4mm SWA - 2 circuits

Question

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&rsquo;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.

gkenyon · Answer

vantech said: I did a few calcs, and based on a 70 degree cable, the maximum grouped current would be 29.6A (37A before the grouping calculation looking at table 4E4A). 
 I agree on the 37 A without grouping factor, but just in case anyone is reading this thread for educational purposes, the correct table to maintain 70 deg C is Table 4D4A (d being very close to E on the keyboard). (Explanation for educational readers ... Table 4E4A is the correct table for thermosetting cables, but the current-carrying capacities are based on conductors operating at 90 deg C; Regulation 512.1.5 NOTE 3 tells you to use the equivalent table numbered 4Dx if you want to run 90 deg C cables at 70 deg C, so that the conductors can be connected to accessories and junction boxes which are normally rated for connection of conductors operating at no more than 70 deg C.) 
 I'm not sure about Cg of 0.8, though ... Table 4C2 applies for buried cables (Reference Method D) and with zero separation, Cg = 0.75, giving a current-carrying capacity with grouping factors of 27.75 A (being less than the hot tub rating of 28.9 A ... and ratings to common appliance standards can be 5 % more than the rating plate rating, so the hot tub could well be pulling 30.3 A max at nominal voltage, and more at maximum utilization voltage). 
 
 I also agree that, with existing load assumptions on the second circuit, you might ignore the second circuit if it were carrying less than 30 % of the grouped rating (8.32 A) 
 The only remaining question (assuming thermal effects ... adiabatic ... is OK, which assuming instantaneous tripping on earth faults, it is likely to be) is whether overload protection is provided in cases where the second circuit is well loaded on the "odd occasion". 
 I assume that the hot tub has at least one concentric mineral insulated heating element, in which case there is a not uncommon (like immersion heaters) to have a fault in which the element shorts to Earth nearer the Neutral end, so you end up with an overload current to Earth. In my opinion, therefore, concentric elements require overload protection (they are not "fixed load"). In reality, the RCD (required by Reg 411.3.3) will operate, but we can't rely on that for protection against overload current unfortunately - meaning protection against overload is definitely needed for the hot tub. I agree that protection against overload would be provided if the second circuit's load current were small enough to be ignored (<8.32 A), BUT a 32 A overcurrent protective device will not provide protection against overload when grouping factor brings the current-carrying capacity below 32 A.

4mm SWA - 2 circuits

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