Secondary consumer unit from SWA cable

[tvrulesme]

All of the work described below will be completed by a qualified electrician, I am just curious and using the info for planning!!

I am planning a garage conversion. While I was bringing across services for water/gas etc I laid a 16mm2 3 core SWA between the main dwelling and the garage (all at the correct depths and signed of by building control) which will feed a secondary consumer unit.

In terms of connecting the SWA, I am trying to determine the best way to achieve max load to the secondary consumer unit in the garage conversion because it will draw significant power including:

• Washing machine
• Tumble dryer
• EV Charger

The SWA is 15 meters long and according to TLC Direct voltage drop calculator I have a max load of 87 Amps so I was thinking of protecting using something around 80 Amps:

Required Cable Size	16mm
Voltage Drop	3.26 Volts
Percentage Drop	1.4%
Current Load	87.0 Amps
Max Cable Load Max Cable Load refers to the maximum allowable current for the cable used in the configuration given above and will vary considerably under different configurations.	94.0 Amps

Very very basic diagram looks like this:


As far as I see it I have the following choices:

• Following the supply isolator, Henley block to split into fused main switch (80 amp) and the other side into the consumer unit
  • Problem: fused main switches do not seem to have cutouts for SWA glands so I would need another ugly box where my meter stuff is to terminate the SWA
• RCBO using a spare way in the consumer unit to feed SWA
  • Problem: Hager RCBOs seem to go up to 40 Amp, I would have to mix and match RCBO brands to get 80 Amps
• Something else inside the consumer unit (MCCB or RCCB?)

What would be the cleanest and best solution?

 

[Julie.]

There is way more than this than you are considering.

Firstly since you have water services, you may indeed have to deal with bonding issues, this will depend on your incoming (supply) cable size.

If you have a substantial load such as you describe, you may very well exceed the maximum demand acceptable for your supply.

If an EVCP is being fitted, given the MD above, you need to apply to the DNO for permission to fit an EVCP. If you are indeed fitting an actual EV charger, this will certainly exceed the supply capacity, do you really intend to fit a charger and not a charge point?

If you do have an EVCP then the requirements for rcbo/rcd may be very specific.

You do not need a RCD or RCBO at the feed point, In fact this would nullify any RCBOs further down the line. A MCB, or fuse switch would be better.

I would check your load calculations, if you need 80A plus your home cu then you would almost absolutely be exceeding the maximum demand permitted by your supplier.

Also to note, if this is the case, one way out is to limit the EVCP via a CT on the supply lines into the property, this would generally require communication cables from the EVCP through to the CT at your incoming supply point, also note that almost all new EVCPs need an Internet connection to allow controlled switching anyway.

So much more than just "do I fit a rcbo or kmf?"

 

[plugsandsparks]

As above, needs a proper design of all necessary parameters , if you are "planning" at this stage i would just lay 2 ducts one for power, one for control and leave it at that.
Interested in what EV charger you have in mind

 

[Aaron b]

I would expect the EV charge point to be 16 or 32A . As Julie said there's usually a current transformer, this monitors the load drawn by the home and limits charging based on the whole house use. You will need an additional cable for that. Your sockets would likely be on a 20a radial so it's unlikely you will need 80amp as this is more than a lot of homes.

 

[Intoelectrics]

Problem: Hager RCBOs seem to go up to 40 Amp, I would have to mix and match RCBO brands to get 80 Amps

Don't mix and match the mcb's. Also a 80A circuit in addition to the other circuits may potentially overload the capacity of the CCU dependent on its rating.

My advice, I think you should get a qualified sparky in to design this installation properly.

 

[tvrulesme]

There is way more than this than you are considering.

Firstly since you have water services, you may indeed have to deal with bonding issues, this will depend on your incoming (supply) cable size.

If you have a substantial load such as you describe, you may very well exceed the maximum demand acceptable for your supply.

If an EVCP is being fitted, given the MD above, you need to apply to the DNO for permission to fit an EVCP. If you are indeed fitting an actual EV charger, this will certainly exceed the supply capacity, do you really intend to fit a charger and not a charge point?

If you do have an EVCP then the requirements for rcbo/rcd may be very specific.

You do not need a RCD or RCBO at the feed point, In fact this would nullify any RCBOs further down the line. A MCB, or fuse switch would be better.

I would check your load calculations, if you need 80A plus your home cu then you would almost absolutely be exceeding the maximum demand permitted by your supplier.

Also to note, if this is the case, one way out is to limit the EVCP via a CT on the supply lines into the property, this would generally require communication cables from the EVCP through to the CT at your incoming supply point, also note that almost all new EVCPs need an Internet connection to allow controlled switching anyway.

So much more than just "do I fit a rcbo or kmf?"

What an awesome reply. This is why I love this forum.

