"jim_in_sussex" wrote in message
om...
[...]
IEE Table 4D5A shows 6mmsq cable has a max rating 47A for ref method
1. The voltage drop is 7.3mV/A/m x 5m x 45A = 1.64V or 0.7%.
Apparently OK so far.

Yes, all OK so far.

But then a check on IEE On-Site Guide table 7.1 (conventional
circuits) shows a 45A radial circuit needing a 10mmsq cable. In fact
the highest rating table 7.1 can offer for 6mmsq is a 32A radial
circuit. Which rules out my 6mmsq cable.

OSG Table 7.1 is provided for convenience; the non-appearance of 6mm^2 cable
on a 45A circuit in this table doesn't /necessarily/ rule out its use,
but...

Am I missing something?

Yes - two things: (1) CPC sizing, (2) use of rewireable fuses in circuits
liable to overload.

1. CPC sizing. As with all twin-and-earth cables 1.5 mm^2 and greater, the
CPC is smaller then the L & N conductors so reg. 543-01-04 and Table 54G
cannot be applied. Therefore you have to 'size' the CPC according to reg.
543-01-03. This requires knowledge of the fault level and disconnection
time, which in turn requires knowledge of the installation earthing (TN-S or
TN-C-S) and the type of overcurrent protective device for the circuit.

2. Rewireable fuse. If the overcurrent device is a 45A rewireable fuse and
the circuit is liable to overload - most free-standing cookers draw ~12kW
flat out, so this doies apply - then the cable rating required is 45A/0.725
, which is 62A. 10mm^2 is again required if 'clipped direct' conditions
apply, or 16mm^2 if not...!

Having said all that, the question we should actually be asking is does your
cooker really need a 45A circuit? The usual diversity rule for a domestic
cooker is 10A + 30% of remainder of FLC. For a 12kW cooker on 230V the FLC
will be 52A. Therefore the circuit design current is 10 + 0.3 * 42 A = 23
A. A 32 A circuit will be fine and you can use the 6mm^2 cable (within the
limits listed in OSG Table 7.1).

HTH
--
Andy