Inverter connnected to garage CU - testing

[dougalthedog]

Hi,

I am doing an install where the inverter is connected to a cable feeding an existing garage CU - we have split the cable and fitted a 2 way CU for the inverter only. Cabling is 6mm which is fine for the inverter supply, however I suspect it may be on the small side for the number of garage circuits. Do I require to calculate this and replace the cable if necessary and if so would I need to test all circuits fully - i.e. how far does my responsibility go? I am not modifying / connecting anything to the existing garage CU.

Thanks.

 

[Markc]

Million Dollar Question.

IMO I see you having no responsibility over something you have not installed or modified however if you are using an existing supply cable which you feel is currently undersized for it current load I think you should make your design allowing for this to be upgraded.

There is an argument for PV however which has been debated but not confirmed and it should be left to your own judgement on how to proceed.

The example being similar to this:

Your full load in the garage is say 40amps. You full output of the PV is say 16amps. The supply cable to the garage is rated at 40amps also. Should this cable be upgraded to accommodate 56amps?

The thought is if on full load in the garage the draw from supply is 40amps however when the PV is in generation this could be producing 16amps of the 40amp required meaning the draw form supply is 24amps. If the load in the garage drops to 0amps then the supply cable is only carrying 16amps. In any account the supply cable cannot pull or carry more than 40amps and the balance is maintained by supply and own generation.

This is not considering other factors such as voltdrop etc etc.

Food for thought but don't consider this as correct.

 

[moggy1968]

have you applied diversity to the garage load. 6mm should be more than ample for a garage.

 

[dougalthedog]

Mark, I agree that the PV install would in practice not increase the load on the cable - presumably it should be dimensioned for whichever has the higher maximum current, the PV system or the garage system, with the proviso of ensuring the PV voltage drop is within tolerance.

Moggy, the "garage" board looks to feed several outbuildings on a farm type scenario and whilst it is unclear exactly what is served and I have yet to do the full calcs I suspect if I did a design exercise from scratch 6mm may be underrated.

So the bottom line is, do I protect the cable with a suitable breaker and not worry as long as the garage load doesn't cause this to trip, or do I map out exactly what is served and do the design calcs for the whole system as if I was installing it from scratch?

 

[Markc]

Mark, I agree that the PV install would in practice not increase the load on the cable - presumably it should be dimensioned for whichever has the higher maximum current, the PV system or the garage system, with the proviso of ensuring the PV voltage drop is within tolerance.

Moggy, the "garage" board looks to feed several outbuildings on a farm type scenario and whilst it is unclear exactly what is served and I have yet to do the full calcs I suspect if I did a design exercise from scratch 6mm may be underrated.

So the bottom line is, do I protect the cable with a suitable breaker and not worry as long as the garage load doesn't cause this to trip, or do I map out exactly what is served and do the design calcs for the whole system as if I was installing it from scratch?

Option 3 consider installing your own dedicated supply cable solely for the PV system.

 

[dougalthedog]

Like the fact that "option 3" is the least satisfactory solution all round, but at the same time probably the "best" option in the sense that I can ignore the CU. However, if I was going to do this, I'd probably give the customer the option of paying for the difference in cost to upgrade their existing 10mm.