UK inverter breaker sizes and tails

[ns31]

Hi all, I'm confused about grid connection breakers with inverters (in hybrid or off grid) and the AC input/ grid connection.

Taking a 7.5kw solax inverter for example, the max AC input is 40a. Therefore, a 50a breaker in the consumer unit would be suitable, but my confusion is mostly around cable sizes as most properties in the UK use 25mm tails to feed the main board.

If the inverter AC input is supplied from a 50a rcbo for example, most rcbo's only accept 10-16mm tails. Therefore an inverters AC out can never power the main board/CU in the house if that CU previously used 25mm tails for the input.

Does this mean that inverters can only ever power a new CU with a small amount of new circuits rather than a whole house?

A house has a main feed -> main board, it seems that main feed -> inverter -> main board isn't possible? Instead an additional board must be added?

 

[ns31]

Racking my brains I think I've figured this out. At 40a, the inverter would only require a 10-16mm cable, should more than 7.5kw be required, a second inverter also has it own an rcbo on a 10-16mm cable. The AC outs of the inverters then combine at a parralel box, this box then has 25mm tails to a main CU. So the rcbo itself doesn't need to take 25mm tails

 

[dslrx9254]

I can tell you Sunsynk only recommend 8mm2 cable with a 50A breaker for both grid / load ports. This is for the 8.8kW ECCO inverter. 10mm2 is the next cable size up which I think is suitable for loads up to 64A (clipped direct - assumed typical mains cable). The neutral fly lead on a 50A RCBO is definitely a lot less than 16mm2.

Please bare in mind you are supposed to apply a de-rating factor and work out the voltage drop of the cable. I think the voltage drop is probably mostly applicable to long runs. The de-rating factor is about how you mount the cable so if it is in an insulated cavity wall or a conduit it's rating needs to be dropped. Realistically it is just another component that will heat up so you just need to make sure the cable and connectors aren't getting too hot. The other nonsense that gets applied in reverse is diversity which I believe allows you to increase the rating based on the average current over time crudely put as less time on or off per 30 or ? mins.

If I look inside an 18th edition CU there is typically only 16mm2 cable from the 100A main switch to the neutral bar. I think that might be the best case for the cable. It's a short run not in a conduit or insulation and is supposed to be well terminated.

25mm2 cable is normally req'd now for the meter tails but I've seem some arguments it's a little over spec'd. I think however the normal ratings still puts it at around 100A when in an insulated wall i.e. after de-rating. So perhaps they are making allowances that you may change the wall insulation for example.

 

[MisterB1959]

I've used, or rather my electrician has, 6mm cable for a 5kw Sunsynk inverter via a 40A RCD (in a consumer unit) which has 16mm tails to a Henley Block (in the UK)