6mmsq electrical cable rating

[jim_in_sussex]

Renewing cooker cable - les than 5m run from CU to the cooker (45A) outlet - run from a 45A MCB type 2.

This will be clipped direct to joists (1.5m) then buried in cement/lime/sand plaster on a brick wall. According to IEE regs this is installation method 1 + method 2, so cable rating ref method 1 applies.

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.

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.

Am I missing something?

I can see that 6mmsq would have the draw back of being run near its limit, which is rarely a good thing engineering-wise, but that doesn't explain why table 7.1 doesn't feature it.

Any comments appreciated.

TIA

[Kalico]

Not sure about why the difference, but I would run it in 10mm.

Also, why not put a nice bit of conduit in there for any future cables/services you might want to run once the concrete is in?

Rob

Replace 'spam' with 'org' to reply

[Andy Wade]

Yes, all OK so far.

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...

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.

1. 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