Three Phase Cable Sizing

[GeoffatMM]

I am an enthusiastic retired engineer rebuilding a barn in France from scratch and am having problems sizing my incoming electrical cables. I have had answers from 4mm to 50mm cross section from various pages on the internet so you can understand I am a bit confused.

There are two main cables to consider. The first is the incoming main that is about 50m long. It is a 36kVA three phase installation with the Linky on the external garden wall and the cable buried in a flexible duct underground. The second is the feed from my 15kW PV panel generation fed back to the Linky then onward to the electrical mains. This is about 35m long also buried.

I calculate (using some software I have) 16mm2 for the incoming main and 6mm2 for the PV panels. Can you let me know if these sizes seem right or not please?

Finally, what should I be assuming as the max current for a domestic installation. Some say 60 Amps, some say 100 Amps which of course makes a difference in the required cable size. I know we have 100Amp breakers in the UK but I am not sure (and cannot definitively find) what the requirement is here in France.

For the house, 36 kVA divided by say 410V (assuming loss from 415) gives a max load of 88 Amps. For the PV feed 15kW divided by 415 V (post inverter) gives 36 Amps.

Below is my software output for the main cable:

50m Length Calculation for Three Phase cable



Any and all advice welcome, TIA,

Geoff

 

[plugsandsparks]

will/is inverter 3 phase?

 

[GeoffatMM]

will/is inverter 3 phase?

Yes it will be three phase.

 

[plugsandsparks]

36KVA will work out at around 50 amps, so is the distance between where your meter and electricity supply fuses are located and where the barn is 50M ?

 

[GeoffatMM]

Hi again,

OK, the mains meter will be installed in the external garden wall by the main entrance (Point A).

The supply will then go to an intermediary fuse box in a workshop with a run of 25m (Point B).

From the workshop to the main fusebox in the house (Point C) it is 48m and these figures include all risers (I have been generous).

From the house a supply will go to the pergola with the PV on the roof (Point D) for lighting and small power but also an 11kW electric car charger over a 43m run. Finally the power from the pergola PV panels returns to the external meter box over a 42m run.

I have attached a schema. All cables (if you can see them) are red.

Geoff

 

[plugsandsparks]

OK, was wondering how this all hung together. So the electricity supply ends at your wall at the edge of the property boundary and you will be responsible for everything beyond the wall. I am not going to state a cable size as you are in France and i don't have their regs, but fortunately electricity does not know what country it is in and behaves predictably all over the world, lol

So these are relatively long runs so volt drop will factor more than current carrying capacity. You will need to size the cable from C to D based on the higher load of full solar going to the house / workshop.
15KW solar equates to around 22amps, however solar is more sensitive to volt drop. So even though a small 2.5sqmm cable could carry the current, the voltdrop would probably max out the inverter to convince the solar power to feed the house. If it was in the UK 10sqmm would be the minimum i would consider. I know you mention running the solar back to the meter wall but you don't need to, it can share the same cable to the house.
The cable from house to workshop to meter is some 73M, absolute minimum will be 16sqmm, i would probably use 25sqmm as you may want your solar to reach the workshop at full power.

Thing is though, those distances are not that great that you could not duct it first, make the ducts minimum 150mm, then you can pull in what you like after the groundworks have been finished. That would be about the right time to get a local electrician in to confirm your calcs to meet local regulations. There will be specific earthing requirements to all these buildings, again local regs apply, so you also need to take that into consideration. Hence ducts are a good step at this stage. Hope this helps

 

[GeoffatMM]

Hi Dave,

I did a reply last night, but the system would not let me post it! I was not logged in and then it lost everything I had typed.

It was just to thank you for your time and effort, helping me and to let you know that I have studied the French regulations so earthing et cetera should not be an issue.

Also the power from the PV panels goes directly back to the Linky box which has a second connection for generated electricity. It then calculates the difference between generated and used and either charges me or pays me for the surplus.

Your thoughts have confirmed my calculations and I had planned to use 16 mm² cable for the main line and 10 mm² for all other uses , both to and from the pergola area.

Again, many thanks for the time you have invested in helping me, it is much appreciated.


Geoff.