Discuss Is this how it's done ? in the Solar PV Forum | Solar Panels Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

Hi all, could some one let me know if this is how PV is connected please ?

I was called out to a job with intermittent tripping today and after much ****ing about have sorted the problem.

My question is: they have had Solar PV installed in the garden this is apparently a 4kW system, and is 50 - 70 meters from the house. The inverter is placed along with the panels and feeds back to the house in 4 or 6mm cable ( I think it may be swa but can't see any armour in the fuse board) incoming feed within fuse board runs out through flex conduit into a a check meter and then from the meter back into the fuse board
and into the top of a 20A mcb.


1) should there be an isolator between the the inverter and the meter in the house ?
2) should the output from the inverter be connected in the top of an mcb, meaning that if a fault develops and trips the main rcd the inverter is still supplying power to the board via the buss bar ?

Perhaps someone could explain the correct way to connect solar.

Many thanks.
SuperlecDirect - ElectriciansForums.net Electrical Suppliers
This official sponsor may provide discounts for members


fig. 712 in BGB is worth looking at.


4 or 6mm sounds a little small for the job to be honest.

In answer to your questions:

1. Yes, there should. One fitted at the consumer unit and another fitted at the inverter.

2. Output from the inverter should be fitted to the top of a circuit breaker BUT not on a shared/communal RCD. It needs to be on its own seperate RCD. As you rightly point out, in the event that the RCD operates, the PV system could, in theory, still be supplying power to the circuits on the same busbar.


Just to add, if the supply cable is too small and has significant volt drop, for the inverter to match the mains voltage back at connection it will have to go overvoltage to match the mains.
Cable volt drop for PV should be less than 1% for the AC, if the incoming supply from the mains is on the high side then it is even more critical.
Do the cable loss calculations over that length of cable.
If you can identify the problem it may be best to call the PV company back to rectify it.

I hope this helps.
  • Thread Starter Thread Starter
  • #6
many thanks for info. I was only asking for my own information re solar. The fault was on another board. The PV is connected into the lighting board but also supplies the garage. The wylex board will not allow an rcbo or rcd to be fitted
so should they have provided a separate rcd in its own enclosure and taken that into the wylex board. I will tell the clients
about the omissions and suggest they get back the installer. They are also having their oil combi boiler removed and replaced with air source heat pump to provide heating and hot water, connected into the wylex pv board.


Do you know what inverter is being used? If it's a SMA Sunny Boy 3600TL, you might want to refer to page 25 of the installation manual for guidance on cable sizing:

Cable sizing
The grid impedance of the AC cable must not exceed 1 ohm. Otherwise, the inverter will disconnect at full feed capacity due to excessive voltage at the feed-in point.
The conductor cross-section should be dimensioned such that cable losses do not exceed 1 % at nominal power. Use "Sunny Design" (SUNNY DESIGN) for this.
The maximum cable lengths relative to the conductor cross-section are shown in the following table:


Gavin A

tbh, if the issue is the tripping of the RCD, and the inverter feed is coming in on the load side of the RCD, then that's the issue to focus on - it just needs to not be on the load side of the RCD, as TL inverters in particular have an earth leakage current in the region of 7-12mA as standard, so add that to any household circuits with a slight earth leak and you get nuisance tripping.

The best way to solve this IMO is to split the main incoming tails between the suppliers meter and the consumer unit, fit a henley block, fit a second set of tails to a separate garage unit / mini consumer unit, then connect this to the meter and on to the SWA to the inverter (with an isolator fitted at the inverter). Some will fit seperate isolators at the meter point as well, and this might not be a bad idea as it provides somewhere solid to terminate the SWA to anyway, but IMO a dual pole RCD or breaker in the garage unit provides the dual pole isolation at this point adequately.

As for the cable from the inverter... Yes the cable from the inverter should be bigger for that distance, BUT it's not a safety thing, worst case is the inverter tripping out regularly due to overvoltage, and if the local grid voltage is medium to low, then this probably won't be an issue anyway (assuming it's 6mm not 4mm)... so I'd maybe mention it, but not stress them out about it unnecessarily. They could ask the installer to return and provide a print out of the error logs since the installation, which would show if the inverter has been tripping out or not, which would then tell them if the cable actually needed replacing or not.


Gavin is right that it's not a safety issue with the cable sizing and it probably won't affect the system at all. However, the grid voltage may be low now, but could rise in the future which will make it a real and present problem.
  • Thread Starter Thread Starter
  • #10
Many thanks for all information supplied. I am going back today to sort out some other issues within the house and will have a look at the solar installation and take some pics. Very interesting about cable size and possible issues with supply voltage,
I noticed this was low but it is the last house down a lane in the middle of the countryside. I will make a note of it today.

My real concern was the pv being on the load side of the rcd, and on the lighting circuits, this is a thatched cottage and some of the cables are run within the loft space. The property has down lighters fitted upstairs, I need to check if they are fire rated or not although they have smoke hoods but one has fallen off so I was told and they have mice in the loft !

I will attempt to find out who is installing the solar.

I don't know what you all think but I would recommend that the Solar be disconnected until they return and provide rcd protection for it, this is based upon the state of the rest of the installation that I have seen so far and the thatch.



shouldn't cable in the loft of a thatch be run in FP or something similar?
  • Thread Starter Thread Starter
  • #13
Stepped out the run and it is at least 85-90 M. cable is 6mm max. Client had a reply from installer who said:
''it is not necessary to have an isolator in the house, nor is it necessary to have the supply run into an rec because the inverter has its own internal RCD and if the house rcd trips the inverter will disconnect the output to the house.''

Inverter is a sunny boy, not sure what model.

Does this sound ok with you ?


''it is not necessary to have an isolator in the house
The Dti guide suggests otherwise. Although I have always thought it was a bit pointless.

nor is it necessary to have the supply run into an rec because the inverter has its own internal RCD and if the house rcd trips the inverter will disconnect the output to the house.''
It will disconnect the output to the house but not necessarily within satisfactory disconnection times.

I'd be amazed if the 6mm cable isn't causing issues on an 85-90m cable run. At the very least it is outside of SMA's guidelines. And as I pointed out before, even if the grid voltage is low now it may not be in a couple of years time. This isn't a safety issue though.
What a bunch of **** ! Tell him to get his installer back and do the job properly. Also tell your client to remind his installer of disconnection times (if he knows what they are) .... Rant over

The run you've described would suggest the cable is largely undersized to be honest. It's true the installer may have thought he could get away with it due to the low nominal voltage but it wouldn't be a good idea to size your cable to the supply parameters of when you install the system especially with such sensitive parameters are being worked too. SMA even make it easy for you by giving maximum suggested cable runs (so you get an idea) in their installation manuals. 6mm for a 4000TL is 23.3 meters, 10mm on the same inverter is 38.8 meters.

Test Meter - Forum Sponsors since 2007!
This official sponsor may provide discounts for members

Reply to Is this how it's done ? in the Solar PV Forum | Solar Panels Forum area at ElectriciansForums.net

Aico Carbon Monoxide Detectors
This official sponsor may provide discounts for members
Top Bottom