Planning ahead for possible 4kWp solar panel array...... what cable size should I lay?

I currently have the hall ceiling PB down, all the plasterboard out of the airing cupboard on the landing so have access to the stud & partition walls within said airing cupboard. The Consumer unit is in the hall and has 5 spare RCBO ways. (a shower booster pump had exploded spraying hot water all over when it was under a previous owner so all the plasterboard had failed).

I may want to one day put a solar PV array on the roof.

Now it seems a good idea for me to lay some twin and earth cable from a spare RCBO way in the CU (in hall) through the hall ceiling, up the airing cupboard walls into the loft and put a junction box on the end with a suitable label.

The total cable length to the inverter (which I assume will be in the loft) will be no more than 15 metres.

Now my questions are this.....

If I assume a PV array up to 4kWp, that makes it up to 18.18 amps (assuming lossless 100% efficient inverter of course) so that suggests a

20A RCBO and according to
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2.5mm2 cable should be adequate.

Given the 15m cable length and the narrow margin of 2A between the peak current output of the inverter and that the 2.5mm2 cable is rated up to

20A, I am inclined to lay 4.0mm2 cable while the plasterboard is down.

Is my choice of a 20A RCBO OK for a PV array or should I be using the next rating up and hence increase the 4.0mm2 to 6.0mm2 cable?

I have room to leave a few metres of cable in the void behind the CU so the cable can be pulled further into the hall if it has to go into boxes of gubbins mounted next to the CU before finally being wired into the CU's spare RCBO way.

Can a RCBO be used the "wrong way round" or does PV specific RCBO's have to be used? (my CU is a 2 pole isolator main switch with 13 RCBOs so no seperate RCD or MCBs are used.)

Will a FIT accredited installer be happy to use this pre-laid cable or will they refuse to use and lay their own cable in horrible white trunking?

Do I have to make the entire cable run visible to them for inspection purposes?

Will any extra boxes be needed next to the CU such as a generation meter and isolation switch or can all these go into the loft?

Looking forward to the comments...

Stephen.

Reply to
Stephen
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T&E's ratings decrease a good bit if the cable is in a hot area, eg a summer loft.

NT

Reply to
meow2222

Agreed. It also gives you the option to use a 6kWp array if FIT rules change (assuming that panel efficiencies will rise too).

I would have thought that if you find a small firm, and talk confidently about inverters and RCBOs and 4mm cable, they'd be prepared to use a pre-installed cable (whatever the rules say).

Reply to
Martin Bonner

Have a look at the installation instructions for say a SMA SB4000 inverter and it covers cable size, mcb rating, rcd.

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The aim under mcs rules is to keep voltage drop to 1% so you are better laying 4mm or 6mm cable. For 15m on a 4kW inverter SMA recommend 6mm cable. It is best to form the cable run so that no rcd is required. But if one is required (eg TT) you will see SMA say at least 100mA, which makes it difficult to bury in a domestic wall unless you use armoured or protected cable. ABB inverters recommend a 300 mA rcd where necessary. The mcb may have to run for many hours at 16A, so you are better oversizing it a bit. SMA say 'up to 32A', ABB say 20A for a 16A inverter.

MCBs and RCBOs will work the wrong way round as you put it. You connect the PV circuit as any other in the CU. You cannot have anything other than PV on a final circuit with PV on it.(ie no lights or sockets etc)

I am an mcs installer and have used previously installed cables. Make sure you do a proper insulation test before and after you cover the cable in.

Reply to
BruceB

2.5mm is good

In practice, it will not get anywhere near the peak rating. You will be lucky to see 3.8Kw on the odd cold day.

Youi don't want an RCBO, you will have endless nuisance tripping. Just an MCB 20A

And ordinary MCB. Works with current going either way.

They accept anything that saves work and money.

You don't want anything in the loft. Not the meter for reading, nor the inverter, it shortens the life as it's so hot up there (escpeciallt because it will be at full load in sunny weather). also you can't see what the display says

Reply to
harryagain

That is an interesting comment.

taking your point about the inverter not being in the loft, the only practical place it could go is in the airing cupboard and to be honest with you, it will not get past SWMBO who has her own plans for the airing cupboard.

There is no room beside the CU for a big inverter box as there is a door below the CU and the front door is next to this door.

Furthermore, I had understood that the inverter needed to be as close to the PV panels due to the low voltage and high current going from the PV array to the inverter via the thick DC cables. These apparently should be kept as short as possible to minimise cable voltage drop.

voltage drop is of course proportional to the current flowing through the cable rather than the actual voltage in the cables.

Lets assume the copper cable has a resistance of 0.1 ohms from the PV array to the inverter and again from the inverter to the CU

Lets assume we have two possible locations for the inverter. One being at the PV array and the other being adjacent to the CU.

First case:

Consider the PV array cable to the inverter and that the inverter is adjacent to the CU. As I understand it, voltages of around 50V are typical from the PV array. So for a 4kW array, thats a current of up to

80 Amps.

So the voltage drop in the cable at 0.1 ohms will be 8V. That is a power loss of 640W which is 16% of the solar power lost to the DC cable before it reaches the inverter.

Furthermore the DC voltage at the inverter will now be 42V instead of

50V, and you will now have a peak power of 3,360W

Second case:

Now let us consider the inverter being located at the PV array instead. cable between the inverter and the CU. Assuming that the full 4kW reaches the inverter and it has 100% conversion efficiency ..... the output will be 220V at up to 18.18A. So the voltage drop is 1.82V so at the CU, the power transferred will be (220 - 1.82) V x 18.18A = 3966 watts.

