Cable from loft to CU

[moggy1968]

OMG, I can't believe what I'm reading on here!!!!!!!!!!!!!!!!!!!!!!!!!!1

solar from a SFS!!!!!!!!!!!!!!!!!!!!
Solar always run in 2.5mm!!!!!!!!!!!!!!!!!!!!!!!!!!!

what the hell is going on out there!!!!!!!!!!!!!!

some inverter manufacturers recommend using 100 or even 300ma RCDs so try and route your cable so it won't need an RCD (e.g. externally in SWA or conduit). I have used 30ma and got away with it but you do increase the risk of nuisance tripping and on one install early on in my career I had to go back and reroute the cable to avoid a 30ma RCD as it was tripping, lesson learned!

earthstore - you have fallen into the trap of thinking people out there are doing installs properly and know what their doing, DOH! It's no wonder I'm finding it hard to compete on price if some of the practices above are what companies are up to

 

[DNS1]

OMG, I can't believe what I'm reading on here!!!!!!!!!!!!!!!

Me neither!

I've never ever seen Murdoch ask for help before!

 

[Octopus]

I've never ever seen Murdoch ask for help before!

Asking on this forum is far too hit and miss these days as there are too many "arm chair" bodgers on it.

This is something I will be doing for me, so wanted to tap into the solar guys on here but the advice is varying somewhat so am left wondering what to do!

 

[SolarCity]

Ignore the 2.5mm 'advice'.

4mm at least, and possibly 6mm - which will allow for a 4kWp system.

Make sure it goes on its own RCD (if required) and not a shared one.

 

[Octopus]

Ignore the 2.5mm 'advice'.

4mm at least, and possibly 6mm - which will allow for a 4kWp system.

Make sure it goes on its own RCD (if required) and not a shared one.

The board will be hi integrity or RCBO's so a dedicated circuit is catered for.

 

[moggy1968]

I would suggest having a flick through the appropriate literature such as the DTi Guide for definitive answers, but, as above, minimum 4mm cable depending on the length of the run. Have a look at SMA sunny design for an easy on line calculator. where people go wrong is it's not current carrying capacity thats the limiting factor, it's voltage drop. The limits on solar are different to normal electrics (1% max). You'll need a couple of AC isolators, one at the CU and one at the inverter. You'll need DC isolators when the time comes and, as I said before, if possible wire it so you don't need and RCD. If you do need one then as solarcity says above, it needs to be on it's own RCD or RCBO.

 

[Earthstore]

earthstore - you have fallen into the trap of thinking people out there are doing installs properly and know what their doing, DOH! It's no wonder I'm finding it hard to compete on price if some of the practices above are what companies are up to

No not at all, I believe that all trades should work to decent standards, a few will be exceptional.

As you are aware I am not an electrician, but if I can work out the volt drop on an AC cable and understand the reasons why it has to be less than 1% for PV, then it would not seem unreasonable for an electrician that is installing PV to be able to work this out too.
2.5MM cable... I despair at some of the poor installations that have happened, and you are right "how can you compete with it", still you can sleep at night and I dare say you will survive.

As to the OP's post, you now have several posts from decent installers telling you the same thing, min 4mm, but 6mm to be on the safe side.

 

[moggy1968]

No not at all, I believe that all trades should work to decent standards, a few will be exceptional.

As you are aware I am not an electrician, but if I can work out the volt drop on an AC cable and understand the reasons why it has to be less than 1% for PV, then it would not seem unreasonable for an electrician that is installing PV to be able to work this out too.
2.5MM cable... side.

yes, it's not rocket science is it! Unfortunately though it doesn't seem all trades work to acceptable standards. I think this is partly due to the overselling of PV by the industry to the industry. Lots of people jumped on board thinking it was just another kind of electrics without actually doing their homework on what was involved. There are some in the electrical industry who think because they were 'properly trained' and have been doing it for years there is nothing new to learn and nothing someone can teach them.

before someone lauches in telling me how good they are, I did say 'some'!! certainly not the majority but there is a significant minority

 

[Worcester]

Lost count of the number of 'qualified' electricians I've fired over the last 3 years .......

 

[Octopus]

May be I need to add a poll to the thread so we can decide that way. lol

2.5mm
4mm
6mm

 

[Earthstore]

An inverter has to equal the grid voltage, well a gnats tallywacker over so it can push the generated electricity back into the grid.

So if the grid voltage is at its maximum allowed 253V and you have a 3% AC cable loss then the inverter will need to get to 260.69V to equal the grid, this is outside the allowed parameters of the inverter, so it will shut down.

If you keep the AC loss to 1% then the inverter will be 255.53V, just within the parameters.

Of course if you DNO always run your voltage low, and you never have any peaks then use some speaker wire....

6mm for me...

 

[Gavin A]

OMG, I can't believe what I'm reading on here!!!!!!!!!!!!!!!!!!!!!!!!!!1

solar from a SFS!!!!!!!!!!!!!!!!!!!!
Solar always run in 2.5mm!!!!!!!!!!!!!!!!!!!!!!!!!!!

what the hell is going on out there!!!!!!!!!!!!!!

some inverter manufacturers recommend using 100 or even 300ma RCDs so try and route your cable so it won't need an RCD (e.g. externally in SWA or conduit). I have used 30ma and got away with it but you do increase the risk of nuisance tripping and on one install early on in my career I had to go back and reroute the cable to avoid a 30ma RCD as it was tripping, lesson learned!

earthstore - you have fallen into the trap of thinking people out there are doing installs properly and know what their doing, DOH! It's no wonder I'm finding it hard to compete on price if some of the practices above are what companies are up to

we must have 150 TL inverters on 30mA RCDs (not shared) and never had a single report of a trip, other than after there's been an actual grid fault / power cut, where the RCD seems to sometimes actually see the fault in the 'load' out in the street and trip before the inverter cuts out.

Power-One technical manager agreed there should be no problem when it's a single RCD per inverter (slightly grudgingly).

 

[Gavin A]

May be I need to add a poll to the thread so we can decide that way. lol

2.5mm
4mm
6mm

honestly, use 6mm2 as per manufacturers recommendations - SMA, Power-One for their TL inverters.

what's the additional cost, a couple of quid at most.

 

[moggy1968]

we must have 150 TL inverters on 30mA RCDs (not shared) and never had a single report of a trip, other than after there's been an actual grid fault / power cut, where the RCD seems to sometimes actually see the fault in the 'load' out in the street and trip before the inverter cuts out.

Power-One technical manager agreed there should be no problem when it's a single RCD per inverter (slightly grudgingly).

I think the one I had a problem with was a fronius
I've used 30ma on several different ones since without problems but advise the customer to keep an eye on it and if it's a problem we'll swap it out, but I try and ensure thats an option by not wiring it so a 30ma RCD is necessary. Generally the only reason we need an RCD is on TT installs.

If you wire it so it needs a 30ma RCD and tripping is an issue then your a bit stuffed!

 

[SibertSolar]

Or just use a transformered inverter....