AC Cable length question

[Radian]

Looking at installing 3 or 4 kW on our detached garage as it has the only suitable unshaded roof available.

The Garage has a small consumer unit cabled in 6mm2 SWA back to house on a 32A MCB. This cable is something like 20m long.

Does this sound OK when most of the load is in the house? The inverter would be in the garage of course.

[Marvin]

26 minutes ago, Radian said:

Looking at installing 3 or 4 kW on our detached garage as it has the only suitable unshaded roof available.

The Garage has a small consumer unit cabled in 6mm2 SWA back to house on a 32A MCB. This cable is something like 20m long.

Does this sound OK when most of the load is in the house? The inverter would be in the garage of course.

Not quite that simple.

The book for my solar grid tied inverter shows this:

 

 

 

Marvin.

 

[Radian]

9 minutes ago, Marvin said:

Not quite that simple.

 

Thanks Marvin. Ah, I see, they say no loads between inverter and main breaker.

 

Now I already realised that there would need to be an RCBO in the garage consumer unit, for local loads, to provide protection from the current coming out of the inverter if that was connected direct to the SWA. But I can't read any explanation into why they're saying no intermediate loads. A local load will look no different to the inverter if presented before its connection to the RCBO or on the other side. Sure it will create an intermediate voltage drop that will lead to less current going back into the house/grid, but this is balanced exactly by the local load so there's no nett loss. I wish all statements such as the one they're making were accompanied by an explanation ?

[SteamyTea]

You have to be careful if voltage drop. I think PV has a 2% limit.

Can you run the DC side longer?

Edited December 12, 2021 by SteamyTea

[Marvin]

All I know is it works like that.

 

I think you can put the inverter and RCBO before any load. So both in the garage before connecting to the house cables. You need to check what your equipment says is required. The RCBO requirement was a real bind. 

[Radian]

24 minutes ago, SteamyTea said:

You have to be careful if voltage drop. I think PV has a 2% limit.

Can you run the DC side longer?

 

Not really practical to extend the DC side. But from the Chinese table the loss in a 20m long 6mm2 cable is given as 0.5% 

Looking at some tables for SWA, 8 mV/A/m is the volt-drop in a 6mm2 cable so I'd be looking at 2 x 2.56V drop @16A (4kW) which is more like 0.2%

 

21 minutes ago, Marvin said:

All I know is it works like that.

 

I think you can put the inverter and RCBO before any load. So both in the garage before connecting to the house cables. You need to check what your equipment says is required. The RCBO requirement was a real bind. 

 

The inverter must (will) have some form of current limit so the RCBO between it and the grid will be there to protect the cable in between.

[ProDave]

The issue with the ac side is volt RISE.  Because the inverter is feeding in to the house loads, the voltage at the garage CU will RISE as generation increases.  If it gets as high as 253V most inverters will shut off.  It all depends what your mains voltage is normally.

 

It will probably be okay but you won't know until you try it.

[Radian]

3 minutes ago, ProDave said:

The issue with the ac side is volt RISE.  Because the inverter is feeding in to the house loads, the voltage at the garage CU will RISE as generation increases.  If it gets as high as 253V most inverters will shut off.  It all depends what your mains voltage is normally.

 

Yes, that's what the paragraph about cable resistance was all about. It's a separate issue to the 'no intermediate load' stuff - indeed such loads would help keep the AC volts down. I can only guess they say don't do it because there would be no residual current protection for intermediate loads.

[TonyT]

No problem in using a sub main for a PV Install.

 

the existing consumer unit will probably need to be swapped over to suit appropriate RCDs for the PV supply and then for the outbuilding itself.

 

 

[Radian]

Thanks TonyT, I just came across this very old iscussion on the IET forum which explains it quite well.

[Marvin]

Watch out for the technical details on the RCBO. Mine had to be 0.1amp  

[Radian]

I'm still scratching my head about the issue of connecting an inverter to the grid supply via a cable that already links a sub-main consumer unit in our garage to the consumer unit in the house that connects to the meter tails.

 

The issues as I see them:

 

Cable safety.

The SWA cable joining the house and garage can be protected by having a 32A MCB at both ends. Overloads from either the grid supply or the inverter generation ends will disconnect the cable.  Cable safety ✔️

 

Earth Leakage protection.

