.

I was told the volt drop wpuod need to be 3% from dno to end of lighting circuits.

My calculations are showing.
100m 95mm swa
25m 35mm swa
50m 35mm swa

But other firms have told the customer that they would use 25/35mm swa for the entire run due to diversity.

Even with divesity of the dwelling im coming up to about 45A for that alone.

Any advice would be most appreciated.

Thank you

 

[westward10]

Your volt drop theory is correct it is calculated from source to the end of each final circuit, people often use a rule of thumb of 2% for submains.

 

[brianmoooore]

I've always advised the customer to spend a few more £££s on the submain than the calculations required. It can pay dividends further down the line when any additions are made to the final circuits .

 

[oscar21]

Why is the DNO connection so far away from the dwelling? Do the DNO expect you to run your own submain from the substation nowadays or something.

 

[2piesFC]

I can only assume for initial cost reasons. The TT lines are on their land and have had it put it as close as possible to the transformer.

 

[pc1966]

If it is TT then you can use the armour of any of them as CPC but will almost certainly need an up-front delay RCD at the sub-main feed point to allow disconnection on a fault.

If TN-C-S then the bonding size of (typically) 10mm copper equivalent can be an issue for many SWA cables as it translates in to about 80mm of steel, you might find it significantly cheaper to run a parallel 16mm CPC or whatever instead of going 3C if you are looking at 95mm cable, but again check Zs levels as that might also push you in to needing up-front delay RCD fault disconnection which might negate a lot of any cost savings.

Aiming for 2-3% VD on the feed makes sense, though that is on the realistic max load which may be less than the OCPD size. If future use is not known then sizing based on the OCPD makes sense. If you are running big loads for a lot of the time then it can pay off in terms of electricity savings vs. copper price over 5 years or so to size the feed bigger, but for many domestic cases the usage duty cycle (and so I2R losses) are unlikely to make it worth pushing down to 1-2% or below.

These days the VD on domestic LED lighting circuits should be very small so meeting just below 3% of the feed ought to be enough, but as it is expensive to do they really need to think forward to anything they might want in the future and if that could push for a 3P supply then you might be better going with something like 4C 50mm paralleled for single phase now, instead of 2C 95mm, if it might meed the drop for a future 3P supply as well without needing a cable change.