AC current - load vs solar generation

[aesmith]

Hi,
This relates to questions I have been asked by the installer. I have suggested the inverter connects to the garage DB which is fed from the house by SWA. The installer has asked for details of existing loads in the garage to confirm the cable has capacity for the "extra" current from the inverter.

But to my mind, unless there's something weird about AC then surely generated current will be subtracted from the load current rather than added to it. For example let's say stuff in the garage is drawing 10A and solar generates 5A then the cable would be carrying the difference, 5A, from the house. If solar generates 21A then the cable would be carrying 11A but in this case back to the house.

Does that sound correct? Just wanted to sanity check, i dont want to come across as a smart arse to the installers especially if I'm wrong. But also when the actual technically knowledgeable people turn up to install, I don't want to find then walking off because I've not filled in the questionnaire correctly.

 

[EFLImpudence]

I see your point with the way the question is worded, but -

What if there is no load current in the garage?

Is the cable large enough for all the solar going to the house?

 

[aesmith]

Yes it is. It's 25sq.mm and the sparky who installed it rated it as 80A.

As it happens there's probably head room even if you did add actual garage load onto solar instead of subtracting. But maybe not if he went round trotting up every power tool or appliance and wanted to allow for everything being in use simultaneously.

 

[EFLImpudence]

Fair enough. That's bigger than most garage supplies will be.

 

[EFLImpudence]

Might even be bigger than the main house supply.

 

[aesmith]

Might even be bigger than the main house supply.

It is. Our transformer is 64A according to the data plate.

We went for his 80A option for future proofing and to minimise voltage drop for circuits run on to the other outbuildings. Wouldn't have saved that much if we'd gone for 60A (presumably 16sq.mm)

 

[aesmith]

And just to be clear, I am correct that they get subtracted, rather than added?

 

[EFLImpudence]

Yes, the current is drawn by the load; not pushed by the supply.

 

[ericmark]

Yes your correct, I would assume standard form, I would have thought the designer would want to see for himself, earthing methods, RCD type etc. I would not want to rely on what the owner said.