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).

Hi, 

The current 16A outlet has a built in isolator so pretty safe, in that it cannot be energised without a socket plugged in. The new tub will connect directly to a rotary isolator, so the issue with shutters goes away. 

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). The first circuit would be protected by a 32A breaker, with the tub actually pulling 28.9A assuming worst case (all features ON). The second circuit, would be limited by means of a 16A (or even a 13A) MCB.

Calculated with a CG = 0.8 to get to the 29.6A grouped current. 

The underloading of circuit 2 (13A compared to 28.9A) would mean the overall cable is under loaded with a positive impact on heat dissipation. I’d also argue that in most situations, the socket circuit would be less than 30% of the grouped calculation, so this circuit could be disregarded 90% of the time but appreciate it could be running at full capacity all of the time with a heater etc, but again, the whole cable would still likely be less than the calculated grouped limit.

Pretty sure my calcs are correct, took me a while to figure this out! 

It is distinctly unusual (!) Given that you know circuit 1 will be limited to 16A by MCB, the proposal is to load the second circuit 2 to carrying 32A max.  You don't say if the cable is the 90C type or the 70C type, but running it that hot is not really great and you may need to split the ends early so that they are not short hot tails into  terminals not really rated for it.

Now if it was a 3 phase 4mm cable, you'd be looking at about 35-36 amps a core, for a 30C ambient and a 40C rise, maybe a bit more depending what it is actually buried in, as the annex in the regs assumes dry desert sand, rather than wet clay, so while it is near the limit, you ought to be all right in practice.

As commented above, making it one '8mm' circuit and splitting it at the destination makes it run cooler and share better, but it sounds like there are good reasons not to do that.

Mike.

PS https://intrelec.co.uk/2020/10/21/swa-cable-sizing-charts/
has some interesting de-rating figures for multi core cables with very many current carrying cores. In terms of cooling one multicore cable is not quite the same as a number of cables with fewer cores side by side.

It is perhaps worth comparing the trends.
"For cables of 5 cores and above it is assumed only 2 cores are loaded simultaneously (i.e. live and neutral) and the 2 core rating should be taken. In instances where several cores are loaded simultaneously, the following rating factors should be applied to the 2 core current rating."

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. 

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.