All other circuits, i.e. those coming off the two consumer units have their own RCD protection. Current sourced either from the grid or inverter must be balanced through L and N. User safety ✔️

 

Over-voltage through generation.

The 20m long 6mm2 SWA has a resistance 3.08mOhms/m so 0.123 in total. WIthout intermediate loads (loads connected to the garage consumer unit) the maximum current of 10A from the inverter (proposed 2.4kWp PV) would create a 1.23V rise at the output of the inverter. Safe levels of Over-voltage ✔️

 

The only other issue I can think of is how export metering might be affected. In this diagram from smartme.co.uk the meter sits in parallel with all house circuits so meters everything coming out of the PV whether consumed locally or fed into the grid. I'm a bit confused by this aspect. Perhaps someone can explain how the amount being exported is determined? My circuit diagram would look similar to the one below except for another consumer unit connected in parallel with the main one at the break between 'New AC installation' and 'Existing house AC installation'.

[SteamyTea]

20 minutes ago, Radian said:

Perhaps someone can explain how the amount being exported is determined?

Traditionally it wasn't. Why the FiT system originally used 50% of total generation as the "deemed" export amount.

I think most smart meters do now show exports.

Assuming the use of a couple of dumb meters though, you can sense the permanently on LED that shows the main meter is working in reverse. Then read the pulsed in the generation meter, assuming no local load, or fit a third meter on the incoming tails to measure exports. As far as I know, dumb meters allow flow in both directions, but only log in one direction.

[Radian]

Ok, so no additional metering is required - just the supply companies smart meter.

 

I'm just trying to figure out why a PV installer I've asked to set us up with a system has come back with this statement:

 

Quote

If the inverter is to be mounted in the garage with no separate connection to the mains / meter position, then your only option would be an AC coupled battery at the house 'end'.

 

That makes no sense to me at all.

[ProDave]

1 hour ago, Radian said:

Cable safety.

The SWA cable joining the house and garage can be protected by having a 32A MCB at both ends. Overloads from either the grid supply or the inverter generation ends will disconnect the cable.  Cable safety ✔️

Don't go putting an MCB at both ends, they are not directional.

 

29 minutes ago, Radian said:

Ok, so no additional metering is required - just the supply companies smart meter.

Do you not want a generation meter so you can see how much you generate?  I have one for my own interest.  Agreed now the supplier is only interested in export now we have the smart export thing.

 

BUT to claim the smart export payment you must have an MCS install so all notions of you designing it and choosing how to do it go out of the window.

[SteamyTea]

35 minutes ago, Radian said:

Ok, so no additional metering is required - just the supply companies smart meter.

 

I'm just trying to figure out why a PV installer I've asked to set us up with a system has come back with this statement:

 

 

That makes no sense to me at all.

I think this goes back to cable losses.

The generation meter had to be within a fixed distance if the main meter.

It is a decade since I was involved in all this, so memory may be failing.

[Radian]

3 minutes ago, ProDave said:

Don't go putting an MCB at both ends, they are not directional.

 

 

I know, however if current can be fed into the cable from both ends then surely it's necessary to have a breaker at both feed points as a fault can occur anywhere along the length?

 

6 minutes ago, ProDave said:

BUT to claim the smart export payment you must have an MCS install so all notions of you designing it and choosing how to do it go out of the window.

 

Yes I'm talking to an MCS installer but as I mentioned above, they say it can only use the existing cable between the garage and the house if we have an AC coupled battery in the house. This makes no sense to me so I was trying to list the issues I knew about in case anyone could spot something I was missing. 

 

I would not buy something that I was being told I needed without understanding why I need it.

[Radian]

5 minutes ago, SteamyTea said:

I think this goes back to cable losses.

The generation meter had to be within a fixed distance if the main meter.

It is a decade since I was involved in all this, so memory may be failing.

 

I've realised now that the generation meter is purely for personal interest. You can bet I'd meter the hell out of it!

[SteamyTea]

6 minutes ago, Radian said:

You can bet I'd meter the hell out of it!

Count the pulsed.

And you may want to get some of these. 

My next project.

 

No idea how to turn the image over on a phone. Shall see if I can put it right in PC.

Hopefully sorted.

