16mm tails 100A fuse - EV & ESS

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. 

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. 

Does the installation method for the new 16mm tails mean they can safety carry 75A?  I don't think you can rely on the car charger for safety functions (i.e. overload protection in this case).

The charger cannot pull more than 7kW. The load curtailment on the charger is attached to the main meter tail (before the Henley block) to ensure the total building demand remains below 60A, it isn’t being used to control the load on the 16mm tails and the CU this supplies. 

The maximum demand will only ever be as high as 74A on this second CU fed by 16mm tails. 

With respect to the tails:

• if they were in free air then they would be fine (assuming that overall loop impedances are low enough for ADS).
• if they were wrapped in insulation then they probably wouldn't be fine.
• reality is probably somewhere between the two.

The 60A load curtailment is a red herring although I do wonder why it isn't set to something higher given the apparently 100A supply.

There is a separate discussion as to whether both CUs are now operating within their rating, the odds are that the old one will be outside its rating (sum of circuit breakers in it greater than 100A with a 121A capable supply) arguably the new one is OK.

They will be contained within 1m of copex between the meter cupboard (external) and the new CU. 

The current install has a kWh and ammeter installed. Even with the current EV, electric cooker, hot tub, and an existing ESS (3kW - to be replaced by the new version) the actual demand from the whole property has never gone above 50A in 2 years. 

I don't know the rating of 16mm sq tails in Copex so can't comment on their rating.  Hopefully that data is available somewhere.

The relevant regulation (536.4.202) doesn't permit the use of diversity to protect the CU (specifically the main switch or RCCDs covering several circuits) against overload - either there needs to be an external device (e.g. the DNO fuse or separate switchfuse) or the sum of downstream breakers needs to be less than the switch (unlikely in most domestic CUs).  In your case, the DNO fuse plus the ESS can deliver sufficient current to exceed the main switch rating (assumed to be 100A).

As gkenyon has helpfully referenced the relevant table of the regs, 16mm tails are rated at 76A in conduit but 61A in conduit in a thermally insulating wall.  You are right on the edge of the cable rating - if the conduit is just clipped to the wall then it should be OK, if it has to pass through a wall then it is a lot more debateable.

The ESS when it’s providing power to the main CU, will only provide as much as is required. The DNO fuse can in theory provide 100A, with the ESS providing a further 27A. That said, the main CU has never pulled more than 55-60A over the last 2 years with an ammeter installed on the supply. 

Does the fact the ESS is upstream of the main CU, need to be factored in as a separate source for the main switch rating in this main CU?

Ps the copex loops out the bottom of the meter box and into the new CU. No insulation, fixed to the wall with saddles. 

The tails sound OK to me from a rating perspective.  Will the Copex provide adequate mechanical protection?

My understanding (and I would recommend that you get opinions from others as well on this as I wouldn't say I was 100% confident) is that you need to ensure that the CU current is limited to its rating (which is probably 100A) and the use of diversity is not an acceptable way of doing that.  Normally in a domestic situation that limitation is provided by the main fuse but the addition of generation in parallel with the main fuse may mean that that is no longer the case, depending on fuse rating and generation rating.

Look at Wiring Matters from November 2024 (https://electrical.theiet.org/wiring-matters/years/2024/103-november-2024/how-does-the-installation-of-microgeneration-affect-the-rated-current-of-a-consumer-unit/) and the BEAMA guidance on consumer unit ratings (https://www.beama.org.uk/static/uploaded/6861a2e1-d8d4-4d81-b5fa710ba60ca4a7.pdf, which whilst written primarily around RCCDs also applies to the main switch).

The "simple" fix is to install a 100A switchfuse to protect the old consumer unit (only).

DShutt said:

In your case, the DNO fuse plus the ESS can deliver sufficient current to exceed the main switch rating (assumed to be 100A).

vantech said:

the actual demand from the whole property has never gone above 50A in 2 years. 

In which case, the DNO's fuse could be down-rated.

Down-rating the supply fuse is the simplest solution but the point of this exercise is to add 12.2kW of load to the property (car + ESS charging).  One might expect that charging would occur at a different time to cooking / hot tub use but I think it would be unwise for the electrical design to mandate that.

Having said that, we are told that the car charger has load curtailment to 60A (but we don't know why), in which case you might get away with a 60A fuse at the cost of car charging being slow for some of the time.  An 80A fuse results in a maximum supply current of 101A to the old CU - arguably not acceptable but one might form a view about the 1A exceedance.