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Old 30th Jun 2019, 9:54 pm   #25
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 1,765
Default Re: 13 amp fuse for 1.5mm mains flex?

Your rule of thumb may serve you well; I prefer to stick to the published tables and derating factors based on extensive research, dealing with all sorts of operational parameters such as installation method, grouping, characteristic curves of protective devices, ambient temperature, etc.

However, what the OP needed to consider specifically was the rating of general purpose flexibles such as H05VV-F and H07RN-F, for which there are standardised ratings when used for portable appliances (as opposed to being part of an electrical installation).

0.5mm˛ : 3A
0.75mm˛ : 6A
1.0mm˛ : 10A
1.25mm˛ : 13A
1.5mm˛ : 15A
2.5mm˛ : 20A

Provided that overload protection is included in an appliance to which the flex is attached, only short-circuit protection is required from the plug fuse. All sizes from 0.75mm˛ are adequately protected against short-circuit damage by a 13A fuse (and indeed by a 16A, hence 0.75mm˛ cables are fully acceptable when used with unfused plugs in Europe protected by 16A OCPD in the distribution board).

Where overload protection is not provided at the load, such as in an extension cable, then either a suitable fuse must be fitted, or preferably, the cable should be able to withstand 13A continuously, for which a minimum of 1.25mm˛ is specified. Most commercially made 13A extension leads use 1.25mm˛ cable, although heavy-duty industrial types are typically 1.5mm˛. 13A plugs and trailing sockets to BS1363 are not designed to accept a larger cable than this.
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