Sparks help. Armoured cable mm2

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  • Sparks help. Armoured cable mm2
  • Premier Icon Stoner
    Subscriber

    What diameter armoured cable for:
    Max 13A load / 2kW peak
    20m
    Buried for half that length.

    according to this http://www.cable-calculations.co.uk/test/psc.htm
    1.5mm2 would be sufficient. Which would be good as it is easier to handle and cheaper too…but better check with STW 😉

    cheers

    Think this comes out the same too
    http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

    Premier Icon wwaswas
    Subscriber

    If footflaps were specifying it for his bike shed;

    Premier Icon Stoner
    Subscriber
    Premier Icon footflaps
    Subscriber

    I did some work with Norweb years ago and have a load of cut section of monster power cables as paper weights somewhere at home.

    Scary thing is they joint everything live, dig up road, lay rubber mat under 415v cable, cut open cable, fit junction box, connect it all up, pour in resin and bury cable.

    Another interesting factoid, from Norweb, was that each phase normally has a 600A fuse at the sub station, but if your house is more than 200m from the substation, the cable resistance is such that the fuse can’t blow as the load won’t get to 600A, so it just sits there arching until someone comes along and pulls out the fuse in the substation!

    Premier Icon Stoner
    Subscriber

    probably makes their hair stand up though.

    Just thinking, I would be OK using earthing glands on my junction boxes, using the armour as earth and twin core for the live wouldnt I?

    Premier Icon footflaps
    Subscriber

    I’ll have to dig out some photos of a sub station, the bus bars have massive clamps on them every 50cm as the current is so high the magnetic field can rip them out of the wall and short them together if they’re not really secure.

    Then there are the oil filled switches with huge springs that will break your arm if you don’t complete the switch cycle – they load a massive spring which suddenly goes to get enough velocity on the contacts to keep arching to a minimum. However, if you let go of the lever before the spring has released, it will just fire back up at you at about 100 mph and god help you if your arm / head are in the way!

    Premier Icon Stoner
    Subscriber

    found some inexpensive glands on fleabay

    getting fed up running extension lead from the shed out to my IP66 socket in the rain and then back in again before the cows eat it…

    donks
    Member

    I make it 2.5mm for a 13A load, this was buried all the way. Calculated on HEVACOMP electrical design software.

    Premier Icon Stoner
    Subscriber

    interesting donks. I wonder why your calcs come higher.

    If I use 3kW (13A) peak, for SWA twin in PVC, I still get 1.5mm with a small v drop (<4%)

    Ah! Im speccing non lighting circuit. If I select lighting it says 2.5mm. Why’s that? I will be running some CFL bulbs on the supply. Does it matter?

    donks
    Member

    The design package takes other things such thermal resistivity/impedance and volt drop from the rest of the system into account. The volt drop for a multi core armoured run 20m (buried direct in ground)with a 13A design current is 4.68V.

    In all fairness if it is for the garage (power and lighting) I would tend to lean towards 2.5mm or bigger even if its only backed up on a 16AMCB.

    If the load were 13A for just lighting then you get a fair amount of reactance in the circuit which will necessitate the use of a bigger conductor.

    Premier Icon Stoner
    Subscriber

    I think I have 10x 9W CFLs on the lighting circuit. And the biggest power tool is the bench grinder rated at 1200W I think.

    It would be protected by a 16A MCB anyway as it’s going to be taken off the end of a domestic spur. And I have an RCD at the workshop.

    £10 (25%) more expensive for 2.5mm over 1.5mm hmmmmmmm

    K
    Member

    Just stick some 4mm and future proof 🙂

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