Good cable size for 4kW & Why 4 core ?

[peterpiper]

I've been asking a few installers for UK grid PV system quotes, saying I'd trench or lay in the cable from panels & inverter to outbuilding containing consumer unit, as it's around a 180m run.

One has said the cable should be 10mm² 4 core, or two x twin core.
Another suggested 16mm²

I reckoned that the extra cost of 16mm² over 10mm² would take over 11 years to recoup. A bit too long if I've got the sums right. That was for 3 core cable, run 200m so used core length 400m.

Also did calculation for cost of 10mm² over 6mm², repay time 5.5 years. Seemed reasonable.

Why use 4 core?
Would it to be for doubling up, 2x live & 2x neutral feeds, or LNE & signalling? I'm new to this & haven't come across any reference to 4 core power cable. (I'll be using thin 4 core for RS485 comms - if I get an Aurora inverter).

If it's for doubling up, to reduce cable loss, does that mean an earth connection isn't required between the inverter and metering /consumer unit.
I suppose that over a long distance an earth wire might be superfluous, or even cause unwanted loop ripple current. Also incoming grid supply is just 2 core.

[guydewdney]

Just a note - you can use 'old' coloured cable - available from your local scrapyard - as long as you colour the ends appropriately - save a bloomin fortune!

I have reels of 3 core and 4 core swa wire - most of it current colours - all from the scrappy. That might alter your payback times.

Still dont see why 4 core - unless you are running 2 inverters @ 3.6kw each - in whcih case they are looking at 2 phases.



edit - your avatar is really creepy....

[peterpiper]

The old colours were much nicer, but no luck finding at Steptoe's yet.

Einstein/Marilyn close/distant; still/moving... it's better distant.

[clivejo]

If it is 3 phase, the fourth one would be neutral or earth, which are kinda the same thing.

The old colours used to be Red, Yellow, Blue, Black

The new euro colours are Brown, Black, Grey, Blue

[BruceB]

google tlc cable calulator if you have not found it already.
Putting your numbers in (4kw, 180m and SWA 2 core) gives a cable size of 16mm2 and voltage drop of 3.9%.
At that length you are limited by 5% voltage drop rather than cable capacity.
So what they are suggesting is use 16mm2 at 2 core.
Or double up by using 10mm2 4 core.
Have a look at the prices on the TLC site - not that much difference in the options.
Provided your total generation meter is at the inverter end of the cable then the voltage drop/power loss is not that much of a problem in monetary terms I think.
Then there is the discussion about a third or fith core for your earth or use the armour...

Regards
Bruce

[peterpiper]

Thanks for suggestions.

__________________________
It's just single phase. So I guess it's 4 cores to double up conductors.

__________________________

Found the tlc site. Also see branches stock lengths up to 1,000 mtrs in some sizes, would be good to avoid having a join.

The tlc calculator for 4kW, 240V 200m comes up with
Required Cable Size    16 mm
Voltage Drop    8.33 Volts.
Percentage Drop    3.5%
Current Load    16.0 Amps
Max Cable Load*    75.0 Amps

8.7V at 17A gives 0.51ohm, power loss 148W.

200m of three core SWA (using per 100m price) is £988 for 16mm², £702 for 10mm². A £286 difference.

____________________________
My calculation (used BASEC spec as had all the figures):

The BASEC 16mm² cable spec http://www.neweysonline.co.uk/basec-standard-cable-black-100m-copper-16mm-three-core/1050300610/ProductInformation.raction
quotes resistance of 1.15ohm/km@20C. So 400m is 0.46ohm (similar to above)
For BASEC 10mm² resistance quoted 1.83ohm/km. So 400m is 0.73 ohm.

BASEC costs, for 200m, are £874; £626. That's £247 more for 16mm².
 
To make that up, at top FIT solar rate, would mean supplying £247/ £0.413 = 598 kWh more.
 
4kW at 240V = 16.6A RMS.
Loss at 16.6A over 200m run (400m wire) I²R : 16mm²  127W; 10mm² 200W
 
At loss difference 73W, to feed in 598kWh more would take  598kWh / 73W = 8,192 hours.

I believe 700-750 return hours is quoted for here (North Scotland). Using 725 hours gives annual production of 2900kWh. Twould mean over 11 years for return on cable investment.

Looking at it another way, 725 hours annual production is 2900kWh, with annual cable loss of 92kWh/£38 (16mm²) or 145kWh/£60 (10mm²).
 

