What size AC cable from loft to consumer unit? - Do you ignore the regs?

[Ramjam]

I have been installing 2.5mm T&E from loft mounted inverter to consumer unit. Cable protected by 16amp MCB

Have you seen page 140 of the regs where it has the formula Iz Greater than or equal to In + Ig where Iz is the current carrying capacity of the conductor and Ib is the MCB rating and Ig is the inverter output?

By that formula we should be using a 4, 6 or 10mm cable depending upon the fixing method.

I can't see the reason for this and I don't think this is common practice, is this just one of those regulations that get ignored???

Perhaps this can be explained away by someone more conversant with BS7671?

 

[telectrix]

you don't say what the inverter output is

 

[BruceB]

Read the regulation carefully - it is not that it is ignored, rather that the second part that you are quoting from does not apply in most cases because the inverter is on its own circuit rather than on a circuit with other loads.

Regards
Bruce

 

[MCS Renewables]

We do EVERY PV system in 6mm or bigger.

manufacturers instructions are more important than the regs.

If you read the sunny boy install manual it gives you a maximum cable length for cable csa.
page 24 allows 4mm to max length of 23.5 m for a 3000Tl
4000TL must be 6mm.
5000TL allows 6mm to 18.6m then needs 10mm.

Basically there are loads of chancers putting PV on 2.5 twin and earth on every project and then loft mounting the inverter......

Our nearest competitors only do systems in 2.5 and always loft mount the inverters but then again their electrician is a one week wonder ...ex lorry driver. laughable really.....

 

[MCS Renewables]

Page 14 of the same manual .......

The ambient temperature should be below 40 degrees at all times to guarantee optimal performance.

20 years of clambering around lofts in the summer with sweat dripping onto whatever I was doing as an electrician leads me to think lofts are unsuitable for inverters...... however the fact that so many start up PV ECO companies are putting inverters in lofts and using subbie sparkies to connect them in 2.5mm cable could well indicate that I have no idea what I am talking about ....time will tell.

 

[DH Electrical]

we always use 6mm YY Cable from CU to Inverter Isolator, that will remove any voltage drop issues, 2.5mm is prone to volt drop onlong runs
DH

 

[JD6400]

At least 4 mm for < 16 A , T+E , surface , not through thermal insulation . Still the margin in pv jobs , why cut corners . Not that i would anyway .

 

[Graeme Harrold]

Got my course coming up soon, and from all the reading Im doing, VD is an important factor and would concur with 4mm min even on short runs. After all its a small cost on the installation value.

 

[Ramjam]

BruceB - I re-read that but I'm still not seeing what you say but what you say makes perfect sense, thank you

I've been following what I was taught on my PV course which really angers me now. I've been considering existing installs and am very pleased that the two loft installs that I've done so far have been less than 1m from the consumer unit (bungalows) so I'm not so worried about the 2.5mm.

MCS Renewables - Do you use the 5000TL? If so in what scenario, surely customers want less than 4kWp or significantly more, do you find a market between 4kWp and 5.3kWp?

I use SMA inverters and I'll get busy with the installation manuals - thanks for that.

Loft mounted inverters - If possible the garage is my preferred location for inverter. I take your point about hot lofts but I understand that loft mounting is very common practice, does anyone know what typical summertime loft temperatures are and correspondingly the operating temperature of common inverters?

dpelectricalltd - I agree 100% - whilst I have clearly made myself look a fool, I've learned something, thanks everyone!

 

[Graeme Harrold]

Have a read through the guide posted here.....


http://www.electriciansforums.net/photovoltaic-solar-panels-green-energy-forum/23511-pv-guide.html

 

[JD6400]

Ramjam , i would say you are far from a fool !
Was told to use no bigger than 2.5 from my so called expert course provider !
On a separate note . Had a look at your fits calculator , reckon it to be the best I've seen yet .

 

[Ramjam]

Hi Graeme,

I just re-read the DTI guide and it offers nothing on the subject of AC cable sizing other than to refer to BS7671.

It does say that the PV plant should be on a dedicated circuit. That then brings us back to sizing the cable for the protective device of the circuit (16amp for G83) which is according to page 282 of the regs means that in most installation methods 2.5mm is sufficient. We obviously need to factor for volt drop <1% additionally.

I will re-read the SMA instructions because as MCS renewables stated the manufacturers instructions override the regs.

 

[Ramjam]

dpelectricalltd - thanks!

 

[edexlab]

I think there's going to be a lot of this going on, course I did was a good introduction to pv and thats all it was!
I went away thinking I knew a bit, and did some more reading etc, and soon realised how inadequate the courses were for installers and that this was'nt something you can just pick up and put down
the more you read, the more you realise how big a subject PV is, that is if you are going to try and do it properly

Incidently I've got a copy of the Dti guide from early 80's and it has'nt changed at all apart from some (i thought) quite useful info was omitted from the 2nd edition, and although the technology has changed little it does'nt seem to have taken any notice of lessons learnt by installers experience since the first one was published