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4mm SWA - 2 circuits

Hi All, 

I have an install with a 4mm 5 core SWA buried that is setup as follows: 

L&N - circuit 1 (2x 13A sockets) - 16A MCB

L&N - circuit 2 (1x 16A socket outlet) - 16A MCB

CPC - for circuits 1 & 2

Maximum distance 30 metres. The client wants to upgrade the 16A socket to a 32A variant for a hot tub with a 32A MCB. 

Will this suffice and how will grouping factors affect this? The other circuit with 2x double sockets are used for lawnmowers and some lighting, and highly unlikely will have a sustained load.

I’m looking at grouping factors given that this is a multi core cable. Will the 4mm handle the increased load or would a 6mm be a better option? There is also capacity for reduction given the socket circuit would likely be loaded a lot less than its overall rating (the 30% grouping factor rule).

The cable is buried and running a new cable would be difficult so would prefer to use the existing 4mm. 

Thanks. 

Parents
  • If you're using the BS 7671 "in the ground" figurers and need overload protection, don't forget the 0.9 factor (as the tables assume 20 degree ambient rather than 30 and the extra CCC due to the reduced temperature doesn't carry through into the same margins for overloads)

       - Andy.

Reply
  • If you're using the BS 7671 "in the ground" figurers and need overload protection, don't forget the 0.9 factor (as the tables assume 20 degree ambient rather than 30 and the extra CCC due to the reduced temperature doesn't carry through into the same margins for overloads)

       - Andy.

Children
  • In terms of 90C rating there is some advantage to having a a few feet of cable not buried, before it reaches the terminals - the fact that the fresh air current rating is higher, can be re-phrased as the section in the fresh air is cooler for the same current as the buried section is perhaps nearer its 90C.
    the way to estimate that is to convert the current rating and voltage drop to watts 'sweated off' per metre, and the ratio of the two figures is more or less proportional to the ratio of the temperature rises for the same current. Heat does flow along the wire to some extent, but the transition distance between hot and cold regions is a few tens of cable diameters.
    Mike

  • How does this apply when calculating at 90 degrees instead of 70?

    Trying to figure out how this would be applied. 

  • How does this apply when calculating at 90 degrees instead of 70?

    Trying to figure out how this would be applied. 

    For 90 degrees, as I read it, it just a matter of using the 90 degree tables, then applying 0.9 to that. I might be wrong though.

       - Andy.