So for water services, it's all plastic MDPE pipe so I don't think I would need bonding for this BUT I do have gas (feeding an already installed boiler in the garage) which is already bonded in 10mm. Will leave it to the spark to decide whether to bring across earth with the 3rd core of the 16mm SWA or install an earth rod.

My calculations were very much based on "I have this cable which is capable of this, what should I do to protect it" rather than looking at the actual loads. Will take a look at this.

Limiting the EVCP sounds very sensible.

There's internet in the garage (in fact this is where it enters the property) so I can get ethernet or wifi there very easily.

 

[tvrulesme]

Don't mix and match the mcb's. Also a 80A circuit in addition to the other circuits may potentially overload the capacity of the CCU dependent on its rating.

My advice, I think you should get a qualified sparky in to design this installation properly.

Yeah 100%. I just like to know what the sparky is talking about hence the question here

 

[Intoelectrics]

Yeah 100%. I just like to know what the sparky is talking about hence the question here

Good, and yeah it is useful to get an idea of what to expect when the electrician suggests a design. Many people just assume its just a matter of slinging a cable across and job is a good 'n. Unfortunately there are always many considerations that have to be taken into account, especially so to satisfy the current BS7671 regs.

 

[tvrulesme]

Interested in what EV charger you have in mind

No idea yet. Just want to know that it would be possible to have one. I have a mini electric on order which won't arrive until Jan 2023 so I have a bit of time. if it has to be 13amp would be a shame but so be it

 

[Julie.]

May seem like a minor point, but please realise the difference between an EV charge point, and an EV charger.

An EVCP presents the supply "13A", 16A 32A etc, and uses the charger within the vehicle, so the charge is limited by the lower of the EVCP or vehicle itself (eg a vehicle with a 5kW charger internally could only charge at ~20A when on a 32A EVCP or 16A when on a 16A EVCP)

An EV charger is usually something like 50kW or 75kW plus and provides DC directly to the vehicle which bypasses the charger within the vehicle.

 

[pc1966]

Others have already covered the main points. However, on the specifics of feeding the SWA cable you don't need or indeed want a 30mA RCD/RCBO at the supply point as it is just asking for spurious trips, and due to the earthed armour you are already protected from shock on a cable penetrating accident (e.g. nailing a buried section).

But how you protect the new sub-main against faults or overload depends on the characteristics of your incoming supply. If it is of adequately low Ze (fault impedance) then a fused-switch is you best choice and you do get metal bodied ones that the SWA cable can be directly glanded on to.

If you cannot meet an adequately low end of sub-main Zs to achieve fast enough disconnection times (5s for sub-main on TN earth, or 1s on TT earth) then you would need a RCD, but that could be a 100mA or 300mA delay type so you achieve selectivity with downstream RCD/RCBO in the garage CU and don't have everything going off on a single fault on a final circuit.

Finally if going with a switched-fuse you might want to consider your supply fuse (in the DNO cut-out) both for the above points about the total load you can safely draw, and also so you can chose the sub-main fuse to be selective with that so a major fault such on the garage circuit will not take out the home's supply and need a call to the DNO to replace the sealed fuse in their equipment. For BS88 series fuses that is usually achieved at a 1.6:1 ratio, so if your main supply is 100A then a 63A sub-main fuse should be totally selective with it, if your supply is 80A then a 50A sub-main, etc.

You could use a MCB to feed the sub-main but generally they have much poorer selectivity with downstream faults, and going for a high magnetic trip curve to help mitigate that normally results in a harder to meed end of cable Zs, compared to a similar rating of fuse.

 

[tvrulesme]

May seem like a minor point, but please realise the difference between an EV charge point, and an EV charger.

An EVCP presents the supply "13A", 16A 32A etc, and uses the charger within the vehicle, so the charge is limited by the lower of the EVCP or vehicle itself (eg a vehicle with a 5kW charger internally could only charge at ~20A when on a 32A EVCP or 16A when on a 16A EVCP)

An EV charger is usually something like 50kW or 75kW plus and provides DC directly to the vehicle which bypasses the charger within the vehicle.

Thank you Julie. Shows my naivety. I thought the name was interchangeable. I do indeed mean an EVCP. The ones I have been looking at are 7.4kw 32 amps

 

[Julie.]

Thank you Julie. Shows my naivety. I thought the name was interchangeable. I do indeed mean an EVCP. The ones I have been looking at are 7.4kw 32 amps

Most cars still have onboard chargers around 7kW, but PHEVs are usually around 3.3kW

There are newer vehicles with 11kW chargers onboard.

 

[timhoward]

Make sure any plan fully considers the EVCP as this as it can affect several factors of this design.

I have a mini electric on order which won't arrive until Jan 2023 so I have a bit of time. if it has to be 13amp would be a shame but so be it

You certainly don't want a 13amp EVCP. (In my view these should really be banned!)
I'd be fairly certain there will be a way to fully utilise a 7Kw EVCP but you'll need your sparks help to arrive at this. There are solutions available for some charge points to wirelessly connect a current monitor.