Cable size on cert

[Traineeboy]

If you are signing off a circuit that has more than one cable size on the circuit. Say it’s a radial 4mm2 but there are fused spurs that are only 2.5mm2 ( only for a short distance) or it’s a 2.5mm2 ring but then a fused spur is used to power an outside light in 1.5mm2 - What size do you write on the cert ? 
 

Would it be the majority of the cable size used ? Or is it the cable that is installed in the CU ? 
 

any thoughts ? 

 

[Murdoch]

That is a great question!

I use certificate software, so in your example of a 2.5 mm ring with a 1.5 mm spur it'll flag an issue if I state 1.5 mm on the cert. What you need to remember is that the OCPD and cable size are VIP so you declare the minimum size of the cable on the OCPD in the CU. The fuse in the fused spur is the OCPD for the spur.

So if the cable at the CU is 4mm and later changes to 6mm or vice versa, you declare the 4mm

Hope this helps

 

[Traineeboy]

That is a great question!

I use certificate software, so in your example of a 2.5 mm ring with a 1.5 mm spur it'll flag an issue if I state 1.5 mm on the cert. What you need to remember is that the OCPD and cable size are VIP so you declare the minimum size of the cable on the OCPD in the CU. The fuse in the fused spur is the OCPD for the spur.

So if the cable at the CU is 4mm and later changes to 6mm or vice versa, you declare the 4mm

Hope this helps

Ok thank you. So if you used 1.5mm2 for lights due to voltdrop. 
 

1.5mm2 at the CU ( on a 6A MCB) but then it changes to 1mm2 down the line ( there is no OPCD between joining the two)  , even though at the CU it’s 1.5mm2 you would put down 1mm2 as it’s the smallest ? I think that’s what your getting at but what to he sure. 
 

This has also raises another question if say you install a FCU with some sockets. 
 

When taking your R1 + R2 readings are you doing these between the last socket on the spur and the fused spur ( as the fuse in the fused spur is the OPCD) or as I was taught you take the R1 + R2 readings from the CU and the last point on that MCB ? It just seems that would a FCU be treated as a Zdb ? Hope that makes sense ? 

 

[Murdoch]

Yes the issue of FCUs is a thorny one with different people looking at it differently.

 If you add a fcu then a long load my guessing is that the new load will exceed the OCPD of the MCB but won’t exceed the OCPD of the BS1361 fuse

 let’s see what others say

 

[Sidney]

I use the cable size from the OCPD that supplies the circuit. If a spur has been run off in a different sized conductor I make a note of this on the certificate and also the circuit chart.

The smaller conductor must be sized to avoid risk of overloading it so either an FCU or socket-outlet depending upon the nature of the circuit.

 

[Phoenix]

As above, in the example given, I'd fill in the cable size of the circuit that MCB is protetcing, so 2.5mm in OP, after the fused spur is not really relevant to that cirucit, you could however make the case that you should fill in a separate schedule for the 'circuit' fed from teh fused spur, in realility, no one really ever does....

In terms of a circuit that changes size at a joint box, I'll try and state both, e.g. 4/6 for line and 1.5/2.5 for the cpc, with the first number being the one thats visible at the board

 

[Traineeboy]

Great thanks chaps good insight, very helpful. 

 

[SPECIAL LOCATION]

If you are signing off a circuit that has more than one cable size on the circuit. Say it’s a radial 4mm2 but there are fused spurs that are only 2.5mm2 ( only for a short distance) or it’s a 2.5mm2 ring but then a fused spur is used to power an outside light in 1.5mm2 - What size do you write on the cert ? 
 

Would it be the majority of the cable size used ? Or is it the cable that is installed in the CU ? 
 

any thoughts ? 

It depends upon if you consider a fused spur as a point where an accessory is connected to a circuit, rather than part of the circuit itself...?? :C

e.g. Think of an older property with just a single 2.5mm socket ring circuit...

it could have:-

Fused spur connected to central heating wiring, flex, surface clipped/trunking.

Fused spur connected to a wall mounted heater, flex, surface clipped/trunking.

Fused spur connected to a pair of bedroom wall lights, flex, surface clipped/trunking.

Fused spur connected to a security alarm, T&E, part buried, part surface.

Fused spur connected to three conservatory walls lights, T&E, all buried.

Fused spur connected to some outdoor lighting, mix of T&E & flex buried & surface.

etc.

etc.

It is not uncommon for circuits to have multiple fused spurs in various places..

You would end up with a massive list of circuit details if you have to include everything.? 

I would work with the logic that often a fused spur has been used as it is neater than a plug & socket..   

and all of the above items could have been wired via a plug & socket..

in which case they would obviously be an accessory not part of the fixed wiring..

So if it could have been a plug & socket I wouldn't mention it..

You don't mention a 6-way multiblock adapter powering all the TV or computer gubbins, even if it was connected to a fused spur.   

Basically you just need to apply a bit of common sense 

Guinness