B
brman
I'm doing a mod for a 3 phase circuit already installed. The only trouble is I can't work out why the existing cables are as they are so i am doubting my sums.
Existing final circuit.
3 pole memshield 2, D32 rated feeding 10mm[SUP]2[/SUP] 4 core swa with a seperate 16mm[SUP]2[/SUP] G/Y as cpc.
Measurements at the DB are max 5.6kA single phase. Zs 0.04
Cable run is about 20m so I don't think voltage drop is an issue. So why has someone fitted an additional 16mm[SUP]2[/SUP] cpc?
If I use table 54.7 I need a cpc of 10mm[SUP]2[/SUP].
If I use the adiabatic if find:
1) max Ipf across phases is 1.73x5.6=9.7kA
2) let through energy for the mem D32 MCB is 43000 at 10kA according to their specs.
3) 43000 A[SUP]2[/SUP]s and a k of 143 (assuming xlpe) gives me a minimum cpc csa of 1.45mm[SUP]2
[/SUP]
So if I wanted to use a seperate cpc I would have gone for either 10mm[SUP]2[/SUP] if using the table values or 4mm[SUP]2[/SUP] (min unprotected) if using the adiabatic.
In fact I would probably have used the pyranha nuts and the armour of the swa as 10mm2 4 core has a armour csa of 43mm[SUP]2[/SUP] which is much more than 8 times 1.45mm[SUP]2[/SUP]
Have I got my sums right as someone spent good money putting in 16mm2 so I am thinking there must have been a reason?
The only thing I can think is that using the adiabatic simply with a 5.6kA ipf and t=0.1 gives 15.4mm[SUP]2[/SUP]
but a) which select a size bigger than table 54.7 requires and b) the 5.6kA is the single phase fault current.
Plus, why 10mm[SUP]2 [/SUP]SWA? Voltage drop at 32A would be about 2.5% I think even if I used 4mm[SUP]2[/SUP]and Zs should not be a problem either, even with a type D breaker.
My job is to put a spur on this circuit but am not keep to match the cable sizes already used for no good reason.........
Existing final circuit.
3 pole memshield 2, D32 rated feeding 10mm[SUP]2[/SUP] 4 core swa with a seperate 16mm[SUP]2[/SUP] G/Y as cpc.
Measurements at the DB are max 5.6kA single phase. Zs 0.04
Cable run is about 20m so I don't think voltage drop is an issue. So why has someone fitted an additional 16mm[SUP]2[/SUP] cpc?
If I use table 54.7 I need a cpc of 10mm[SUP]2[/SUP].
If I use the adiabatic if find:
1) max Ipf across phases is 1.73x5.6=9.7kA
2) let through energy for the mem D32 MCB is 43000 at 10kA according to their specs.
3) 43000 A[SUP]2[/SUP]s and a k of 143 (assuming xlpe) gives me a minimum cpc csa of 1.45mm[SUP]2
[/SUP]
So if I wanted to use a seperate cpc I would have gone for either 10mm[SUP]2[/SUP] if using the table values or 4mm[SUP]2[/SUP] (min unprotected) if using the adiabatic.
In fact I would probably have used the pyranha nuts and the armour of the swa as 10mm2 4 core has a armour csa of 43mm[SUP]2[/SUP] which is much more than 8 times 1.45mm[SUP]2[/SUP]
Have I got my sums right as someone spent good money putting in 16mm2 so I am thinking there must have been a reason?
The only thing I can think is that using the adiabatic simply with a 5.6kA ipf and t=0.1 gives 15.4mm[SUP]2[/SUP]
but a) which select a size bigger than table 54.7 requires and b) the 5.6kA is the single phase fault current.
Plus, why 10mm[SUP]2 [/SUP]SWA? Voltage drop at 32A would be about 2.5% I think even if I used 4mm[SUP]2[/SUP]and Zs should not be a problem either, even with a type D breaker.
My job is to put a spur on this circuit but am not keep to match the cable sizes already used for no good reason.........
