16mm tails 100A fuse - EV & ESS

Thanks, that’s a really useful breakdown.

Just one correction on CU2 — the ESS branch is on a 50A MCB, not 63A.

So on that basis, the maximum non-fault downstream loading of CU2 would be 32A for the car charger (40A MCB) and 45A for the ESS branch (50A MCB), giving a total of 77A.

That strengthens the case that CU2 itself is not at risk of overload, because the maximum downstream loading is bounded by the connected equipment and their protective devices, rather than simply adding incoming supply and generation capability.

On CU1, I agree that an 80 A fuse upstream is the sensible approach given the PV on the board and possible ESS contribution. This would be 100A, 16A, 22A = 138A without the upstream 80A fuse.

Your point on the tails is also fair. If the CU2 tails are 16 mm² in conduit, then the real question is whether the downstream arrangement and inverter limits genuinely ensure they can never be overloaded. In my case, the ESS output/import is inherently limited by the inverter, but I agree that this needs to be treated as a hard documented limit rather than just an assumption.

I also agree on car charger load curtailment — I wouldn’t rely on that as the sole means of overload protection.

So I think the position becomes:

CU1 protected conservatively with an upstream fuse. Fed from the DNO cupboard to the switched fused, and then onto CU1 using 25mm tails. 
CU2 maximum downstream non-fault load is 77A. I will go for 25mm tails. 

I was not factoring in the actual realistic loading on CU2 before, in that it cannot discharge and charge at the same time and the maximum possible load is set by the device and can only be reduced through settings and not increased. 

Thanks. 

Just reading  this  sorry if I have missed information.

Presumably both the solar and esss can export to the grid, will dno require this to be limited and how is it going to be achieved with two inverters. Seems to be much more export capacity than is normally approved.

Would it be simpler to have a hybrid inverter with solar and essential on same inverter, makes export easier to manage and if limited to say 25a output resuce max supply to cu1 atom some extent.

Cu1 can then either be protected by switched fuse or a unit rated at 125a used. 125a units are available from the likes of schneider  although price at least doubles and space required increases by at least 50 percent. I considered this for one of my projects but going with a switched fuse to.protect the cu to keep costs down.

Personally would avoid trusting user not to.play with max demand settings on ev charger  way to easy to change. Although I have done it with garage submains.

Further thoughts

Most people with house batteries and ev charger want to avoid the car discharging the batteries, how will this be achieved. Probably a bit easier with ac coupled battery and the right ev charger.

Hi, 

No problem, thanks for the reply. 

The DNO has approved a 5kW export and there is a G100 compliant scheme installed.

I have decided on an 80A upstream fuse for CU1 and calculated the CU2 for the EV/ESS cannot go beyond 100A in any scenario. 

Thanks. 

Hi, 

The EV can charge basis off peak tariffs (cheaper rates) as can the ESS (eg Octopus have API support). You can also discharge from the ESS into the EV by setting the max charge demand on some EVs to match the output of the ESS (to prevent grid pull). 

It’s all very configurable. 

Yes all very configurable, difficult to get operating correctly especially if Octopus intelligent go is involved and there are charging slots when the battery is charging the house.