50 kW PV System a.c cable sizing info

[Nickyrox]

Hi there,
I am installing a 50 kW PV system on a barn roof. The barn is supplied by a 4 core 16mm SWA which is partly run underground and is 100 meters in length.
The three phase DB in the barn only supplies the lighting in the barn, all the old circuits have been removed becuase they wasn't being used. So this means most of the PV power will go back down the cable to the origin. There are three other buildings fed off of this point, so the PV should go to good use and not to the greedy energy companys.
the system will be around 16.6 kW per phase and about 80A per phase,I have been trying to calculate whether the cable is big enough to carry all this power over this length.
Getting a little bit stressed with it because it looks to me to be boarder line but don't wont to get this wrong. Anyone got any advice?

Nick

 

[SolarCity]

I'm not sure that 16mm is borderline. I haven't done the sums but I reckon volt drop is the issue here. I would have thought you'd need 35mm2 at the very least.

 

[Gavin A]

stick it in sunny design, use the cable sizing tool, see if it says yeah or nay... don't forget to report back pls cos it does sound borderline, so I'd be interested to know if it is ok for future reference.

I suspect it probably isn't quite as well.

 

[dansk]

Well i used 16mm 3 core over 60m on a 4kwp -so i doubt 50kwp is going to ok over 100m.

 

[SpitfireW'sale]

Hi there,
I am installing a 50 kW PV system on a barn roof. The barn is supplied by a 4 core 16mm SWA which is partly run underground and is 100 meters in length....the system will be around 16.6 kW per phase and about 80A per phase...
Nick

4-core 16mm2 SWA cable is not really suitable for 50kW system because it's rated at 75A. As long as your current is about 80A, I would recommend 25mm2 (rated at 96A) keeping the losses about 2.6% or even better 35mm2 (rated at 115A). The selection of the cable is also based on the MCBs that you gonna use.

 

[Gavin A]

computer says no, as well.

actually, computer says 50mm2, which is going to add a fair wack to your costs.

fwiw, 16mm2 is ok up to around 30m

25mm is, ok upto 45-50m

35mm is ok upto around 70-75m

50mm is ok upto around 100m



(taken from Sunny design, might be worth checking with other software as the SMA inverters tend to be more sensitive to voltage rises than some)

 

[sedgy34]

Correct me if I'm wrong but just read post and feeding back down the swa it's split feeding other stuff so how does that comply to it being on its own circuit

 

[Earthstore]

I feel we might soon have another thread "Why is my inverter going over voltage"
or "why is my system tripping" or "The voltage is too high, should I get the DNO to turn it down":clap:

 

[BruceB]

As long as you have separate mcbs for the lighting and the inverter at the inverter end, then the swa becomes a distribution circuit rather than a final circuit. The inverter must be on its own final circuit.

 

[Gavin A]

we've just been discussing an install we did last November in the rush, which we're thinking is going to be tripping out this summer, as it was a 4kW install on the end of what we now reckon must have been nearer 200m of SWA that was already laid, we think either 16mm2 or 10mm2. We weren't really sure of the cable length when we did the site visit, did test the circuit, and were a little surprised to find that it seemed ok, but our site visit was at the end of a long day installing elsewhere, and we reckon we probably made a mistake with the testing somehow as it's so far outside the sunny design criteria that we can't see how we got the readings we did.

I think I got distracted by the fact that the spark who'd fitted the cable had nipped it in the screw fitting for the enclosure, so there was bare live cable showing when I opened it up to test it which I felt the need to fix.

Think we're going to take pre-emptive action and go down for a visit to see how it's been coping with the days of good weather, and test the circuit again. Could work out as a very expensive error if we have to replace that length of cable though.

erm doh.

 

[pvman]

Another way, and possible cheaper is to run separate swa for each inverter.