Edited March 3, 2022 by SteamyTea

[Wil]

6 minutes ago, Radian said:

if we have an AC coupled battery in the house.

Have you spoken to them about battery storage (now or future)? If not, this is irrelevant. They may be telling you that to add batteries, you'd only be able to do AC coupled in the future rather than direct DC?

 

Put the gen meter between the AC Isolator and the MCB in your garage DB. That way it's only metering PV generation. Not sure why the gen meter would need to be within a certain distance of the Utility meter? They're not on most of the commercial buildings I've seen (or my house).

 

Edit: for metering I use Shelly EM or 3EM products to provide metering direct to my phone so I can get the smug feeling of knowing my PV is working wherever I am.

Edited March 3, 2022 by Wil

[Radian]

1 hour ago, Wil said:

Have you spoken to them about battery storage (now or future)? If not, this is irrelevant. They may be telling you that to add batteries, you'd only be able to do AC coupled in the future rather than direct DC?

 

Yes, I asked for a costing with and without battery storage. The saving in VAT (5% vs 20%) makes it somewhat attractive to have it done during the PV installation.

 

The statement "If the inverter is to be mounted in the garage with no separate connection to the mains / meter position, then your only option would be an AC coupled battery at the house end" doesn't make it clear if it means that if we wanted a battery system it would have to be the AC connected variety, located in the house - or that if we wanted a PV on the garage roof at all, then it would require us to have an AC coupled battery in the house!

 

Electrically speaking, A DC coupled hybrid inverter would look no different on the AC side to a regular PV inverter so I'm unable to see how the statement could mean that the only type of battery system available to us is an AC coupled one in the house. On the other hand, if the meaning is that the only way we can have PV connected to the garage sub-main at all is by having a battery in the house then that'd be mighty mysterious.

 

I've asked and am waiting for a reply, I'm just impatient and a little frustrated that I can't work out what's going on myself - i.e. am I missing something that should be obvious to me.

[Radian]

2 hours ago, SteamyTea said:

Count the pulsed.

And you may want to get some of these. 

My next project.

 

I think you were the one extolling the virtues of the Currentcost meter? I have a few homebrew current clamp meters but I've also got my original Scottish Power branded CC-128 metering the 100A incoming. Still going strong since 2009 although I did desolder the RJ45 and glue in an ESP8266 to relay the serial data to MQTT for logging. Handy that. A while ago the clamp packed up so I dismantled it and found the winding was open circuit. It all looked OK on the surface, but not being able to see the connection going into the middle of the coil I took a punt and broke away a corner of the former then pushed a header pin all the way in under the windings. To my delight this remade the connection and got it working again. As far as I can tell it didn't affect the calibration at all.

[SteamyTea]

2 hours ago, Radian said:

got my original Scottish Power branded CC-128 metering

I got the posh one with the optical counter.

[Dillsue]

A few observations...

If youve got 2.4kwp and make even the smallest effort to self consume, youre not likely to be exporting much so is it worth paying the premium for an MCS install? Youll be paying a premium and maybe constrained in how you configure things

 

Not sure what inverter youre using but ours gives a readout of generation by day, week and total since install so you may not need a generation meter if the inverter gives that info.

 

I cant see what the issue is with connecting into the garage CU. Weve got several consumer units all fed from the same incoming service fuse/meter and our MCS installers were quite happy to feed into one which wasnt nearest the incoming supply. They just fitted the dual supply warning labels to the other CUs

[ProDave]

7 minutes ago, Dillsue said:

I cant see what the issue is with connecting into the garage CU. Weve got several consumer units all fed from the same incoming service fuse/meter and our MCS installers were quite happy to feed into one which wasnt nearest the incoming supply. They just fitted the dual supply warning labels to the other CUs

My inverter is in a shed and connected to the shed CU which is a submain direct from the meter box CU.  My generation meter is in the shed next to the inverter.

 

There was an odd comment when getting the home report done for our last house.  The assessor made some comment that he could only take the solar PV into account when calculating the EPC if he could see the generation meter next to the main meter.

 

I think that was just the assessor being silly.  The EPC (full SAP) for our new build was done without anyone visiting and so there was nobody to be bothered about where the meters were located.

 

I agree than MCS is pointless unless you have a very large PV array and there is no way you will self use all of it.