1. What size meter cable do you think I have encountered here?
2. What is the minimum diameter permitted (the meter is no more than 3m from the consumer unit, and the main fuse is a 100A BS1361).
Answer: Cables that are made to the relevant British Standard (generally BS 6004, Table 7) will follow the regular conductor size ranges.
The next size smaller than 25mm2 is 16mm2 (then 10mm2). It may be 16mm2, or your sample may be a particularly generous 25mm2, in which case it might still be 25mm2. The actual parameter we measure is the conductivity of the cable, not the physical dimension, which is purely nominal.
Both sizes (16mm2 and 25mm2) are commonly used for meter tails, depending on the load. Clearly 25mm2 is a better bet for possible future increase of the load. Depending on the load applied, 16mm2 may or may not be adequate. For advice on this matter, consult a body such as NICEIC or ECA that can advise on the interpretation of the Wiring Regulations (which should be followed) in this respect - Dr Jeremy Hodge, BASEC.
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