Primarily, the cable needs to be sized to not overload- but the load is limited by the fuse at the far end.

Secondly, if there is a credible failure mode that may cause a problem on the 'undersized' cable the upstream fuse or breaker and the cable size need to be able to operate to clear that.

But as the credible fault may well be a near zero ohm 'short' due to mechanical damage, that 6mm cable only needs to hold out for the time it takes the 100a fuse to pop - which it will, unless the cable is so very long that its resistance means not enough current will flow to blow the 100A fuse at at all.

In fact on that basis anything more than about 2.5mm2 will do on a typical 100A fuse, for short runs.

Also if the dead short fault is incredible, perhaps due to the type of cable enclosure or location, and/or if it being blasted to bits would not matter, as the bits would be contained and then  you would replace  it anyway, you may be able to go even thinner.

Mike.

If it's the internal wiring of a panel it probably doesn't come under BS 7671, but under a different machinery or assembly standard (others here will likely know which off the top of their heads than me).

The underlying principles are pretty standard though, and if it were based on BS 7671 there are a number of options.

1. Cables may validly rated much lower then the upstream protection, if they're still protected from faults by the upstream device and downstream protection prevents overloads. (6mm² is probably good for fault protection on a 100A fuse if loop impedances are low enough (figures vary with fuse type but something in the region of 0.22Ω for a BS 88-2 or 0.15 for a BS 1361/BS 88-3).) Actually this option is quite common - unfused spurs from domestic ring final circuits fall into this category (single 20A cable off a 32A MCB) - also for industrial motor circuits where overload protection is provided next to the machine.

2. Even smaller cables are permitted (i.e. where the upstream device won't provide adequate fault protection) under some specific circumstances:

• not longer than 3m
• installed to reduce the risk of faults to a minimum (inside a robust enclosure probably counts here)
• be installed to reduce the risk of fire or danger to persons to a minimum (again, a earthed, fireproof enclosure might be adequate)

In BS 7671 terms see section 433 and 434.

   - Andy.

Mike and Andy thanks very much for your helpful comments. We will have a look through the 7671 sections you mention below. All things considered it is good to know this is not necessarily an immediate contradiction of the regs.

Thanks

Ed