Multiple SWA Cables through ducting?

[jfgomint]

I have had a kitchen extesion built on an old property (c.1810), which has an unusual consumer unit situation. Part of the property has only a first floor, with a driveway underneath (originally to get carts under for deliveries), with the consumer unit, meter, gas meter etc in a cupboard on the neighbouring building's wall under cover of the first floor.

The extension builder, who said he knew what he was doing installed 50mm corrugated ducting from the extension to the consumer unit approx 12 metres, a metre underground with the gas pipe on gravel, with sand over the top and tape, then buried. I thought okay, we just have to get the t+e through for the cooker (10mm), sockets (2X2.5mm) and lighting (1X1.5mm), may be tight but it was buried by the time i saw it...

Upon investigation and talking to a domestic sparky mate of mine, (I only have experience with manufacture of lab equipment panels), I believe i will have to run SWA through the trunking...

First off, will I get 10mm SWA , 2X2.5mm SWA and 1.5mm SWA through a single 50mm trunking???

Would I be better running 10mm SWA for the cooker from a 50A breaker and 6mm SWA on 45A breaker on existing consumer unit to a garage type consumer unit in the extension and splitting with an RCD to 2.5mm for the socket ring and 1.5mm for the lighting on 2 seperate MCBs, rather than messing around with 4 cables from the consumer unit?

What is the best way to get it through? One at a time, together, method?

From most stuff I have read, pull blue polyrope through with the wire, tie off rope on cable, lube cable , a person pushing and a person pulling, hope sweat and a few choice words.

Any advice much appreciated.

 

[James]

I think I would go for 16mm 3 core swa and put a small consumer unit in the extension.
it will be better and cheaper than trying to pull 3 different cables through.

 

[ipf]

As above, Kitchen CU.

Are you sure you need 50A for the cooker, seems a bit OTT.
I'd probably be looking at a 45amp supply to sub board supplied by 10mm 3c swa.

Obviously, you know the situation, so I'm just speculating.

 

[littlespark]

Corrugated ducting?
That’s going to be fun pulling a cable through… all those ridges to catch on.

Smooth internal walls to the duct would be better.
Corrugated ducting sounds more like field drain pipe.

 

[UNG]

Corrugated ducting?
That’s going to be fun pulling a cable through… all those ridges to catch on.

Smooth internal walls to the duct would be better.
Corrugated ducting sounds more like field drain pipe.

It could be twin wall ducting corrugated on the outside smooth on the inside it should really be orange as well for electrical services

 

[Jimmint]

Thanks for the quick replies, to address a couple of points…

Corrugated trunking is double wall - relatively smooth on inside - black, but warning tape in trench. It is pretty skinny, close to 40mm on ID!

The kitchen extension has 3 circuits:

1. Cooker - 11kW dual fuel range - 10mm2 T&E to proposed kitchen CU point - 6m long

2. Socket ring - 8 doubles - Kettle, MWave, Toaster, TV(maybe) - fused spurs to DWasher, WMachine, FridgeFreezer, Cooker hood - Soon adds up! Approx 25m ring

3. Lighting - All modern LEDs - MAX 1kW - with Smoke and Heat Alarm (should I/do I have to put this on a separate breaker?)

Chatted to another mate of mine, he thinks best option may be to use Visco block on incoming mains from meter, then 16mm2 SWA through trunking to separate new CU in extension, not running it off a breaker in the existing CU. Will be neater and less hassle to wire up.

He said through the trunking one cable not two smaller ones, even 20mm2 if it fits, but not if my incoming mains is only 16mm2. He reckons if I can pull a 10mm2 I can pull a 16mm2 through just as easy, so long as it fits!

Confirms what I think you had all said so far. Can anyone see any issues with this approach?

 

[Jimmint]

Thanks for the quick replies, to address a couple of points…

Corrugated trunking is double wall - relatively smooth on inside - black, but warning tape in trench. It is pretty skinny, close to 40mm on ID!

The kitchen extension has 3 circuits:

1. Cooker - 11kW dual fuel range - 10mm2 T&E to proposed kitchen CU point - 6m long

2. Socket ring - 8 doubles - Kettle, MWave, Toaster, TV(maybe) - fused spurs to DWasher, WMachine, FridgeFreezer, Cooker hood - Soon adds up! Approx 25m ring

3. Lighting - All modern LEDs - MAX 1kW - with Smoke and Heat Alarm (should I/do I have to put this on a separate breaker?)

