project solar (Ie 50w or under) 20w in this case

[MR GUS]

 Just a point that needs clarifying from learned solar types please.
 
.5  3 core flex suffice for setting a small set of panels together?, i'm looking at a direct comparison to the Navi item,

PVCABLE4MMX100M
PV Cable - 100M X 4mm

(the reel is for comparison, not a length that's required)

roast me as required!  

I do need to get my Mike Boxwell book back though ..as i'm bound to be making a tit of myself! ...memory retention (shot)

[Ivan]

Depends what you want to do with it. The PV cable you'll see on the Navitron website is double-insulated and high-voltage rated - so suitable for grid tie systems. Mains cable is fine for low voltages in terms of insulation. Work out what the highest current you are likely to generate from your PV, and make sure that the cable you're using is rated for this continuous output, bearing in mind how long the cable run is going to be. I don't know what cable you're using (is that 0.5mm2 cross-sectional area?) or what length of cable or what wattage/voltage PV you're using, so I can't tell you if your cable is good enough or not.

[Justme]

Work out what the highest current you are likely to generate from your PV, and make sure that the cable you're using is rated for this continuous output, bearing in mind how long the cable run is going to be.

Whilst what Ivan has said is correct its not the complete picture.

Cable ratings are based on its capacity to carry the amps without heating it up to much. You should be more concerned with its resistance & thus its volt drop over the distance & amps that you are using. You still need to ensure that its amp rating is above what you need (which with a low volt drop it will be).

A very quick & simple way to work it out is A x D / 12 = cable size in CSA mm2, where A is amps D is distance in meters one way & / is divide by.

For easy of working out in your head instead of dividing by 12 you can divide by 10 & get even lower V drop.

Oh & online cable calcs are normally set up for a 6% volt drop over the distance. With PV & especially low volt DC you want to be aiming for sub 1%

So a load of 8 amps over 15m would need 10mm2 csa (or 12mm using the easy calc).

[MR GUS]

Thanks folks, i'd prefer to over-bodgineer rather than under, so will do math when I get some more bits & bobs, currently reconditioning the freebie battery for a few days then will rootle around the garage, Typically my sparky mate range me up to announce he was off on holiday in 3 hrs (yesterday) so cannot get my book back & do proper math, so bear with me on the "stupid" front*

*Stupid is not asking questions when wholly unqualified! 

[SimonHobson]

And just to add another factor, the "current capacity" of a cable depends on the installation method and conditions. So for example, a calp clipped direct to a wall aqnd exposed to free air has a higher capacity than one buried in insulation, one exposed to high temperatures (as you might get around your panels on a windless sunny day) has a lower capacity than one that will never be exposed to external heat.

And of course, for every type of insulation, there is a maximum operating temperature which you must never exceed (this is in fact the primary limitation on carrying capacity). So ambient plus self heating effects must never raise the insulation above it's limit.

And you have to consider all that under fault conditions, taking into account the rating and characteristics of any circuit protection.

It's not for nothing that "properly qualified" electricians (which I am not) can take several years to pass all their C&G exams.

[acresswell]

It's not for nothing that "properly qualified" electricians (which I am not) can take several years to pass all their C&G exams.

I understand where you're coming from, but in reality I reckon most electricians doing domestic work never do a single calculation themselves. 

They rely on using the same six types of cable (1.0 / 1.5 / 2.5 / 4.0 / 6.0 / 10mm sq T&E) and then the same few sizes of MCB (6A / 10A /16A /32A /40A / maybe 50A) and just know for each of those how far they can go when surface-clipped, in a wall or under insulation. They've memorised about 50 different possible combinations (in theory there are 108 combinations - 6 x 6 x 3 - but nobody is going to use a 50A MCB with anything less than 10mmsq cable), and then just select from those.  In reality, this could be put as a 1-page summary on a sheet of A5, but the electricians remember it because it's convenient to do so. 

In my experience (3 re-wires in the last decade... different houses!), electricians doing domestic work have to "phone a friend" or "look it up" when they get outside these normal situations... they're not actually doing the calculations. I haven't had a re-wire in the last couple of years since iphones became so commonplace, but I suspect that most electricians with an iphone have a cable calculator website on their list of favourites e.g. http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

The situation for non-domestic work is probably different: because of the wider range of scenarios that electricians may come across, they're probably less able to just refer to a quick table (mental or otherwise)

I definitely agree that the regs are complex and complying with them takes training and appropriate qualifications are important,  but I don't think cable selection is one of the particularly complex bits on a day-to-day basis.

[SimonHobson]

It's not for nothing that "properly qualified" electricians (which I am not) can take several years to pass all their C&G exams.

I understand where you're coming from, but in reality I reckon most electricians doing domestic work never do a single calculation themselves. 

They rely on using the same six types of cable (1.0 / 1.5 / 2.5 / 4.0 / 6.0 / 10mm sq T&E) and then the same few sizes of MCB (6A / 10A /16A /32A /40A / maybe 50A) and just know for each of those how far they can go when surface-clipped, in a wall or under insulation.

I wouldn't disagree with that - it seems you can become "Part P qualified" with a 5 day course which rather devalues the whole idea Note that I did say "properly qualified".

When I had a PIR (Periodic Inspection Report) in my flat, the electrician (who is one of the brighter and qualified kind) said he'd have to do some calculations (when he got home) - he thought the size of fuse protecting the submain from meter to CU was too large. He was right - his calculations showed that it should have been a 63A fuse, not the 80A installed. I guess on-site he knew it wasn't within his "crib sheet of regular figures" but didn't have everything to hand to calculate it there and then.