Sizing DC cables, PV Array to Inverter. Confused. Help please!

[screwdriver]

You may find that engineering for the best days will mean the inverter is always in limits with voltage and current so the tracker can work efficiently, I have seen cases where the AC has been under engineered ( 2.5mm 40meters through long loft) and the inverter has been tripping on the brightest days with output overvoltage so there has to be a limit set to how close you get to the edge!

 

[Mark42]

…
On the double tick, I suggest you are over-interpreting:
- you do use Voc, but only to check insulation levels;
- you do use Isc, but only to check it will not melt;
- and you use mpp voltage and curent for the voltage drop calc.
…

Then it should bloody-well say so, because ‘over-interpreting’ is exactly what non-practical inspectors with their clipboards do!

… look at an output curve for a panel and you will see Voc and Isc can never occur together. Where would the power come from? (my emphasis) …

Exactly! Well said, Sir!

 

[Mark42]

You may find that engineering for the best days will mean the inverter is always in limits with voltage and current so the tracker can work efficiently, I have seen cases where the AC has been under engineered ( 2.5mm 40meters through long loft) and the inverter has been tripping on the brightest days with output overvoltage so there has to be a limit set to how close you get to the edge!

I agree, over-voltage tripping is a well-known phenomenon, and the reason I have (during my one-week steep learning curve!) come to definitely support long DC over long AC runs.

The problem of keeping voltage drop down on a long AC run on a good day, especially in an installation where the mains voltage tends to be on the high side, is a considerable engineering challenge, and one that can only be solved in one way: copper, and lots of it.

But I was talking about my long DC run. And so what if a little power is wasted on the best days? The chances are the inverter wouldn't be able to use it anyway.

 

[dpelec]

I always refer to sunny design version 2 and on there you can enter your cable sizes for both the ac and dc side and also length of run and the rest is worked out for you.Less than 1% on dc and ac side a thumbs up.

 

[Mark42]

OK. Let’s try to get a consensus here.

From the DTI guide:

2.1.2: Use (total) Voc and Isc to specify ‘components’, ie switches, connectors, cable type, etc, using safety factors of 15% for voltage, and 25% for current.

2.1.4.1 Cable sizing: Apply the safety factors as above (plus any BS 7671 derating). It’s not specified which parameters to use for the calculations though, which I think it should.

As Screwdriver says in his excellent post #5, Imp and Vmp are used here.

I hold the DTI guide is wrong not to state this clearly.

2.1.4.1 also states, clearly, that the Voltage Drop between array and inverter should be less than 3% (one tick therefore a recommendation.) So working on a 2.99 % drop for your dc calculations is fine.

2.1.6 is titled ‘Main d.c. cable’ but is actually (I think) talking about panel interconnects and other local array wiring, which is confusing. Using, again Voc and Isc, it applies only to cable type; it’s nothing to do with cable sizing per se. This is where I went wrong in the beginning.

Does anyone have a referenced formula, in metres and mm[SUP]2[/SUP] (not, like most on the web, AWG & feet) for DC volt drop calcs in underground SWA please?

 

[infinity]

Recommended is 1% on ac and dc.

3% on Dc in guide? 1% AC

 

[yellowvanman]

3% on Dc in guide? 1% AC

Yes I see that now. The Logic PV Course recommends 1% on each, thats were I got my info from!

 

[screwdriver]

OK. Let’s try to get a consensus here.

From the DTI guide:

2.1.2: Use (total) Voc and Isc to specify ‘components’, ie switches, connectors, cable type, etc, using safety factors of 15% for voltage, and 25% for current.

2.1.4.1 Cable sizing: Apply the safety factors as above (plus any BS 7671 derating). It’s not specified which parameters to use for the calculations though, which I think it should.

As Screwdriver says in his excellent post #5, Imp and Vmp are used here.

I hold the DTI guide is wrong not to state this clearly.

2.1.4.1 also states, clearly, that the Voltage Drop between array and inverter should be less than 3% (one tick therefore a recommendation.) So working on a 2.99 % drop for your dc calculations is fine.

2.1.6 is titled ‘Main d.c. cable’ but is actually (I think) talking about panel interconnects and other local array wiring, which is confusing. Using, again Voc and Isc, it applies only to cable type; it’s nothing to do with cable sizing per se. This is where I went wrong in the beginning.

Does anyone have a referenced formula, in metres and mm[SUP]2[/SUP] (not, like most on the web, AWG & feet) for DC volt drop calcs in underground SWA please?

Imp and Vmp are only to be used to confirm voltage drop, the minimum cable size must be worked from isc x 1.25 x correction factors for bunching/heat/insulation etc as that is the current that will flow under fault conditions.

Basic formula for volt drop?

the chart is in the BS7671 for DC volt drop

metres x Imp x volt drop(in mV/A/M which can be found in BS7671 )

so a 150Metre long 2 core 10mm pvc swa cable carrying 8.43 amps is going to give:

150 x 8.43 x 4.4 = 5563.8mV or 5.56v of volt drop which you can work minimum system voltage by dividing by 3 and multiplying by 100, it comes to 185v which assuming your panels were 30vdc each would mean you would need more than 7 to have a high enough voltage to comply with voltdrop limit of 3%





The table for 2 core swa is

1.5mm 29mV/A/M
2.5mm 18
4 11
6 7.3
10 4.4
16 2.8
25 1.75
35 1.25

Now I haven't used a corrected Imp in my calculations for volt drop because I believe that the inverter will keep the panel at it's optimum power voltage/current and therefore will compensate for temperature differences my moving the mpp around to suit although I would have thought that for the majority of the time the vmp/imp is where we should be looking for normal running conditions, I really don't mind being corrected on this one, although the increase in current is so low in comparison to the voltages I doubt it would begin to make much difference unless you encountered a massive cable run!

 

[Gavin A]

4mm double insulated single core cable is the maximum you are likely to ever need to use on the DC side of a sub 4kWp installation unless it's in a field or something silly like that.

in open air it can take 32amps, and the volt drop is negligible particularly on this system because the voltage is so much higher than standard AC runs.

Most situations will be fine with 2.5mm, but as the price difference is minimal we tend to stick to 4mm cable across the board to avoid any potential for a cable ending up undersized by accident.

http://portal.segen.co.uk/reseller/docs/Datasheet 2491X Cable Spec.pdf

do the calcs anyway, but you'll swiftly come to the same conclusion.

 

[SolarCity]

Yep, we use 4mm as standard. We use 6mm every now and then.