Ring Main

[gary moore]

I know that you can have as many sockets as you like on a ring main circuit, in 100sq meters of floor space.Is it also correct that you must not exceed

60 meters of cable on the circuit

Thanks in advance G M

[BigWallop]

[Owain]

"gary moore" wrote ... | I know that you can have as many sockets as you like on a ring main | circuit, in 100sq meters of floor space.Is it also correct that you | must not exceed 60 meters of cable on the circuit

It's not a case of 'must not' but you have to comply with regulations in respect of volt drop and earth impedance etc. If your cable length is such that those requirements aren't met in 2.5mm, you have to wire in 4mm (or even bigger in some awful examination-question scenario of grouped cables in high ambient temperature protected by a rewirable fuse).

60m is probably a reasonable working rule for whether calculations are required.

Owain

[Andy Wade]

Not quite. The length limit varies, depending on the type of protective device in the consumer unit, and possibly on the type of earthing of the installation (i.e. whether TN-C-S (PME) or TN-S (cable sheath)).

Here, for reference, are the limits for the most common (domestic)devices, assuming 2.5mm^2 T&E cable (1.5mm^2 CPC).

============================================= Max. total cable length for ring circuits ============================================= Earthing Overcurrent protective device PME TN-S

----------------------------- --- ---- Fuse 30A rewireable (BS 3036) 91m 91m Fuse 30A cartridge (BS 1361) 90m 90m MCB 32A Type B (BS EN 60898) 84m 84m MCB 32A Type C (BS EN 60898) 68m ***

*** not permissible, earth fault loop impedance too high.

NB this excludes unfused spurs, which may need to be individually calculated.