Good cable size for 4kW & Why 4 core ?

[Iain]

Hi
AC and DC losses
I believe that for our use there is basically no difference between them but for high power long distance the DC works out with less losses. I think part of the high power AC limit/problem is cable capacitance, not a problem with DC. But for household use I don't think there is much difference. There is a bit more info here if people want to read further.
http://www.solarec-egypt.com/resources/Larruskain_HVAC_to_HVDC.pdf

Iain

[Stuart]

Sorry EA, I was forgetting it was several hundred volts DC, So you can use a thiner cable and still need Type B RCD(s?) rated for the voltage.
 

[peterpiper]

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 is that 5% voltage drop limit due to this? :

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.

from dti http://www.dti.gov.uk/energy/reliability/quality-continuity-supply/index.html
In Great Britain domestic supplies must be maintained within the range 216 volts to 253 volts, corresponding to a nominal value of 230 volts, minus 6% to plus 10%.

The inverter I was considering, Aurora PVI-3.6-OUTD, has spec:
Nominal AC voltage range 200-245V
Maximum AC voltage range 180-264V (may vary to comply with country regulations)

So don't really know what the inverter might allow, need the spec for UK grid tie.
If it was 253V +5% that would explain that '5% voltage drop limit' for the cable (in order to always deliver full power).
With 4kW system and cable Voltage Drop 8.33 Volts (3.5% @230V), if inverter limits at 253V that means the power delivered would reduce as grid voltage rose above 245V.

[peterpiper]

Realised one good reason for 4 core cable, when feeding dc, is using 2 strings.

[BruceB]

The 5% comes from BS7671.   It is the voltage drop you are normally allowed in a circuit.  In general terms, lighting circuits are allowed 3%, power circuits 5%.  When designing a circuit and selecting the cable you might be limited by voltage drop or by current carrying capacity of the cable.  The point I was making was that with those numbers it was the voltage drop that was the limiting factor rather than the current carrying capacity.

The UK spec for grid tie inverters is in G83/1 and the over-voltage figure at which inverters must cut out is 264V

Regards
Bruce

[peterpiper]

Thanks Bruce. Glad someone has all the figures!

Had just come across your post on topic 'Grid Voltage Too High'.
 The (Country) over-voltage limits and disconnection times vary significantly. Some examples from BS EN 50438:2007 (G83/1) are:
GB 264V    1.5s

Where inverters trip >264V and grid limit is 253 that's a 264/253 = 4.35%

Transposing that tripping limit from @264V to 230V 4.35 *264/230 = 4.99% @230V

So cable to BS7671, 5% voltage drop limit, makes sense.

_______________________________________________

The tlc cable length calculator works in a way which suits calculations for loads fed off the mains - a known voltage. I guess for the UK we'd put in 230V. Is that the way BS7671 applies?

When the source is generating power the source voltage isn't initially known. What's needed is a result from the tlc calculator where (voltage drop + 230V) = source voltage.
Calculating to find the longest 10mm² cable run for a 3.96kW system gave (via 6 iterations / guesses):
Temp. 20°C; Power 3.96kW; Voltage 242V; Length 184m; Voltage Drop 12V; Percentage Drop 5%.
This compares to a maximum length of 166m, when Voltage 230V is given.

[BruceB]

The TLC calculator is a rough and ready way of doing the calculations and makes various assumptions on cable temps, installation methods etc.  Rather than have to do iterations, depending on your knowledge of electrical stuff you could set up a spreadsheet to work out whatever you need.  BS7671 has tables which give resistance of cables in milli-ohms per meter which allow the calculations. Two things to be careful of from your previous post:

- Not many places in this country is the actual voltage 230V.   Mostly it is nearer 240V. 
- Be aware that when you are operating the cables near their maximum current then temperatures can go up which increases the resistance which affects the calculation etc.

I suggest you do not try to over-finesse it.  If it looks like you might be operating a cable near its limt then just go to the next size up.  In a simple domestic set-up the price difference should not be enormous and the whole life cost difference marginal.

Regards
Bruce