I got the return on 10mm² cable over 6mm², to be 5.5 years. That seems reasonable.

Doubling up 10mm² gives a loss of 100W, and costs £378 more than 16mm².
£378 at £0.413per kWh is 915 kWh. At the 27W loss difference this would take 33,898 hours - nearly 47 years! Don't think I'd see any saving.

Also ground temperature is usually below 15C, and 4kW (without tracker) would only be produced at peak hours. Would guess maybe >3kW for <3hours a day. So losses will be lower than above for most of the time.
________________________

The only problem I can imagine, is that the grid-tie inverter (anti-islanding protection) might detect resistance / voltage drop as a fault and so cut out.
Any ideas on UK grid-tie limits?

I was considering the Aurora PVI-3.6-outd-s, as its IP65 and includes RS485 comms, good over the distance involved, and there's the free Aurora communicator pc prog.

[stephendv]

Couldn't you move the inverter next to the consumer unit so that the long cables would carry 500+ VDC?  Much thinner cable needed.
(I'm not an electrician, or familiar with UK electrical regs - so take suggestions with more than 2 grains of salt)

[Stuart]

Any buried cable has to be RCD protected these days.
Earth the armour of the SWA at the inverter end.
You don't need a earth conductor in the cable as you will have to ensure an adequate earth at the inverter end.
As Guy says double up cores no problem, just make sure there colour marked at the other end.

so 16mm 2C SWA would be a resonable price. Think you'll find a bit Newey's cheaper than TLC mind.

Stuart

[camillitech]

Any buried cable has to be RCD protected these days.
Earth the armour of the SWA at the inverter end.
You don't need a earth conductor in the cable as you will have to ensure an adequate earth at the inverter end.
As Guy says double up cores no problem, just make sure there colour marked at the other end.

so 16mm 2C SWA would be a resonable price. Think you'll find a bit Newey's cheaper than TLC mind.

Stuart

Think you'll probably find the quality of the cable better too, I got a roll of TLC to finish off my hydro as it was cheaper than Newey due to extortionate delivery charge.

The outer PVC sheathing was thinner, the SWA was thinner and looked like BZP rather than galvanized, the insulation also appeared to be inferior. It was 'fit for purpose' but certainly a few notches below in the quality stakes.

Cheers, Paul

[peterpiper]

stephendv
Hadn't thought of that - more used to dc systems with voltage < mains RMS. I suppose the 4 core could then be for two strings.
Havn't done calculation but putting 4kW @500V &200m into the tlc calculator comes up with:
Required Cable Size    4 mm
Voltage Drop    16.0 Volts.
Percentage Drop    3.2%
Current Load    8.00 Amps

Max Cable Load*    36.0 Amps
So even less reason for a doubled up (4 core) 10mm²


Stuart
So do you have a dc RCD at panel end if inverter is by consumer unit?


camillitech:
Nice to hear about quality.
Newey's nearest branch not too far, so could save on carriage.
Not sue though if Newey's do lengths >100m, are there alternative suppliers who do?
I've no experience with 'poxy potted joins.

[mespilus]

Don't forget resellers are going to want any sales they can get,
as they are probably, (at Head Office), on volume related retrospective rebates.

Ask for quotes in the last week of a month,
or,
better still if you have the time,
the last week of a quarter and let some desperate
salesman(ager) meet his target by giving you special 'one-time-only' discount.

[JohnS]

Another thing to consider is grid voltage at your site.

The inverter will run at grid voltage plus cable loss voltage.  If grid voltage is high, it might cause the inverter to trip out due to over voltage protection.

You will probably fijnd it cheaper to install a big cable than to get the DNO to lower the grid voltage.

John

[Stuart]

yes you would need a RCD in that case, As DC losses are higher best to have inverter close as possible to panels.
bigger cable is going to be cheaper than DC RCD's  

All proper wholesalers sell SWA by the metre, but phone and get price, they usually cut the cable at the bigger branches and deliver next morning to the branch. glands come in two's I'd recommend indoor ones, less faff

[EccentricAnomaly]

As DC losses are higher...

We keep having this discussion but I don't recall anybody giving a good reason why this should be true, particularly not why there should be lower losses for 500 V DC vs 240 V AC.

[Other-Power]

As DC losses are higher...

We keep having this discussion but I don't recall anybody giving a good reason why this should be true, particularly not why there should be lower losses for 500 V DC vs 240 V AC.

Agreed.

Only thing required for DC cable is thicker insulation.

Jonathan