Cable Sizing

[synergy power]

Hi everyone,

I've just had my electrician run the AC Cable from the inverter to the consumer unit but i think he has used the wrong cable size.

Background Information- Its a 4 kW (Sharp NDR-250A5) system split over two strings running to a SMA SB 3800 inverter on a farm outbuilding. From here the electrician has used 2.5mm SY cable suspended overhead to the farm house about 20 Metres away. I would say the total AC length is around 30-35 Metres, my main concern is the 2.5mm SY cable not being adequate enough for the distance.

Off my head i would have thought 6mm would be enough? Could i have your opinions please

Thanks

 

[yellowvanman]

Using your dimensions and using Sunny Design, at full wack the voltage drops are as follows:
2.5mm 2.76%
4.0mm 1.73
6.0mm 1.15
10.0mm 0.69%

The recommended voltage drop is 1% max, but 6mm is certainly better than the 2.5mm he's installing.

 

[BruceB]

Page 22 here in the manufacturer's installation instructions says 4mm is good for up to 16m. So you are going to need 6mm2 or 10mm2 as per yellowvanman's calc.
http://files.sma.de/dl/5691/SB33_38-IEN111851.pdf
Regards
Bruce

 

[fiardor]

The IEE regs say 5% volt drop.
The DTI guide say 1%.
And SMA say if i remember rightly 6mm for 3800/4000TL.

Obviously the manufacturers guidelines should be adhered to first, but as long as you can show that you have made all efforts to comply with the regs (5%) you would not be in the wrong.

However, i always err on the side of caution and would use 6mm.

Can i assume SY 2.5mm cable as you call it is steel wired armoured?

Suspending anything else across that distance would be a no no for me.

 

[Earthstore]

Hmmm, cables are not my thing, however I am always interested to find things out.
I would suggest you Google SY cables, one point is, they are not suitable for outdoor use as they do not comply to regs(just what I read).
Also I would take note from the experts on here, they know their onions..

 

[fiardor]

OK, after Earthstores advice i googled SY and found this:-

Used as interconnecting cable for measuring, controlling or regulation in control equipment for assembly and production lines, conveyors and for computer units. Suitable for fixed installations or for flexible use in conditions of light mechanical stress. Can be used outdoors when protected (which is a clear plastic coating), and in dry or moist conditions indoors.
The braided screen offers the best possible protection against mechanical damage and offers a level of Electro-magnetic shielding. The galvanised coating helps protect against corrosion.

So?

The cable construction seems ok but the cable size is still in question and for 6mm SY cable 50 metres is around £200 which is expensive or 3 core swa is around £75.

 

[yellowvanman]

The IEE regs say 5% volt drop.
The DTI guide say 1%.
And SMA say if i remember rightly 6mm for 3800/4000TL.

The 5% in the regs is quoted for 'general' circuits. You need to keep the voltage drop on ac circuits for PV to a minimum because the inverter will only have a finite voltage band above the maximum from the grid, and if you work on the basis of 5%, the maximum grid voltage can be 230V + 10% = 253V add another 5% (@230V) then that adds another 11.5V, so in worst case conditions the inverter will need to have a terminal voltage of 264.5V. Nomimal voltage range of the SB3800 is 180V to 265V so getting close to the limit and it wouldn't take much for it to 'trip out'. On a bright summers day it potentially will be kicking more current than used in the calculations and so incurring a bigger voltage drop over the cable and I would be pretty sure it would trip out just when you don't want it to .

Personally I wouldn't use SY cable for such an application - SWA or NYY. If he has used SY then the braiding needs to be earthed!

 

[synergy power]

Thanks for all the information everyone has written. I am back on the job tomorrow taking the 2.5mm SY cable down and changing it for 6mm SWA.

 

[synergy power]

Also, anyone know of any good suppliers of SWA cable?

 

[moggy1968]

try these guys
Electrical Supplies from Electrical Wholesaler - Discount Electrical
as yellow van man says, 5% is for general circuits not PV. PV is 1% and your way over.

 

[Earthstore]

Told you they know their onions....
What a great forum this is...

 

[SolarCity]

Just to add, I would strongly advise sticking to the 1% rate - especially on long runs. I know a lot of installers relax these rules a little but if ever you get issues with high grid voltages, you want to be sure that you've done all you can your end.

Yes, it is expensive but not as expensive as changing the cable at a later date.