Unfused Spurs on Ring Main

[zsoltimatrix]

Hi guys,

What are the rules and common practices for unfused spurs on a standard ring main?

If you use 2.5 for the spur than how is that properly protected with 32A MCB

 

[Sav]

Hi,
Page 362 BS7671 reg 433.1.5

An unfused spur should feed one single or one twin socket-outlet only. An unfused spur may be connected to the origin of the circuit in the distribution board.

So basically only 1 socket allowed.

Best regards,
Sav

 

[Mr Mark Sparks]

yes you are not likely to overload a single bit of 2.5 with ony 1 socket on it

 

[pevvers]

Think how much load a double socket can take... At the very most 26A due to the 13A fuses in the plugs. Then have a look at the current carrying capacities for 2.5.

 

[GT1]

Now, I may be barking up the wrong tree, (this had me thinking too..!)
But I think the answer lays in the fact that because there will only be the 1 unfused spur (per socket outlet) then the limiting factor for the single 2.5mm cable is the going to be the 13amp (or smaller) fuses in the appliances being used from that spur.

OSG page 128 gives best current carrying for 2.5 as 27amps... a double socket fed as a spur will be feeding 2 plugs fused at 13amps each, maximum.. or 26 amps total..
So with that worst case scenario.. ie both appliances going full fault, both at the same time, the fuses will go before the capacity for the 2.5 is met.

Obviously, different reference methods give lower capacities, but we are talking about 2 items of electrical equipement faulting at EXACTLY the same time...! so under most circumstances ie single fault, the 13 amp fuse in the plug top gives ample protection.

I know its downstream of the actual change from ring to single cable... but it appears to answer the issue..?

However.... i may be barking in error..

 

[zsoltimatrix]

Now, I may be barking up the wrong tree, (this had me thinking too..!)
But I think the answer lays in the fact that because there will only be the 1 unfused spur (per socket outlet) then the limiting factor for the single 2.5mm cable is the going to be the 13amp (or smaller) fuses in the appliances being used from that spur.

OSG page 128 gives best current carrying for 2.5 as 27amps... a double socket fed as a spur will be feeding 2 plugs fused at 13amps each, maximum.. or 26 amps total..
So with that worst case scenario.. ie both appliances going full fault, both at the same time, the fuses will go before the capacity for the 2.5 is met.

Obviously, different reference methods give lower capacities, but we are talking about 2 items of electrical equipement faulting at EXACTLY the same time...! so under most circumstances ie single fault, the 13 amp fuse in the plug top gives ample protection.

I know its downstream of the actual change from ring to single cable... but it appears to answer the issue..?

However.... i may be barking in error..

It makes sense to me.

Thanks for answer guys!

 

[Graeme Harrold]

Now, I may be barking up the wrong tree, (this had me thinking too..!)
But I think the answer lays in the fact that because there will only be the 1 unfused spur (per socket outlet) then the limiting factor for the single 2.5mm cable is the going to be the 13amp (or smaller) fuses in the appliances being used from that spur.

OSG page 128 gives best current carrying for 2.5 as 27amps... a double socket fed as a spur will be feeding 2 plugs fused at 13amps each, maximum.. or 26 amps total..
So with that worst case scenario.. ie both appliances going full fault, both at the same time, the fuses will go before the capacity for the 2.5 is met.

Obviously, different reference methods give lower capacities, but we are talking about 2 items of electrical equipement faulting at EXACTLY the same time...! so under most circumstances ie single fault, the 13 amp fuse in the plug top gives ample protection.

I know its downstream of the actual change from ring to single cable... but it appears to answer the issue..?

However.... i may be barking in error..

Its not so much fault current as this will be short lived, its the use of 2 high current appliances at the same time that will lead to overheating of the cable, i.e. a high use kitchen with 2 x 2.9Kw kettles boiling up constantly for thirsty sparks.........