16mm tails 100A fuse - EV & ESS

vantech said:

I currently have job with a 100A DNO fuse with 25mm meter tails feeding the main consumer unit.

Single phase or three-phase?

vantech said:

I currently have job with a 100A DNO fuse with 25mm meter tails feeding the main consumer unit. I’m planning to install an additional external consumer unit via a Henley block to supply:

• 7kW EV charger
• 5kW battery storage system (charging) plus 5kW backup circuit (discharge capability) - total 10kW. 

1. What is the consumer unit rated for?

2. Is there only one consumer unit for the premises?

vantech said:

The maximum potential simultaneous load on these tails would be approximately 74A (5kW charge + 5kW discharge + 7kW EV).

I'm not sure I follow that ... although RDF might lead you there for the consumer unit.

Hi, 

It’s a single phase install. 100A cut out, residential supply. 

DNO has approved a 5kW feed-in. 

The ESS has a 5kW inverter, and a backup circuit (creates a N-E bond during backup, supplemented by an Earth rod). If the ESS is charging its batteries at 5kW, there is also a possibility that the backup (essential loads) output can also pull 5kW so the total demand from the ESS has a theoretical maximum of 10kW (with the obvious lower loading noted above, socket circuits etc, not the kitchen - gas heating, no large loads, plus lighting). 

This new ESS combined with a 7kW EV charger, will connect into a new dedicated external CU. The plan is for the ESS to sit on a 63A MCB, and the EV a 32A RCBO (2 pole). This new CU is then connected via 16mm tails to a Henley block. 

The Henley block is fed from a DP isolator (from the back of the meter). The main house CU (25mm tails) are connected back in, with the new 16mm tails connected to the new CU. 

How long will the 16mm tails be?

There's a regulation that says anything under 3 meters can be protected by a downstream device. Although it's a controversial get out and may not comply with local  dno requirements many installers use 16mm tails.

The 16mm tails will be approx 1.5 metres from the cut out to the new external CU. 

74A (5kW charge + 5kW discharge + 7kW EV).

Is the battery system physically capable of both charging and discharging at the same time? (Systems I've looked at certainly aren't and Ohm's Law tends to make it a bit difficult if there's a single conductor involved).

There's a regulation that says anything under 3 meters can be protected by a downstream device.

Although the 3m limit is for fault protection - overload protection can be anywhere (at the start, at the end, or anywhere in the middle) - as is the case for unfused spurs...

   - Andy.

The ESS has a 5kW coupling (charge and discharge) and a 5kW backup output. 

If the battery is charging, and the backup loads are consuming 5kW, the total demand for the ESS is 10kW. 

So presumably if the tails are less than 3m, I can work on the basis that the connected loads (10kW for ESS and 7kW for EV) are lower than the maximum current rating for the 16mm tails? 

Thanks. 

In summary, the proposal is to feed a maximum load of 7.2kW (car charging) + 5kW (battery charging) + 5kW (ESS output to sockets) a total of 17.2kW ~= 75A via the new tails.

The argument for this is that the car charging load is limited such that the maximum load on the new 16mm tails (or the DNO fuse, not clear which) will be limited to 60A.  The loads not controlled by the current limitation feature total 10kW ~= 43A.

16mm tails have a rating of ~87A in free air.  The fact that routing constraints are mentioned suggests that this rating might not be achieved but doesn't give any indication as to how much it might be reduced by.

It is possible that the tails would be OK without the 60A current limitation on the charger, it is also possible that the tails would be insufficient even with a 60A current limitation although this is unlikely - this will depend tails installation method.

My question is: "Is it permissible to use current limitation features in car chargers or energy storage systems as the only protection against overload or should a more traditional device (fuse, MCB, etc.) be provided?".  It is not clear that these features are intended to fulfil a safety function so my suggested answer is "No".  If that is right then the design needs to assume the tails will be loaded to 75A, in fact, given the protective devices in the new CU will be a 40A MCB for the car and a 40A(?)MCB for the ESS then 80A might be the number to use.

Hi, 

The EV will be curtailed to 60A (total supply demand).

The theoretical maximum demand from the new CU would be 74A maximum. Given the connected loads on the backup, it’ll rarely (if ever) get to this level. 

The main building CU is pre-existing and supplied via 25mm tails. 

The 16mm tails are 1.5m long, from the external meter box enclosure, through a length of flexible conduit, and into the new CU. This new CU will have an SPD, 63A MCB for the ESS and a 40A for the charger. These are overrated as per the manufacturers specs, but the load is always controlled downstream. The maximum can never go beyond 74A. It’s impossible (unless under fault of course). 

Thanks.