Q & A of the Day - What is the maximum distance we can run an electric flex?

Question: I work for an AV installation company and have continuously been unable to find a concrete answer to the following - what is the maximum distance we can run an electric flex?

The reason is that we sometimes have the power socket located in a different part of the room to the screen/equipment rack, so are confronted with running power between the two.

What are the restrictions with regard to running power cable - i.e. cable thickness, length, trunking etc?

Any help or guidance on this will be much appreciated.

Answer: BS 7671 requires that where mobile equipment is likely to be used, provision shall be made so that equipment can be fed from an adjacent and conveniently accessible socket outlet, taking account of the length of flexible cables normally fitted to portable appliances and luminaires.

The restrictions for running excessive lengths of flex, i.e extension leads, are that there could be problems with volt drop and earth fault loop impedance (it could be excessive, resulting in the fuse failing to operate to disconnect under fault conditions within prescribed disconnection times).

Also, in some instances, excessive lengths of flex for extension leads could cause a trip hazard.

I recommend that you obtain 'The IET Code of Practice for In Service Inspection and Testing of Electrical Equipment', which provides some good advice on this. It recommends in the first instance that sufficient socket outlets are installed and that extension leads should be avoided where possible.

However, if supplying equipment from an extension lead turns out to be the only option, it is recommended that the maximum length should not exceed 12m for flexes with 1.25mm2 conductors, 15m where the flex conductors are 1.5mm2, and no greater than 25m where 2.5mm2

Note that 2.5mm2 is too large for standard 13A plugs. Also, if the above lengths are exceeded, the extension lead should be further protected by a 30mA RCD.

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