Chatted to another mate of mine, he thinks best option may be to use Visco block on incoming mains from meter, then 16mm2 SWA through trunking to separate new CU in extension, not running it off a breaker in the existing CU. Will be neater and less hassle to wire up.

He said through the trunking one cable not two smaller ones, even 20mm2 if it fits, but not if my incoming mains is only 16mm2. He reckons if I can pull a 10mm2 I can pull a 16mm2 through just as easy, so long as it fits!

Confirms what I think you had all said so far. Can anyone see any issues with this approach?

Had to re-register, couldn’t get into account, but jfgomint = jimmint

 

[UNG]

Chatted to another mate of mine, he thinks best option may be to use Visco block on incoming mains from meter, then 16mm2 SWA through trunking to separate new CU in extension, not running it off a breaker in the existing CU. Will be neater and less hassle to wire up.

So what overcurrent protection will the 12m cable run have and how will you be able to isolate it at the mains position, I assume a visco block is the same as a henley block

He said through the trunking one cable not two smaller ones, even 20mm2 if it fits, but not if my incoming mains is only 16mm2. He reckons if I can pull a 10mm2 I can pull a 16mm2 through just as easy, so long as it fits!

I assume by 20mm² you actually mean 25mm²

 

[Jimmint]

So what overcurrent protection will the 12m cable run have and how will you be able to isolate it at the mains position, I assume a visco block is the same as a henley block

I assume by 20mm² you actually mean 25mm²

Good point UNG, so I will have to put in Henley Block - Fused Sub-main switch with a 75A fuse (do they do 75A?) to protect 16mm2 cable then new CU in the extension with the three circuits coming off.

Will also be better in the case of firebrigade attending an incident as they can just turn off the CU and Sub-switch at same location (or just pull main fuse).

Also good point about 25mm2, my builder/sparky mate said 20mm2 over the phone… Will have no chance getting that down the trunking!

 

[UNG]

Good point UNG, so I will have to put in Henley Block - Fused Sub-main switch with a 75A fuse (do they do 75A?) to protect 16mm2 cable then new CU in the extension with the three circuits coming off.

Will also be better in the case of firebrigade attending an incident as they can just turn off the CU and Sub-switch at same location (or just pull main fuse).

63A or 80A are your options on fusing if using something like this Metal Fused Box - https://www.liveelectrical.co.uk/Catalogue/metal-fused-box_ID103.html

Also good point about 25mm2, Will have no chance getting that down the trunking!

3 core 25mm² SWA is only 26mm diameter so should easily go through your duct

my builder/sparky mate said 20mm2 over the phone…

Ahh a builder trying to be a spark not a good mix from past experience

 

[ipf]

Also good point about 25mm2, my builder/sparky mate said 20mm2 over the phone… Will have no chance getting that down the trunking!

Mmm!!! As UNG indicates, THAT presents a whole new way of looking at things.

 

[Jimmint]

63A or 80A are your options on fusing if using something like this Metal Fused Box - https://www.liveelectrical.co.uk/Catalogue/metal-fused-box_ID103.html

3 core 25mm² SWA is only 26mm diameter so should easily go through your duct

Ahh a builder trying to be a spark not a good mix from past experience

Thanks for the help UNG. That is what I thought on the fuses. Fused switch box is just what I am looking at. Now it gets technical…

According to my calcs 12m of buried 16mm2 3 core (1 being Earth) SWA should be good for 80A protection with a little overhead on 70’C tables.

That should be enough for a kitchen extension, if not people will need to turn stuff off! I realise it is only 19kW, but the mains for the entire house is only on a 100A fuse.

The 11kW cooker would need to be running 2 ovens, a grill, hot plate/griddle and heating drawer on full. Xmas we may run 2 ovens, but that is only 5kW (2.5kW ea)… The manual calls for a maximum sized connection cable of 6mm2 T&E running on a 40A circuit so they obviously don’t envisage it pulling 11kW for long if at all!

 

[UNG]

While the headline loads seem large compared to the main fuse diversity has to be taken into account, in most properties if you add up the ratings of all the fuses, MCB's or RCBO's they will exceed the rating of the main fuse by some margin while the reality is most circuits are never run anything like close to their maximum and if they are it is only for short periods

With regard to the cooker once it has reached the set temperature the elements cycle using the thermostats to maintain the desired temperature so the current drawn is some what reduced as the full 11Kw is not being used continuously