2.5mm Radial Circuit- Rcbo Question

[Mitch6278]

Hi all,
Having currently finished a new install comprising of offices and an adjoining warehouse we have now been told by our QS that we need to change upto 99 breakers across 17 DBs!!!!
Basically we have installed a lot of 2.5mm T&E radial circuits serving floor boxes. The runs are a max of 50m.
The designer had put B20 RCBOs on the schedule and so we went with this. However the QS has now asked us to change these to B16 RCBOs because B20s are too big?? Am i missing something here?

Without trawling through the regs i was wondering if someone can point me in the right direction?

It seems a bit far fetched that the design would be so far out?

 

[HandySparks]

Hi all,
Having currently finished a new install comprising of offices and an adjoining warehouse we have now been told by our QS that we need to change upto 99 breakers across 17 DBs!!!!
Basically we have installed a lot of 2.5mm T&E radial circuits serving floor boxes. The runs are a max of 50m.
The designer had put B20 RCBOs on the schedule and so we went with this. However the QS has now asked us to change these to B16 RCBOs because B20s are too big?? Am i missing something here?

Without trawling through the regs i was wondering if someone can point me in the right direction?

It seems a bit far fetched that the design would be so far out?

That will depend on the actual current carrying capacity of the cable as installed. What's the installation method? Any thermal insulation? What about cable grouping?

 

[Mitch6278]

Installation method is on a basket.. Cable grouping is minimal as its on a 300mm wide basket with about 8 or 9 circuits on.
Current carrying capacity?? Not sure but only supplying about 4/6 13A sockets?

 

[Leesparkykent]

Is the QS signing for the design on the cert?

 

[Andy78]

Have you asked the QS to detail his reasoning ?

 

[Mitch6278]

Leesparky no he isn't...

Andy.. He has said that a 2.5mm radial should have a max breaker size of B16.. I just can't see it myself.. In my head I was always of the thinking that 2.5mm could easily take 20A?

 

[LankyWill]

cant see the logic or reasoning for this, 2.5 can take 26-27A depending on install method.

 

[westward10]

Leesparky no he isn't...

Andy.. He has said that a 2.5mm radial should have a max breaker size of B16.. I just can't see it myself.. In my head I was always of the thinking that 2.5mm could easily take 20A?

Sounds odd to me as the other posts say ask his reasons, the design suggested 20A but he may have opinions which prove the design to be incorrect.

 

[snowhead]

My first thought is if that's what the Q.S wants then do as he asks.
I assume you're employed so it wont cost you anything.

Second thought, maybe you should get the Designer to talk to the Q.S, it's their issue not yours.

What did the customers Spec, if any ask for?

 

[Wilko]

Hi - for my trainee fun I looked at the good book. My design :
Perforated cable tray reference method E from 7.2 (p339), confirmed as Number 31 in Table 4A2 (p342). If that is the one you have installed, then 2.5mm ref E is 30A from Table 4D2A (p358) which is then derated due to multiple cables using Table 4C1 (p351) where item 3 says use 0.72 factor. So I get ... wait for it ... 21.6A . Of course this could be a load of trainee -------s . Cheers

 

[Wilko]

And now for fun the max earth loop impedance from the OSG Table B6 (updated page 125) says 2.2Ohms for 16A or 1.75 for 20A. With 50m of 2.5mm I think r1+r2 is about 0.75Ohms leaving us with 1 Ohm spare for Ze, which should be easily the case. So I would have answered with 20A rcbo being ok if it was a test. Please let me know if I've made a mistake

 

[Electric Magic]

Maybe the designer is concerned about the potential for voltage drop? 2.5mm2 will only give you almost 32 meters before you go over 5% at with a 20A Ib. At 50 meters the voltage drop will be around 18V (7.8%).

Of course, a 16A Ib would also have a voltage drop above 5% at 50m, but perhaps the actual voltage is sufficiently above 230V that the designer is happy to list this as a departure?