Green Building Forum - Cords from radial circuits Mon, 25 May 2026 22:12:03 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202763#Comment_202763 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202763#Comment_202763 Thu, 29 May 2014 10:01:45 +0100 Ed Davies The Electrician's Guide to the 17th Edition, first edition, by John Whitfield it says (pg 133) when talking about socket outlet circuits:

Generally, 0.5 mm² cords should be the smallest size connected to plugs fed by 30 A or 32 A ring circuits.
Apart from the following sentence, which discusses cords longer than 10 metres, it doesn't say anything more about minimum cord CSA for ring final circuits. Page 136, talking about radial circuits, says:

The minimum cross-sectional area for flexible cords should be:

0.5 mm² where the radial circuit is protected by a 16 A fuse or circuit breaker,
0.75 mm² for a 20 A fuse or circuit breaker,
or 1.0 mm² for a 30 A or 32 A fuse or circuit breaker.
Obvious question, why is is acceptable to plug a 0.5 mm² cord into a 32 A ring final circuit but not into a 20 A radial circuit?]]> Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202775#Comment_202775 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202775#Comment_202775 Thu, 29 May 2014 11:42:53 +0100 owlman Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202788#Comment_202788 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202788#Comment_202788 Thu, 29 May 2014 13:53:09 +0100 Ed Davies
Where the cord length must be 10 m or greater, the minimum size should be 0.75 mm² and rubber-insulated cords are preferred to those that are PVC insulated.
It's not completely clear if that applies just to ring final circuits or all socket circuits - it's in the more general section but immediately follows and qualifies a sentence which is specific to rings rather than radials.

Whatever, it still leaves the question of why it might be acceptable to plug a 0.75 mm² cord into a 32 A ring but not a 32 A radial.

(My interest in all this is that I plan not to bother with any rings in my house - mainly because I won't have the inverter power available to generate enough current to need them. Most sockets will be on low power radials off a small always-on inverter with a few socket pairs, mostly in the kitchen plus a few others, on their own individual cables off the higher-power inverter which is only run up when it's needed.)]]> Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202796#Comment_202796 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202796#Comment_202796 Thu, 29 May 2014 16:05:34 +0100 markocosic
Short 0.5mm^2 cords can draw enough current in a fault condition to trip the breaker quickly before things get too hot. Long 0.5mm^2 cords are too high impedance to trip the breaker quickly enough to meet the regs.

We should be designing for 16A socket circuits as many continental market/chinese market devices commonly sold in shops do not have large enough conductors to trip the 32A breakers used in (UK only) ring circuits. It'll take a few house fires before the regs catch up though.]]> Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202798#Comment_202798 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202798#Comment_202798 Thu, 29 May 2014 17:20:44 +0100 Ed Davies Posted By: markocosicShort 0.5mm^2 cords can draw enough current in a fault condition to trip the breaker quickly before things get too hot. Long 0.5mm^2 cords are too high impedance to trip the breaker quickly enough to meet the regs.Ah, that makes sense: it'd be an awfully long cord to stop 32 A flowing but it's easy to believe that the cable or whatever's actually shorting might catch fire before the breaker tripped at 50 amps or so. At 32 amps the temperature of the cable would initially rise by about 0.7 20 °C per second (¹)until heat loss cut it back but not by enough, I suspect.

Ring mains are an awful idea …
Indeed, a desperate war-time fail-dangerous invention we should have got rid of long ago.

We should be designing for 16A socket circuits as many continental market/chinese market devices commonly sold in shops do not have large enough conductors to trip the 32A breakers used in (UK only) ring circuits.
Yep - who needs or wants more than 6 kW on sockets? I'm planning 6, 10 or 16 amp MCBs on my low power radials and 16 or 20 A on the few high power socket pairs.

(¹) Oops, forgot the squared bit of I²R.]]> Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202800#Comment_202800 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202800#Comment_202800 Thu, 29 May 2014 18:42:41 +0100 Ed Davies Cords from radial circuits https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202801#Comment_202801 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=12142&Focus=202801#Comment_202801 Thu, 29 May 2014 18:53:17 +0100 Gotanewlife