Setting up our new build for PV in the future

[JanetE]

The planners gave us the thumbs down for PV panels and we hadn't the heart to argue our case at the time with everything else going on.  So we plan to install them retrospectively and I'm thinking about cable runs before the plasterers turn up.  It would make sense to put in conduit but we need to know where.  I assume the cable will be taken through the roof which shouldn't be too much of a problem as we have a services room up there.  Where does it go from there?  The meter box? The distribution/fuse box? What size conduit will we need? Any thoughts and suggestions gratefully received. 

[PeterW]

I was advised to put a fused spur at the nearest point to where it would come through the roof and then run that to a spare way on the CU. Make sure there is space to change for a double isolator and any control gear the panels may need near to the spur. 

 

Pre-wired with 2.5mm T&E was also advised as fine as you should never be able to generate anything in the realms of 30A anyway with a domestic array. 

 

Supplier also advised micro inverters as they would need less space internally and all wiring from the panels down was mains voltage. 

[Declan52]

Mine just has the one cable running from the roof straight through to where the meter box is in my utility room. My iboost uses WiFi to connect to the main unit at the immersion do have no cable for it. Mine has the fused spur as it comes through the ceiling is easy got to if I need to.

[ProDave]

The inverter might as well go in the services room in the loft. Provision for a conduit to take the dc cables from the inverter onto the roof.  Include a dedicated cable from the inverter to the consumer unit, 2.5mm is okay as long as it's not a very long run but no harm in putting a 4mm or even 6mm twin and earth in for it.

 

When you chose your consumer unit, get a "high integrity" one. This usually has two banks of mcb's each covered by an rcd and a couple of ways not covered by an rcd. Feed the inverter into one of the non rcd protected circuits via an rcbo.  What you do NOT want is the inverter feeding into an ordinary mcb fed as one of a bank from an rcd.

[TerryE]

Nice to know that my lovely better half is worrying about such things whilst I am in Alonissos.  A miracle might happen and our plasterers start work before I get back on Thursday. 

 

We've got an MBC warm roof, which is sarked, counter battened and battened so wouldn't it make more sense to run any PV cabling under the tiles and into the services room through the gable rather than compromise the roof integrity?

 

But as far as extra circuits go, IIRC a single 2.5mm is rated at something like 20 amps but this is 5 kW and ample for a standard 3 kVA array. But given that we need an extra circuit on the non RCD side we might as well get our electrician to lay an extra 4mm cable to the CU.

 

XX to the O/P.

[SteamyTea]

You have to remember that if you intend to claim FiTs (or something in the future), there is a 1% voltage drop allowance (not the usual 5%).
  This is on the AC (it is to stop the meter showing higher readings than is reaching the CU).

 

As for DC or AC cabling, that all depends on which inverter(s) you go for.  If you go for micro-inverters, then you need to think about AC cabling (4mm), if DC to one inverter (somewhere cold), then just conduit for the DC cabling from the roof.

It is not unusual to route this outside the building.

 

Shame that you have been refused PP for PV.  You could have gone roof integrated , saved some money and made a better looking install.

[ProDave]

The voltage drop issue is nothing to do with fooling the metering.  It's that inverters are set to trip or limit output at 253 volts.  With many UK supplies being 245V there is not much headroom for volt drop (which perversely is voltage "rise" in this case) before the inverter trips.  So although a 2.5mm cable will handle the load, it is wise to fit a much larger cable to minimise the risk of voltage issues.
 

[Nickfromwales]

2 hours ago, ProDave said:

The voltage drop issue is nothing to do with fooling the metering.  It's that inverters are set to trip or limit output at 253 volts.  With many UK supplies being 245V there is not much headroom for volt drop (which perversely is voltage "rise" in this case) before the inverter trips.  So although a 2.5mm cable will handle the load, it is wise to fit a much larger cable to minimise the risk of voltage issues.
 

+1. 4mm2 min as the price difference is chicken feed. 

[SteamyTea]

PD

It is a bit of both from my understanding.

You can't have more than a 1% volt drop between the meter (PV) and the CU (but really the main meter, which s usually close).  This is more the case for ground mounted systems, or you could get the scenario where the modules, inverter and generation meter are several hundred yards from the load.  The generation meter would count every joule being generated, but there could be large losses on the way to the load.

It is true that voltage between the systems has to be within limits as well for reliability and safety.

[Vijay]

12 hours ago, TerryE said:

We've got an MBC warm roof, which is sarked, counter battened and battened so wouldn't it make more sense to run any PV cabling under the tiles and into the services room through the gable rather than compromise the roof integrity?

 

 

I've wondered about this myself, how have people put the cables through the roof space if it's a well insulated room?

[Declan52]

Taped as it goes through the membrane and same with the ceiling.

[JanetE]

5 hours ago, Declan52 said:

Taped as it goes through the membrane and same with the ceiling.

 

 

I would have thought this would be by far the easiest way to get the cable in.

 

This site really is amazing, so thanks to you all for your help. 

Edited October 17, 2016 by JanetE

[TerryE]

@SteamyTea, re in roof PV. We made the same comment to the planners.

 

Water off a duck's back.

 

@Vijay, We've got putting holes through the twinwall off pat. The trick is to have a long pilot drill. Cut a 22mm hole on one side, use the pilot and pull back a few inches; the filler keeps it roughly in place. Cut a 22mm sole on the other side push the pilot through then use it as a guide to push through a 20 or 22mm plastic pipe. Silicon and tape both sides. Or use larger pipe if needed. 

 

Thread cables and when done, foam and silicon seal as needed.

 

I find using a guide pipe straight forward, but there's nothing in principle taping the wire to the end of the pilot drill and using it to draw the bare cable through.

Edited October 17, 2016 by TerryE