4mm SWA - 2 circuits

In terms of the standard forms of diversity provided in guidance, it's a no-go.

Without knowing the power requirements of the hot tub, it's not really possible to comment further - the 4 sq mm might just do that alone to keep conductors at 70 deg C or less (Table 4D4A of BS 7671).

vantech said:

The other circuit with 2x double sockets are used for lawnmowers and some lighting, and highly unlikely will have a sustained load.

That is with the current owner, and makes the assumption that, for example, no-one is going to use the other outlets for beer coolers, bouncy castle, etc., for a party ... a reasonable person might well choose to do just those things?

Are the 16 A and 32 A socket-outlets suitable for domestic use in any case (not shuttered)? (of course, may or may not be domestic).

The cable is buried so hard to tell. I have based the calcs on 70 degrees as it’s worst case.

The maximum load in table 4E4A is 37A, and with derating applied 29.6A using a CG of 0.8. The reality is that circuit 1 will most likely always be <30% of the rated load, so this could be completely disregarded but I’ve left this in again to ensure worst case. As you indicate, the calcs are unusual given circuit 1 will be limited and substantially lower than the maximum rated. 

Essentially, if circuit 1 didn’t exist there wouldn’t be issue. A 4mm would be fine on a 32A breaker and a circa 29A load (intermittent heating / pumps etc). It’s only the inclusion of the other circuit that makes this more complex. 

I’m leaning on this being ok given the realistic use case. 

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.

gkenyon said:

 (assuming thermal effects ... adiabatic ... is OK, which assuming instantaneous tripping on earth faults, it is likely to be)

The reasoning as as follows:

If Zs is low enough to ensure instantaneous tripping, then for a C32, maximum I²t = 20000 A²s for mcb's to BS EN 60898 series and RCBOs to BS EN 61009 series or BS EN 62423 (see Table 8.6 of the IET's Electrical Installation Design Guide).

For copper conductors at 70 deg C, k=115, so S_min = (√(20000))÷115, or S_min = 1.23 mm²

For the steel armour as cpc in 90 deg C thermosetting cable, k=46, so S_min = (√(20000))÷46, or S_min = 3.07 mm²

These values are much less than than the actual cross-sectional areas provided for a 4 sq mm 5 core SWA (armour gross CSA of 25 sq mm).

I see, I was using table 4C1 which covers reference methods A-F however table 4C2 specifically relates to burried cables so supersedes table 4C1 as this factors in soil? Many thanks for the detailed explanation! 

Closer inspection of the cable and its marked BASEC BS5467. A quick search indicates most of these cables are generally 90 degree rated. The cable is dated 2023. 

This I believe is the cable, it’s branded with the manufacturers detail and this is the only cable on their website that matches. 

vantech said:

A quick search indicates most of these cables are generally 90 degree rated.

Yes, they are ... but not the terminals into which the cables are terminated. Hence Regulation 512.1.5

If I can confirm the terminals support 90 degrees, the overall design should be fine then. 

Circuit 1 = 16A MCB

Circuit 2 = 32A MCB 

Table 4E4A 4mm @ 43A and a CG factor of 0.75 would allow a maximum load per circuit of 32A. In this scenario, circuit 2 would be protected by the 32A breaker, with overload protection offered by the hot tub itself and further by the MCB. This rating however is likely OTT given that circuit 1, is underloaded by 50%, so the actual rating of circuit 2, would be higher in practice. 

The reality is that neither circuit will ever reach its maximum capacity because of the loading limitations. 

I checked the and spa has internal fusing for all pumps and heaters, so overload and short circuit protection is built into the product. 

vantech said:

I checked the and spa has internal fusing for all pumps and heaters, so overload and short circuit protection is built into the product. 

But does the sum of all those fuses give you the level of overload protection required? E.g. if a 11A heater is on a 16A fuse and a short part way along the element means it's drawing 15A .... the internal wiring may be protected OK, but overall it's then drawing 36A rather than 32A (or whatever the figures actually are).

   - Andy.

The hot tub itself would be protected by a 32A MCB, so this would offer overload protection from the supply. 

The hot tub itself would be protected by a 32A MCB, so this would offer overload protection from the supply. 

In which case, as you're relying on a conventional overcurrent protective device to provide overload protection, the 0.9 factor (Cc) comes into play (last paragraph of appendix 4 section 4). Although that could be just about reversed if you were sure the cable was buried direct (no duct) 0.7m down in soil with a resistivity 2.5Km/W (or better) (section 7 method D).

   - Andy.

The hot tub itself would be protected by a 32A MCB, so this would offer overload protection from the supply. 

In which case, as you're relying on a conventional overcurrent protective device to provide overload protection, the 0.9 factor (Cc) comes into play (last paragraph of appendix 4 section 4). Although that could be just about reversed if you were sure the cable was buried direct (no duct) 0.7m down in soil with a resistivity 2.5Km/W (or better) (section 7 method D).

   - Andy.