16 amp radial or 20 amp radial ?

[bobbybrown]

quick couple of questions

apart from demand why would you use a 16amp radial rather then 20amp radial?

when are you not allowed a 20amp radial when clipping direct, apart form when covered in insulation and when the floor area exceeds 50m

also is there a installation method where cables are lay'd on top of insulation in a loft?

cheers in advance

 

[Deleted member 26818]

The choice of protective device would depend on the CSA and the length of the circuit conductors.
For instance if the Zs of the radial were too high for a 20A MCB, the options would be to alter the wiring so the Zs would be lower, or install a lower rated MCB such that the Zs would allow the MCB to operate.

 

[PC Electrics]

I'd use a 16A radial with 1.5mm csa to feed an immersion for example. Or a storage heater. Or a bathroom fan heater. Or.....or.....or......any other circuit with a design current under 16A.

However I speak gobbledegook fluently too and so understood what you were getting at. Yes, sockets on 2.5mm, I'd put on a 20A generally (ref method permitting).

 

[bobbybrown]

thanks spin and to find out if zs would be acceptable at the design stage i would use table 7.1 osg (pg 48)
there it also tells me the max length of circuit would be 31 m

is this correct ?

 

[Deleted member 26818]

Yes if you are using 2.5mm² T&E.
Although it may be possible to use a longer cable if the measured Zs are low enough

 

[bobbybrown]

learn something new every day, i used to think 16amp for 2.5 radial max and never even thought of using 1.5 for immersion etc... cheers guys

 

[bobbybrown]

looking at table 7.1 (pg 48) osg again it doesn't give any info for TT systems ??

 

[Deleted member 26818]

That's because the Ze is usually so high that Zs will never be low enough for an MCB or fuse to operate in the event of an earth fault.

 

[BlueToBits]

If the immersion heater cable is grouped with others under floor boards say, it is likely that 1.5mm cable will not be sufficient.

 

[bobbybrown]

so table 7.1 doesn't include grouping factors, maybe a way round this would be to drill a separate hole solely for that 1 cable?
table 6c(osg pg 126) describes the grouping factors, do you need to take this into consideration for cables entering the consumer unit even if there only touching for about 15 cm?

 

[BlueToBits]

You could also fit a refrigeration unit and plenum ducting to the sub-floor to keep the cables cool, but I personally would use 2.5mm, all other things being equal.

I wouldn't pick that out on a PIR unless there was something more significant about it. If you are looking for pricise detail in regulations, I would refer to the regs themselves rather than the OSG. But sometimes life is just tooo short