Your cables are seriously under used.
10mm2 can do 80A DC easy (SANS table show it as 98A at 60Deg)
Make a combiner box close to the panels. Then combine the strings in there using a DC fuse with 80A or lower and have just one set of cables to your MPPT.
In your case you already have all those cables, so you might as well use them.
Eina that must have costed a few bucks.
1.) Isc <> Imp work on +/- 8.5A ish peak.
2.) Current in series panels does not double. Two panels in a series string is still 8.5A.
3.) Three strings = 3 x 8.5A = 25.5A peak.
4.) 6mm wire and existing MC4 connections could easily handle this. I assume you had to use an unnecessary connection ( read: weak point), just to cater for 10mm2. You cost yourself money and the result was not as weatherproof…
5.) With 2 panels in series/ string your Vmp is probably around 80V. So your peak input power is 25.5A x 80 V = 2040 Watts peak. Of which unless, you have perfect tilt and direction you are going to see around 60% of. (1225W).
So working on 1225W/6 panels = 200W/ panel peak.
You have 10mm2 cabling, you may as well use it now.
Without historical measurements, I’d say you will get around 2 strings of 2 more panels in parallel/MPPT using existing cabling.
You’ve stated the new strings are going to face a different direction so maybe 3 strings, but easily 2.
I am pretty sure, you’d still be leaving unused capacity on the table with only one extra string/MPPT.
Sometimes I wish I was an MPPT salesman, people must just throw money at you.
I like to over engineer! I knew I didn’t need that size cables, but I like to prepare for any other possible scenario.
Well at least you don’t need any extra cables if you want to add panels now 
Aaah yes, you are right, I was too hasty in my response.
No idea what you mean here? Please explain? Maybe I’m wrong about the 10mm2 size but I don’t think so. But they fitted just fine into the MC4 connectors. See the pic below with a Bic pen for comparison.
I think I agree with you here, but now space becomes an issue. But this is good to know.
I don’t have a bic pen to compare 
but I think that is 4 or 6mm which would make sense as that is the size cable the mc4 connectors take
I am not familiar with the Bic pen measurement standards, but 6mm2 was the biggest conductor I could fit into an MC4.
Perhaps you used 1 &1/4 Bic’s?
Don’t rush to measure, safety first. No faster than a furlong/fortnight.
There, fixed … … AS die son skyn!
I’ve recorded some exceptionally poor days recently! Especially since my wife decided a heater and tumbledryer are necessities…
They are … trust me on this. Just shut up and pay … it ensures a longer more comfortable life.
See, I tried to go there … nope, not again … now I sit and have a beer … that she brought me. 
Haha! Yeah I didn’t even try to go there. But it is amazing how bad a few days of rain like this can make your PV system seem. At least I harvest ample rainwater… into a Jojo that has been full since the first rains… 
Reminds me of when people tell me "I’m going off-grid and basta, I’ll show Eskom/ANC!
I then sit there silently smiling thinking “Mmm, another one who’s got no idea what shiite they are heading for.”
Since April this year, o man, Cpt has had week upon week of cold rainy spells. I’ve NEVER seen my winter production this low EVER.
Geez man, just last winter, and the one before, I saw I needed 500w more … what a joke … most days I watch and see 300w coming from 4.2kw AFTER I added the extra 500w! 
Offgridders, the real ones, they get this challenge re. fickle generation, as it is in their bones.
Me, it is like naaa, Eskom kan maar gooi boet, Eskom kan maar gooi.
You know what … basta this rainy weather and nonsense … I must get one of these and be done!
Haha! I’d rather just get one generator… but boy… if money was no object, I’d certainly waste it on things like this rather than a super yacht!
General observation to no-one in particular:
To my mind, there is far too much focus on the ancillary bits.
The bits don’t make any power they just cost money.
Make the power first, get those panels in the sunshine, the ancillary stuff follows on from that.
It seems as if often the inverter, the MPPT’s, the batteries etc are the system drivers, and the panels are the afterthought.
The problem should be I have X power how can I deal with it, not I have X something else but not enough power.
More panels, more panels, always more panels first.
so is that what you thought when I said I’m going off grid… ill show you
That fuse board on the right, to me, is the prettiest thing in the whole picture.
YES, it IS!!!
I thought about this … <300w from 4.2kw array peak sun hours in Cpt winters for days on end … naaa. won’t work. 
I think TTT should have built a DIY battery bank for them… far cheaper!!
