Does a Grid tie inverter need a dedicated circuit (UK) ?

[geoff_uk]

Hi all, new here so apologies if this has already been answered, but struggling to find a definitive answer on this.
I've recently had a solar installation at my house, with 5 x 390w panels and a Solis grid-tie inverter.
I was originally supposed to get 7 panels, but the fitters could only get 5 on the roof with the velux windows, etc
So, I've been looking at putting more panels up myself on a shed at the back of the property, connecting to another grid-tie inverter
Question is, as there is already a mains feed to the shed, can I connect the grid-tie inverter directly to this, or does it require a separate circuit / breaker on the consumer unit?
I'm based in the South East, UK if that makes a difference.
Thanks, geoff

 

[Markpc]

Does your shed have a breaker inside it or is it basically an extension lead?
If its a large shed with more than 1 socket and lights I would expect its own breaker box which you can tap into with its own solar circuit (if there is room).

 

[SeaGal]

Welcome @geoff_uk!

Lots of questions for you...
a) how far is the shed from the consumer unit?
b) how far is the shed from the existing Solis?
c) what is the spec of the cable running from the CU to the shed? (distance and mm2 wise)
d) is there a secondary fuse box in the shed?
e) does the shed have its own TT earthing arrangement?
f) what is the spec of the internal wiring of the shed (e.g. is it 2.5mm2 T&E?)

With those answers I should be able to give better feedback.

I see 2 potential issues:

1) It is _not_ within recommendations to feed solar directly into an existing ring mains. The reason being is that ring mains circuit would could then have more than 30A supplied to it (i.e. from grid and solar in parallel) which would exceed the capacity of the 2.5mm2 T&E and could therefore cause overheat/fire if a fault developed on that circuit.

2) If the shed is a reasonable distance from CU and wire gauge insufficient, then the resulting voltage rise (from CU to inverter) could lead to higher than expected / allowable voltages in your shed circuit.

Neither of these are insurmountable, but one (possibly) simpler option (and hence my question (b) above) could be to run dc PV circuit from the panels on your shed back to the secondary MPPT input of the Solis. I am assuming that the Solis is currently configured to have only one string (as you only have 5 panels), so if that were an option, you'd save the cost and complexity of a second inverter for the cost of putting some conduit and PV cables underground from shed to Solis.

If you do go with another Solis in the shed, you would of course still need a circuit to establish an RS485 connection for the Solis to connect to its grid CT clamp / Acrel power meter.

Just a thought.

 

[dmsims]

@SeaGal has made a lot of good points, I suppose capacity will depend on how many panels the OP installs

 

[geoff_uk]

Welcome @geoff_uk!

Lots of questions for you...
a) how far is the shed from the consumer unit?
b) how far is the shed from the existing Solis?
c) what is the spec of the cable running from the CU to the shed? (distance and mm2 wise)
d) is there a secondary fuse box in the shed?
e) does the shed have its own TT earthing arrangement?
f) what is the spec of the internal wiring of the shed (e.g. is it 2.5mm2 T&E?)

With those answers I should be able to give better feedback.

I see 2 potential issues:

1) It is _not_ within recommendations to feed solar directly into an existing ring mains. The reason being is that ring mains circuit would could then have more than 30A supplied to it (i.e. from grid and solar in parallel) which would exceed the capacity of the 2.5mm2 T&E and could therefore cause overheat/fire if a fault developed on that circuit.

2) If the shed is a reasonable distance from CU and wire gauge insufficient, then the resulting voltage rise (from CU to inverter) could lead to higher than expected / allowable voltages in your shed circuit.

Neither of these are insurmountable, but one (possibly) simpler option (and hence my question (b) above) could be to run dc PV circuit from the panels on your shed back to the secondary MPPT input of the Solis. I am assuming that the Solis is currently configured to have only one string (as you only have 5 panels), so if that were an option, you'd save the cost and complexity of a second inverter for the cost of putting some conduit and PV cables underground from shed to Solis.

If you do go with another Solis in the shed, you would of course still need a circuit to establish an RS485 connection for the Solis to connect to its grid CT clamp / Acrel power meter.

