PV - Point of Interconnection

richsogecko · Mar 31, 2014

Hi,

Quick compliance question regarding the AC aspect of a PV circuit...

Imagine a standard domestic <4kWp install on a 3 storey town house. Because of the geography of the house and the internal decoration there's no way to run a dedicated circuit back to the main board.

The alternative would be to sacrifice a top floor socket outlet in favour of a 2W garage board and connect the PV into a 16A MCB within the garage board (along with rotary iso and gen meter)

So protection would be 30mA RCD on main board, 32A Type B on Ring Circuit, 30mA RCD on Garage Board, 16A MCB for PV circuit.

Notwithstanding that a) there is no way to discriminate between 30mA RCDs in series (albeit the Garage board one is only protecting the upstream PV circuit) and b) the Guide to PV would 'preferably' want a dedicated circuit to the main board, is there anything technically not compliant with this method of installation.

We're confident it meets the guide (or doesn't contravene) but a second opinion is always welcome...

Gavin A · Mar 31, 2014

one issue with that would be that you have the potential for part of the ring to overload to up to 48A without tripping the MCB as you'd have the potential to draw 32A from the mains, and 16A from the solar PV from sockets on that ring without tripping either MCB.

I think the only way of making that side of things compliant would be to reduce the main MCB to 20A - I forget where this is, but I'm sure it's covered somewhere between the bgb, mcs etc, even if not though then it stands to reason that you can't do anything that might allow the cable to become overloaded without tripping the MCB.

Obviously this depends on the original cable size, if you do the calcs and 48A would be allowed on a ring, then that would resolve that issue.

The other issue being that you're on the wrong side of the RCD, so could well cause nuisance tripping even if the dual RCD situation does work to protect the sockets from both power sources.

It's far from ideal as I'm sure you're aware, but could be possible to get such a system to be safe and compliant with a bit of thought (I think, others may disagree).

If you need to do something like this, an immersion or shower circuit would be a far better option if available as they're current limited devices anyway.

richsogecko · Mar 31, 2014

Hi Gavin,

I get your initial logic but the PV is backfeeding the circuit and the ring circuit is unaffected?

If the customer plugged 48A of devices into the ring I can see that in theory you might not trip the 32A if the PV was working full tilt and contributing 16A onto the circuit but allowing for diversity there isn't a situation I can think of where >11kW of appliances would be plugged into an upstairs ring circuit...unless it's winter, cold and *sexist* Mrs Customer has brought the kettle upstairs to make a cup of tea while ironing and straightening her hair (*sexist recovery* phew) at the same time while sitting in front of an electric heater

Gavin A · Mar 31, 2014

...with the electric fire going etc.

I don't know, I'm not there, no idea if it's 2 rooms on the circuit or 10, so if you're happy to do the calcs and decide that it's safe enough, then fair enough, as long as you've taken account of the issue - personally I'd be making some notes to reference the calcs on the test sheet to explain my thinking, actually I'd probably be dropping the MCB to 20A to be on the safe side unless the cable was going to be ok to take the full combined potential power draw, as I prefer to idiot proof things due to customers being a little prone to doing unexpected things, but I can see the logic for not doing this as well (increased potential for nuisance tripping being a good reason not to).

SolarCity · Mar 31, 2014

I thought that PV being on it's own final circuit was more than just a 'preference'.

I can honestly say that I've never heard of a new CU being installed on a ring circuit before either.

If you're going to do this then I'd follow Gavin's route and change the breaker for a 20A or even preferably a 16A.

richsogecko · Mar 31, 2014

Hi Both,

The Guide to the installation of PV definitely says 'preferably a dedicated circuit'.

I'm not saying I'm right or anyone else is wrong - in my mind it's like spurring off a ring to run an outbuilding or an outside socket etc ... you wouldn't downrate the main breaker in that situation, you'd add a new local point of protection for the extension to the circuit?

Gavin A · Mar 31, 2014

richsogecko said:
Hi Both,

The Guide to the installation of PV definitely says 'preferably a dedicated circuit'.

I'm not saying I'm right or anyone else is wrong - in my mind it's like spurring off a ring to run an outbuilding or an outside socket etc ... you wouldn't downrate the main breaker in that situation, you'd add a new local point of protection for the extension to the circuit?

but you're adding a point of generation, not a new load circuit.

basically the cable in the circuit needs to be rated to potentially take 48A of continuous load, as this potentially is the combined power that can be supplied along that circuit before any protective device tripped (obviously this would actually be a hell of a lot higher in reality as a 32A breaker isn't going to be tripping at 32A anyway).

Found it - Guidance note 7, 551.7.2 i

The current carrying capacity of the final circuit conductors shall be greater than or equal to the rated current of the protective device plus the rated output of the generating set.

BruceB · Mar 31, 2014

I believe 551.7.2.i is relevant for other generators, but not for PV as it is not allowed to be on the load side of a mcb of a shared final circuit (712.411.3.2.1.1). So you would have to blank off the other sockets on the ring if you want to use it, or turn it into a distribution circuit with individual protection at each socket outlet.