4mm SWA - 2 circuits

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

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. 

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

Trying to figure out how this would be applied. 

Under the conditions of use expected, with minimum load on the 13 amp sockets I suspect that this proposed installation will be fine, it is the sort of thing that I might do for myself of for friends/family.

I would be reluctant however to do this for a paying customer, as patterns of use might change. Nothing prevents the 13 amp sockets being loaded to 16 amps forever or to 20 amps for some hours.

A better solution IMHO would be to double up the cores, so as to give 8mm for phase and for neutral, protect by a 40 amp or 45 amp MCB at the supply end. At the load end, fit a 2 way consumer unit with a 16 amp MCB for the 13 amp sockets and a 32 amp MCB for the 32 amp socket.

Either final circuit may then be fully loaded without concerns, though not both at the same time. The two MCBs in series will probably not discriminate in the event of a short circuit, but that is arguably a price worth paying.

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.

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. 

I am starting to see this as a realistic approach. It is for a friendly customer so should be fine. The socket circuit will be limited to 16A, which would be below the reduced grouping factor loading.