Cable grouping at Consumer Unit

[Toby]

Hi all,

What is the position with calculating grouping for cables at the consumer unit?

In my proposed location, I would have a 50x50mm trunking over the top of the CU, that then meets the same size trunking running from the floor to the ceiling of the ground floor. (It's all going to end up in a corner cupboard, so the trunking won't look as bad as it sounds it might!)

So all cables will be grouped together over about 50cm over the top of the CU (Starting with one cable at the far end, gradually increasing over the width of the CU)

Then the following cables that go upstairs, and are grouped for about

1.4m... Upstairs rung (2x2.5mm T&E) Upstairs lights (1x1.5mm T&E) Downstairs lights (1x1.5mm T&E) Outside lights (1x 1.5mm T&E) Loft Power (1x2.5mm T&E) Emergency lighting (1x1.5 T&E)

Then the following cables that go under the ground floor, and are grouped for about 1m... Kitchen ring Downstairs ring 2x2.5mm Alarm 1x1.5mm Fridge 1x2.5mm Hob 1 x 6mm Oven 1x2.5mm Microwave 1x2.5mm Air Conditioning 1x2.5mm (20A radial)

There is also a separate SWA cable for the garage, but this is on its own.

Thanks for any pointers!

Toby...

[js.b1]

...

Flat Twin & Earth is rated to Medium Impact Duty (AG2). Domestic Environment is classed as Low Impact environment (AG1).

Since "it's all going to end up in a corner cupboard" I would dispense with trunking, or at least use multiple 38x25mm trunking and run a nominal number of cables through each.

As a guide cables loaded to 30% of their grouped rating can be disregarded for grouping purposes. In practice this means that lighting up, lighting down, smoke, outside socket, security light supplying cables may be disregarded from grouping calculations (ie, a

1.5mm cable protected by 6A Overcurrent (OC) device).

Likewise if your main fuse is 60A and it is obvious from cable routing that whilst some (cooker, shower) cables may be drawing 16A (cooker diversity) & 45A (shower has no diversity) then other cables will actually be essentially zero, then grouping can be disregarded. It is common in domestic for cables to be grouped seemingly "against regulations", however a proper consideration can show the decision is quite sound.

Where you can not ignore grouping is where you (say) ran the cooker & shower cable against one another, or through insulation.

For practical purposes I would not consider using 50x50mm trunking, but instead 2-3 38x25mm - thereby automatically spreading cables out between the 2-3 "ducts". Lighting cables are your friend in that they can be disregarded.

I notice you have air conditioning in there, if it is wall mounted (off the ground) it does not require bonding back to the MET, however if it is sat on concrete it does - so consider that when routing cables. I would leave one trunking almost empty in order to add future circuits as necessary.

If this is "all new", fit a larger CU than you might consider originally, and size the cupboard accordingly. If the shed has its own sub-main CU, you may want to put the HVAC on that side of things - re "outdoor stuff". Same goes for security lighting, so it does not trip out the house lights or a ring with the fridge/freezer on it for example.

[ARWadsworth]

Start with

with all the diversity that you have thrown in then there is little chance of anything but the aircon running at or near full load for any length of time.

Why does the emergency lighting have it's own supply?

Cheers

Adam

[Toby]

Thanks Adam - I did think with all the diversity it would be fine. I can route the A/C separately easily too, as it needs to go straight outside to an isolator near the external (wall mounted) unit before going anywhere else.

This A/C is actually a three way split system with three seperate compressors(9,9,18K BTU) and it is unlikley I would have all three units running at the same time, as I have the two 9k units in bedroom 1 and 2 and the 18k unit in the through lounge diner, as they are reversable, I can also use them as a backup or addition to the gas fired boiler for space heating.

The upstairs EL is powered from the upstairs lighting RCBO, and the downstairs for the downstairs RCBO, but I have installed two of the MK secret key switches above the CU, plus two normal switches (all on a 4 way MK Grid plate), so I can put them on test without taking out the lighting circuits and so I can also switch them on if I want to just use them as a light (They are flush fitting, maintained fittings, so don't look too bad in a domestic setting IMO, although the hyper bright greed LED may be dimmed later!). so it is actually three core and earth cable to them from the switch plate above the CU.

Toby...