Sums Check on Cable Size for Remote Solar Array.

[clanky]

Hello, hopefully ive got this right but if someone can check over my maths for me, id appreciate it. Im needing to install a new supply cable from my remote solar array and because of the distance, it needs to go underground for about 35m then into a nearby barn where it will be joined into a JB then run clipped (25m) to a perforated cable tray to another JB, then back underground to the main house for about 15m.

So 4kw solar array : 4000/230 = 17.39A

from table 4D4B, (ref method of D and E) 10mm armoured cable gives 4.4 mV/A/m drop so:

Total VD = (17.39 x 35 x 0.0044)+(17.39 x 25 x 0.0044)+(17.39 x 15 x 0.0044)

Giving a total of 2.68 + 1.91 + 1.15 = 5.74 which equates to a VD of 2.4%

Im assuming of course that theres no penalties for the joints in the two JB's and that the accumulative way ive added up the losses in the separate sections is correct?

I know 10mm is excessive but i have read somewhere (and it has been mentioned on here) that VD should be around the 1-2% for the inverter. Although this is still 2.4%, going to 16mm seems excessive or is that what i should be doing?

Thanks in advance.

C

 

[pc1966]

Unless you absolutely need the junctions, try and avoid them. If you do then look at proper metal adaptable boxes (of suitable IP rating) and glands, and then connections inside that can accommodate your cable size. Most Wagos will only take 4mm or 6mm so for 10 you might need to look at others like spring-loaded DIN terminals if you want something moderately maintenance-free.

Someone asked about the cable size a while ago and I made up a spread-sheet to look at best-cost option. This has not been checked, but here are the results:

Electricity cost​	27.9​	p/kWh​	​	​	​	​
Solar power (max)​	4​	kW​	​	​	​	​
Solar max current​	17.4​	A​	​	​	​	​
Solar average/max for I2R​	0.25​	(ratio)​	​	​	​	​
Solar average power​	1​	kW​	​	​	​	​
Solar average current​	4.35​	A​	​	​	​	​
Expected lifetime​	20​	years​	​	​	​	​
Cable length​	75​	m​	​	​	​	​
​	​	​	​	​	​	​
Cable size​	2.5mm​	4mm​	6mm​	10mm​	16mm​	​
Resistance (per unit length)​	14.8​	9.2​	6.2​	3.7​	2.3​	Ohm/km​
Resistance (installed)​	1.11​	0.69​	0.465​	0.2775​	0.1725​	Ohm​
Loss power​	20.98​	13.04​	8.79​	5.25​	3.26​	W​
Lost annual energy​	179.4​	111.5​	75.2​	44.9​	27.9​	kWh/year​
Annual cost​	50.1​	31.1​	21.0​	12.5​	7.8​	£/year​
Over lifetime​	£1,001.08​	£622.29​	£419.37​	£250.27​	£155.57​	​
Volt drop (max)​	19.30​	12.00​	8.09​	4.83​	3.00​	V​
Volt drop (max %)​	8.39​	5.22​	3.52​	2.10​	1.30​	%​
​	​	​	​	​	​	​
Cable cost (per unit)​	£1.74​	£2.36​	£3.08​	£4.75​	£7.09​	£/m (Superlec LSZH Apr 2022)​
Cable cost (installed)​	£130.50​	£177.00​	£231.00​	£356.25​	£531.75​	​
Total cost (cable + loss)​	£1,131.58​	£799.29​	£650.37​	£606.52​	£687.32​	​

So it seems 10mm is most cost-effective over 20 years. Again, assumptions on solar power pattern (and resulting I2R loss which differes from IV power) are in there and not necessarily true, but probably not crazy.

 

[clanky]

Ok, interesting. But apologies, im not sure what you mean by the I2R loss? Also, where does the 3.7 come from?

Thanks, C

PS joins will probably be done on a din rail or similiar inside a rated box. Makes life simpler i think.

 

[pc1966]

Ok, interesting. But apologies, im not sure what you mean by the I2R loss? Also, where does the 3.7 come from?

I^2 * R since voltage drop is V = I*R and power is I*V = I * (I * R) = I^2 * R or I2R for want of a fancy way to do superscripts.

3.7 in the cold resistance value in mOhm/m = 0.0034, your 0.0044 is the hot and/or AC value.

PS joins will probably be done on a din rail or similiar inside a rated box. Makes life simpler i think.

A decent enough idea if you really must have a join.