Just a thought.hanks for all the responses on this.

Welcome @geoff_uk!

Lots of questions for you...
a) how far is the shed from the consumer unit?
b) how far is the shed from the existing Solis?
c) what is the spec of the cable running from the CU to the shed? (distance and mm2 wise)
d) is there a secondary fuse box in the shed?
e) does the shed have its own TT earthing arrangement?
f) what is the spec of the internal wiring of the shed (e.g. is it 2.5mm2 T&E?)

With those answers I should be able to give better feedback.

I see 2 potential issues:

1) It is _not_ within recommendations to feed solar directly into an existing ring mains. The reason being is that ring mains circuit would could then have more than 30A supplied to it (i.e. from grid and solar in parallel) which would exceed the capacity of the 2.5mm2 T&E and could therefore cause overheat/fire if a fault developed on that circuit.

2) If the shed is a reasonable distance from CU and wire gauge insufficient, then the resulting voltage rise (from CU to inverter) could lead to higher than expected / allowable voltages in your shed circuit.

Neither of these are insurmountable, but one (possibly) simpler option (and hence my question (b) above) could be to run dc PV circuit from the panels on your shed back to the secondary MPPT input of the Solis. I am assuming that the Solis is currently configured to have only one string (as you only have 5 panels), so if that were an option, you'd save the cost and complexity of a second inverter for the cost of putting some conduit and PV cables underground from shed to Solis.

If you do go with another Solis in the shed, you would of course still need a circuit to establish an RS485 connection for the Solis to connect to its grid CT clamp / Acrel power meter.

Just a thought.

Thanks for all the responses.
To add some more details, the shed is around 40 metres from the house. The current Solis unit is in the loft of the house. I don't have the exact details of the cable from the house to the shed, but from what I can tell, it's on a 13a fused spur from another house circuit, so no secondary fuse box in the shed.
I was curious what options there were without adding additional cable(s) from the shed back to the house, so just wanted to see what was possible. Am I right in thinking the limit would be the 13a fuse which would need to cover the load plus the solar generated? E.g. If I have a load of 5a, but I'm generating 10a from solar then that would overload the circuit / spur?
My other thought was to use a hybrid inverter with batteries to just cover the load of the shed, etc, but obviously less benefit of doing it this way.

 

[SamDeleted]

but one (possibly) simpler option (and hence my question (b) above) could be to run dc PV circuit from the panels on your shed back to the secondary MPPT input of the Solis.

That sounds like the best bet to me

 

[rpdom]

Am I right in thinking the limit would be the 13a fuse which would need to cover the load plus the solar generated? E.g. If I have a load of 5a, but I'm generating 10a from solar then that would overload the circuit / spur?

There would only be 10A going through that circuit. The 10A from the would supply the 5A load and the remaining 5A would go back to the house.
I agree with SeaGal that running DC cables to your existing inverter would be the easiest and safest option.

As you are in the SE, I wonder if you had the same installer as me.

 

[SeaGal]

Thanks for all the responses.
To add some more details, the shed is around 40 metres from the house. The current Solis unit is in the loft of the house. I don't have the exact details of the cable from the house to the shed, but from what I can tell, it's on a 13a fused spur from another house circuit, so no secondary fuse box in the shed.
I was curious what options there were without adding additional cable(s) from the shed back to the house, so just wanted to see what was possible. Am I right in thinking the limit would be the 13a fuse which would need to cover the load plus the solar generated? E.g. If I have a load of 5a, but I'm generating 10a from solar then that would overload the circuit / spur?
My other thought was to use a hybrid inverter with batteries to just cover the load of the shed, etc, but obviously less benefit of doing it this way.

OK... you didn't answer all my questions in detail. However from this additional information (and without a proper inspection from a qualified person) then IMHO I would say that the answer to your question of "I was curious what options there were without adding additional cable(s) from the shed back to the house" is a definite there are none. This is for the following 2 reasons:-

1) Feeding solar generated power into an existing ring mains, via a fused spur will not meet IEE wiring regulations. As I detailed above, a ring mains circuit is typically 2.5mm2 T&E cable, protected by a 30A MCB in your consumer unit. An additional supply being fed from your solar generation in the shed would be a maximum of 13A (protected by the 13A fuse in the FCU). Therefore the maximum current that could be supplied to that existing ring mains would increase to 43A (30A from grid and 13A from solar). This will exceed rating of your existing ring mains and is not safe.

