Why so many spurs? (Electrics)

fenwick458

grumbler said:

Yes, but AFAIK,

• it's more than 20A for 2.5mm2 cable, and the regulations say nothing about the cross-section of of a spur. Why can it not be thicker? Why can it not be a new ring (possibly, with one socket) connected to an existing socket without breaking the existing ring?
• an unfused spur can be a double socket;
• having a ring is just a recommendation, not a requirement. If so, if the existing sockets don't make a ring, then, essentially it's a long spur with many sockets.

the regulations have been explained already.

if you really want to add a load of spurs one after the other, go for it, it's called a radial circuit. just swap the MCB for a 20A and you can have as many as you want

and your point about using a thicker cable for multiple spurs, come on. do you not think it would get a bit complicated with multiple cable sizes one circuit? also might get quite difficult to terminate the cables

Mickey666

Ectophile said:

A double socket on an unfused spur is allowed.  In reality, a 2.5mm cable can carry 18A forever, and probably more than 21A for a short time, even buried in the wall.  There's an assumption that nobody is going two plug in two high-current appliances next to each other and then run them for long enough to melt the cable.

Anything more than one double socket needs a fused spur.

I entirely agree it's unlikely but in a country of 25 million (-ish) houses plus all the offices out there, 'unlikely' can translate into many instances of people doing exactly that!

Assumptions concerning the numbers of sockets in a room or fed by a single circuit are one thing, but that's more of a convenience thing, ie to minimise the chances of anyone overloading a circuit and causing a fuse/MCB to trip.  But the point is that the circuit wiring IS protected against accidental overload . . . which is almost certain to happen sometime, somewhere.

By allowing a double socket on a single 2.5mm2 spur the DESIGN of that circuit is compromised because it's possible for 26A to be drawn, which IN A TYPICAL HOUSEHOLD INSTALLATION** would overload the cable feeding the spur socket and the 30A ring fuse/MCB would do nothing to protect that cable.  

Seems like a dangerous assumption to me.



** Typically, cables are buried in a wall, which limits their ability to dissipate heat.  A 2.5mm2 cable buried in an insulated wall is rated at between 13.5A (Method 103) and 21A (Method 102), both of which would result in an overheating cable when 26A is drawn.  Only if the cable is 'clipped direct' to a wall and exposed to the air (Method C) would the cable be rated at 27A and therefore capable of safely carrying the 26A maximum loading of a double socket.

But how many houses have cables 'clipped direct' to the walls?

Ectophile

Mickey666 said:

Ectophile said:

A double socket on an unfused spur is allowed.  In reality, a 2.5mm cable can carry 18A forever, and probably more than 21A for a short time, even buried in the wall.  There's an assumption that nobody is going two plug in two high-current appliances next to each other and then run them for long enough to melt the cable.

Anything more than one double socket needs a fused spur.

I entirely agree it's unlikely but in a country of 25 million (-ish) houses plus all the offices out there, 'unlikely' can translate into many instances of people doing exactly that!

Assumptions concerning the numbers of sockets in a room or fed by a single circuit are one thing, but that's more of a convenience thing, ie to minimise the chances of anyone overloading a circuit and causing a fuse/MCB to trip.  But the point is that the circuit wiring IS protected against accidental overload . . . which is almost certain to happen sometime, somewhere.

By allowing a double socket on a single 2.5mm2 spur the DESIGN of that circuit is compromised because it's possible for 26A to be drawn, which IN A TYPICAL HOUSEHOLD INSTALLATION** would overload the cable feeding the spur socket and the 30A ring fuse/MCB would do nothing to protect that cable.  

Seems like a dangerous assumption to me.



** Typically, cables are buried in a wall, which limits their ability to dissipate heat.  A 2.5mm2 cable buried in an insulated wall is rated at between 13.5A (Method 103) and 21A (Method 102), both of which would result in an overheating cable when 26A is drawn.  Only if the cable is 'clipped direct' to a wall and exposed to the air (Method C) would the cable be rated at 27A and therefore capable of safely carrying the 26A maximum loading of a double socket.

But how many houses have cables 'clipped direct' to the walls?

To complicate things, BS1363 only requires a double socket to survive a total of 20A for 8 hours.  So there's no guarantee that the socket can even handle 26A.