1.5mm or 2.5mm, 3 core SWA cable

[saveasteading]

I asked the Marsh rep what cable we needed and he said "Generally, a 2.5mm 3 core armoured cable will need to be ducting out to the tank."

 

He also attached the installation manual which says "A single run of 1.5mm (or greater dependent on distance/voltage drop considerations) 3 core SWA cable".

and "Cable protection via 10amp MCB/RCD or RCBO, rated 230V AC and tripping current 30mA."

 

 

The length of cable will be about 25m from fuse board to tank.

I don't know what power the pump draws to make air bubbles but I seem to recall it is under 100W.

 

It's only worth about £15 difference in cable. But put it another way and it is 20% and wouldn't that be nice through the whole job.

Plus the skinnier one will be easier to manipulate.

 

And does it really need a duct? I was planning to put it in the drain trench. 

 

Electricians' advice welcomed.

 

Edited 18 hours ago by saveasteading

[Nickfromwales]

15 hours ago, saveasteading said:

Electricians' advice welcomed

Well…..I’m no electrician……lol. 
 

But! That’s less than half an amp.

 

25m of 1.5mm2 will do this 41 times over (@ >18a current carrying capacity) so you are very much fine by a country mile. 
 

My only reason to suggest going to 2.5mm2 is (as the cost difference is not worth the argument) if you’d find it handy to have an outside double socket at that location, and then you can run off a 20a breaker; caveat is that you’d either plug the STP into one side of that or make it off to a switched FCU (to downrate the supply to 10a (or 5a or 3a even)).

 

Otherwise 2.5mm2 is massive overkill.
 

Voltage drop is way less of an issue when the cable is buried (directly) in the ground as the temp of the cable is much lower (eg the resistivity is lower), and “yes” you can bury SWA direct and do away with the duct.

 

If the STP has an option for a remote indoor repeater for status / alarm you may want a small duct to pull that cable back to the house. Some do some don’t, but if it’s 25m away you have no way of knowing if the thing has tripped or popped the fuse. 

[ProDave]

Nick has answered it all.  Mine is wired in 1.5mm buried direct in the ground.  The only reason for using 2.5 as stated is should you wish to use the fact you have a cable there and also feed a socket from it.

 

If you want to save even more, use 2 core and use the SWA as the CPC.

[BadgerBodger]

Is it pumped or gravity drained. I recall needing to put 4 core in for my pumped Marsh tank. I put 4mm SWA laid bare earth with a spare duct. Overkill but I had some from another site and it allowed me to future proof for extension to an outbuilding. 

[saveasteading]

Great answers thanks.

I've never understood voltage drop despite minor modules on electrics at school then uni. " how can you not understand this"?

Whereas for concrete design.... you are the only person to understand it. Must have been the teaching skills.

 

I dislike how people casually overspec.

 

No pump just a fish tank bubble maker, so no other cables.

This is the first for me that is mechanical at all, so I know nothing immediate happens if the power is off for a year or so.

We have an outlet charger so can at least see if it is clear. A stick down the cover will tell us if there is excessive flotsam or sludge.

Good point re an outside socket. I don't see a need but will mention to the team.