Mains Cable to the new Observatory

[Jkulin]

Hi All,

My builder is coming over this weekend to prepare to pour the concrete on Monday and I completely forgot to order mains cable.

I need to order it quickly so can anyone advise what cable I will need to run the main through the conduit underground.

It is going to just plug into the 240v outdoor waterproof plug in the lean to.

At the moment I run 12v cabling out to the pier, the plan is to have sockets in the dome to power the PC, Humidifier and all the peripherals.

The run will be around 35 metres.

Can anyone advise please?

Many Thanks.

[carastro]

If any of this is going be exposed and got at by rodents then I'd get armoured cable. 

Carole 

[Jkulin]

5 minutes ago, carastro said:

If any of this is going be exposed and got at by rodents then I'd get armoured cable. 

Carole 

Thanks Carole, nah it will all be underground in 3" piping

[Dr_Ju_ju]

You must still run it using armoured cable, rats can get anywhere, and safety is the primary importance....

[Skipper Billy]

As long as you have got the ducting in then there is no rush for the cable. Put a couple of mousing lines in to pull stuff through and when you pull a cable through tie another mouse line to it so you still have two.

[Jkulin]

Thanks David, yep I plan to leave 3 lengths in there just in case

[wxsatuser]

On 28/05/2020 at 14:13, Jkulin said:

Hi All,

My builder is coming over this weekend to prepare to pour the concrete on Monday and I completely forgot to order mains cable.

I need to order it quickly so can anyone advise what cable I will need to run the main through the conduit underground.

It is going to just plug into the 240v outdoor waterproof plug in the lean to.

At the moment I run 12v cabling out to the pier, the plan is to have sockets in the dome to power the PC, Humidifier and all the peripherals.

The run will be around 35 metres.

Can anyone advise please?

Many Thanks.

Depends on what your electric supply is.

If you have a metal pier it no doubt will be part of earth and will need bonding for safety.

If your supply is TN-S the cable will need to be a minimum of 6mm².
If it is TN-C-S it will need to be 10mm²

If you happen to be TT, rare in modern properties, then just install an earth rod and make sure you have RCDs.

Edited May 30, 2020 by wxsatuser
typo

[discardedastro]

Just to add to this (perhaps too late!) so long as you have a good drawrope in the duct it doesn't matter now. You'll find nylon 2kN drawrope available at lots of places as lots of utilities use it like BT etc.

I'd always provide for a local earth at the obsy especially on long runs and treat the obsy as a TT system hanging off your house (which could be TN-S/TN-CS/TT, doesn't matter) with a RCD on the supply at the house end. Local earthing will generally give you a better ground for fault currents and means you're not reliant on the cable for a safety ground. It's also cheap to do - bash a rod in, clamp to the top, bond it to your incoming earth. Put a cap on the top and reinspect every 5 years or so to make sure it hasn't corroded away.

This is all notifiable work so you'll need to get a sparky to at least sign it off for insurance etc - may be best to just get one in who can advise properly anyway.

I'd not worry about SWA armouring in duct, but I would make sure the ducts are continous and sealed at each end. Either use a plastic cap over wire mesh or use a rodentproof sealing mastic - there's various ones out there, we use Filoform sealing mastic at $dayjob which is pretty well anything-proof. The mastic approach also stops gas/water which can be a benefit.