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708 and 721

alanblaby

This came up on another forum yesterday, caravan supply cables.

32 amp caravan supply cable that was 4mm flex on a caravan park which supplies an outlet to each pitch.

708, fig. 708 gives a drawing of the supply, Note 2, page 270 says "..for a higher current, the CSA must be chosen so that secure tripping of the over current protective device is achieved..."  But the note above says refer to 721.55.2.6

So, from that, a 4mm 3 core flex of 25 metres would be acceptable - Page 470, Table 4F3A, albeit, volt drop and R1 + R2 will need to be taken into account, but we are assuming the lead is short enough to satisfy those.(ICBA working that out now, but a rule of thumb says 25m will satisfy all requirements)

Then we have 721.55.2.6 ,and  Table 721, Minimum CSA of flexible cables for caravan connection, which, for 32 amp, says 6mm is required.

Why the confusion on this, between 708 and 721?  Surely they would be better as one chapter on their own, rather than separate chapters which need reference to each other?

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    32 amp caravan supply cable that was 4mm flex on a caravan park which supplies an outlet to each pitch.

    Just to be clear - I presume we're taking about the hook-up lead (between site socket and caravan inlet) rather than some flex supplying the site socket.

    Why the confusion on this, between 708 and 721?  Surely they would be better as one chapter on their own, rather than separate chapters which need reference to each other?

    It used to be one section, and I think is simpler to have split. I agree it's confusing that the "hook up lead" seems to be covered by both - I would have said that normally it's part of the caravan rather than the site (following normal ownership rules - tourers always bring their own). That said for statics, the semi-permanent hookup lead is often provided by the site (or their electrician when the static is installed.

    "..for a higher current, the CSA must be chosen so that secure tripping of the over current protective device is achieved..."

    That does seem like a strange requirement - 99.9% of sites will be TT, so the changes of an earth fault operating an overcurrent device is practically nil whatever the length of the lead, and as there's no direct requirement for L-N loop impedances at the socket (or margin for v.d.), I'm not how it would be calculated for L-N faults either.

       - Andy.

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    32 amp caravan supply cable that was 4mm flex on a caravan park which supplies an outlet to each pitch.

    Just to be clear - I presume we're taking about the hook-up lead (between site socket and caravan inlet) rather than some flex supplying the site socket.

    Why the confusion on this, between 708 and 721?  Surely they would be better as one chapter on their own, rather than separate chapters which need reference to each other?

    It used to be one section, and I think is simpler to have split. I agree it's confusing that the "hook up lead" seems to be covered by both - I would have said that normally it's part of the caravan rather than the site (following normal ownership rules - tourers always bring their own). That said for statics, the semi-permanent hookup lead is often provided by the site (or their electrician when the static is installed.

    "..for a higher current, the CSA must be chosen so that secure tripping of the over current protective device is achieved..."

    That does seem like a strange requirement - 99.9% of sites will be TT, so the changes of an earth fault operating an overcurrent device is practically nil whatever the length of the lead, and as there's no direct requirement for L-N loop impedances at the socket (or margin for v.d.), I'm not how it would be calculated for L-N faults either.

       - Andy.

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