2) As the shed has been connected as a spur from a 13A FCU, I would suspect that the cable to the shed is not that heavy duty or, even worse, not of the correct type of cable for external use. To feed a reasonable solar supply across 40m will typically require SWA cable of at least 8mm2, if not 10mm2. It is not just a question of current carrying capacity - you will also need to mitigate voltage drop/rise across that connection.

Hence, my "desktop" analysis / suggestion would be to either:-

a) Have a 10mm2 (min) SWA cable installed on its own circuit direct from CU to the outbuilding, together with cabling for RS485 connection. Install panels and another Solis in the outbuilding.

b) Install panels on the outbuilding and run dc cables back to the secondary MPPT input of your existing Solis in the loft.

Having said that, I can't think of any reason why you would choose option (a) over option (b). Option (b) will be far cheaper cabling wise and you don't need another Solis.

Hope that helps

 

[geoff_uk]

OK... you didn't answer all my questions in detail. However from this additional information (and without a proper inspection from a qualified person) then IMHO I would say that the answer to your question of "I was curious what options there were without adding additional cable(s) from the shed back to the house" is a definite there are none. This is for the following 2 reasons:-

1) Feeding solar generated power into an existing ring mains, via a fused spur will not meet IEE wiring regulations. As I detailed above, a ring mains circuit is typically 2.5mm2 T&E cable, protected by a 30A MCB in your consumer unit. An additional supply being fed from your solar generation in the shed would be a maximum of 13A (protected by the 13A fuse in the FCU). Therefore the maximum current that could be supplied to that existing ring mains would increase to 43A (30A from grid and 13A from solar). This will exceed rating of your existing ring mains and is not safe.

2) As the shed has been connected as a spur from a 13A FCU, I would suspect that the cable to the shed is not that heavy duty or, even worse, not of the correct type of cable for external use. To feed a reasonable solar supply across 40m will typically require SWA cable of at least 8mm2, if not 10mm2. It is not just a question of current carrying capacity - you will also need to mitigate voltage drop/rise across that connection.

Hence, my "desktop" analysis / suggestion would be to either:-

a) Have a 10mm2 (min) SWA cable installed on its own circuit direct from CU to the outbuilding, together with cabling for RS485 connection. Install panels and another Solis in the outbuilding.

b) Install panels on the outbuilding and run dc cables back to the secondary MPPT input of your existing Solis in the loft.

Having said that, I can't think of any reason why you would choose option (a) over option (b). Option (b) will be far cheaper cabling wise and you don't need another Solis.

Hope that helps

Many thanks for the comprehensive answer SeaGal.
I think I need to go back to the drawing board here, as you say best option would be to use the secondary MPPT input, but it would be a lot of work to get cabling up to the loft of the house and probably require more scaffolding. I'd also be limited to adding a maximum of 4 panels (Solis inverter has maximum of 3600W), so not sure it would be worth the effort.
Thanks for the help and bettering my understanding though!

 

[dmsims]

One other option (which may not work for you!) is to keep the shed system off grid

 

[geoff_uk]

One other option (which may not work for you!) is to keep the shed system off grid

So what would I need for that? Is that where a hybrid inverter comes in?

 

[SeaGal]

...I'd also be limited to adding a maximum of 4 panels (Solis inverter has maximum of 3600W), so not sure it would be worth the effort.

The 3.6kW limit will be the Solis output power - it is common practice to 'overpanel' as the panel's peak output (kWp) will rarely be achieved and only in full sun with panels perpendicular to the sun. By having more panels, you will benefit from more power for most of the year. So, I'd go for at least 12 if you can afford them. We have 16 panels (over 6kWp) of panels with a 3.6kW Solis, albeit paired with 15kWh of batteries. So on an on-off sunny/cloudy day like today (with East-West panels) we saw 1kW before 8am and now seeing between 700W and 4400W depending on cloud cover.