So you have lost just 4W to the AC cable if the inverter is at the PV array rather than 640 Watts to the DC cable if the inverter is adjacent to the CU.

Regards,

STephen

Reply to
Stephen

With which I would agree.

Or, you could use a cable section more suited to the situation.

I'm not up to date with the latest inverters, but it is quite common to wire the panels as two parallel strings, halving the current in your calculation.

Chris

Reply to
Chris J Dixon

When we had our 4kW solar array installed 3 years ago, I got quotes from (in the end) 5 different firms. Two (maybe 3) insisted that the inverter had to be in the loft as you lose too much power from long runs of DC cabling. This is simply not true: my panels are in two strings and the peak voltage is over 300 V, so the current is, if anything, lower on the DC side than the AC side. So I was able to eliminate these firms from the running right away as having a totally inadequate grasp of Ohm's law.

Also, as others have said, a hot location is undesirable, and you want to have the inverter accessible to admire the pretty lights and check the dials now and again.

We have our inverter in the garage on a side wall which is entirely suitable - they don't take up much space and at worse hum very gently when working hard.

Reply to
Clive Page

Indeed, but the bigger the cable crosssectional area is, the more expensive it is due to the extra copper needed......

I see.

Reply to
Stephen

Generally speaking the losses in DC cables will be less that that of the AC cables. Inverters typically will want to see 300 to 700v DC according to maker and type and the output nominally constant at 240v AC.

So taking a simple example of equal cable sizes and lengths and the inverter running at 480v dc, The DC losses will be 1/4 of the AC losses.

eg input 480vdc x 8 amps - output 240vrms at 16amps

As cable losses will be proportional to the square of the current and the length you can do the sums to minimise losses by careful positioning of the inverter. Cable losses can be managed/minimise by cable sizes

Reliability of electronic equipment is heavily dependent on operating temperature and by far the most important consideration will be to keep this at sensible temperatures. An airing cupboard will still be at slightly elevated temperatures from normal. Stick it in the garage/utility room etc. Whilst an inverter will operate at loft temperatures its life will be shortened. The economics of PV was based on a 25 year contract (might be shorter now??) having to change a couple of inverters in that period will take the shine off the deal.

Current regulations require that in feed of more than 16 amps per phase will need special permission. Depending on the grid voltage you might get near to 4kW but it is normally limited to around 3.8kW

This is normally managed by the inverter which will be programmed to the relevant standards for the region of use. The installer will just "tell" the inverter the region and it will do the rest.

Reply to
Bob Minchin

that's because of tne higher voltage, not because it's DC.

Reply to
charles

I did not mean to imply that is was. I used to AC and DC descriptors to simply identify where the losses occurred.

Reply to
Bob Minchin

True, but capable of being optimised. However, if you get into angels on pinhead calculations of capital cost against return, you will find that you are having to predict inflation and energy costs for 20 years, which I think we can all agree is a tall order.

My installation does, in fact, have the inverter in the garage. I have 14 x 260 kWp panels, connected as two parallel strings of 7. Looking at the data from the inverter, it tells me that the maximum dc current so far has been 17.95 A, and the average voltage is in the region of 220 Vdc.

An important point to be aware of is that most of the time you are not running anywhere near peak output. Histogram data of those 15 minute measuring periods where some output is present show that it has only exceeded 3 kW for about 1% of the time, and was below 500 W for 50% of the time.

Chris

Reply to
Chris J Dixon

Agreed. If you have a long run between panels and service head it is generally much better to position the inverter so most of the cable length is dc not ac. This keeps the losses down and minimises the voltage rise at the output of the inverter.

There have been many examples of too small cables being used for the ac segment which has caused the voltage to be too high and trip the inverter when sunny.

PV FIT contract is 20 years now.

A useful trick to maximise generation is to put say 5 kWp of panels on a

3.68kW inverter if you have the roof space. The FIT rate is slightly lower but extra production more than makes up for it.
Reply to
BruceB

What is the manufacturers guaranteed life of the inverter?

Reply to
Capitol

The DC voltage from the PV panels can be up to 800volts (depending on how they are connected and the sort of inverter you have. ) There is no transformer. I have two 4Kw arrays, in one the panels are all connected in series. (21 panels) In the other there are two circuits in parallel. (2x8 panels) So obviously there is never more than five or six amps on the DC side.

The panel manipulates volts and current to get the best power output. Efficiency is 96% in one and 98% in the other.

The panels/DC is largely wired in an aluminium cable specially designed for the purpose with male & female waterproof connectors. So you have no control of the volt drop on the DC side. Exceptions are if part of the circuit is underground. Then ordinary steel wire armoured cable is used.

The inverter and meter are usually next to your consumer unit, but not neccesarily so. Usually just connected to a spare wat in the consumer unit There are AC and DC isolators for the inverter.

All the items are quite compact these days.

There are lots of cowboy installers about. Be careful, get someone in with a personal recommendation. You can get everything from roof leaks to cockups with the paperwork (which is quite extensive). I have had all of these an more.

Reply to
harryagain

Usually five years. However they seem to come and go and the technology is changing all the time.

Reply to
harryagain

17.95 x 220 = 3949 W

You bin diddled...

Not impressed but not unexpected... At 500 W what is the DC input voltage to the invertor?

Reply to
Dave Liquorice

It's not always a good idea to put the inverter in the loft. It can get very hot in there in summer and electronics don't like heat.

Reply to
F

The airing cupboard would be less than ideal. Put it in the garage or on an outside wall. It doesn't have to be next to the CU.

Reply to
F

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