3 phase design process

[tomlonsborough]

Hi folks,

I'm installing a 3-ph 32A 60309 socket and just wanted to double check my design calcs are right...

The circuit will be part of a 3-ph sub-board (200A main switch) supplied by our main 3-ph distribution board. The final circuit length is approx 30m. The sub board comprises just power/lighting, no inductive loads, motors etc.

Ib = 32A
In = 3-ph 32A type C (there's already a spare MCB in the board)

Going for an XLPE SWA cable, clipped direct, no other rating factors play a part.

4mm carries 42A, but going for a 6mm because cost isn't a factor and would rather err on the side of too much capacity!

EFLI calcs:
Measured Zs @ DB = 0.05
R1+R2 calculated from table F8A in the IET design guide = 0.2214 ((3.08/1000*30)+(4.3/1000*30))
Total Zs = 0.2714
Max measured Zs for 32A type C = 0.54 ... disconnection times satisfied

Volt drop calcs:
Board supplied by 95mm SWA 109m run, 3-ph.
Pre-diversity total = 463A
Diversity applied as per OSG appendix A = 247A
Diverstiy applied as per IET design guide (0.9 lighting 0.1 power) = 141A

I've based calcs on a figure about 1/2 way between the 2 diversity values = 180A

Volt drop to sub-board = (109m x 180A x 0.45)/1000 = 8.8V

Volt drop on final circuit = (30m x 32A x 6.8)/1000 = 6.5V

Total volt drop = 15.3V ... volt drop less that 5%


Adiabatic calcs:
fault current at end of run:
1.1 x 230V / 0.2714 = 932A
932A = disconnection time of 0.1s for MCB

0.95 x 230V / 0.2714 = 805A

Worst case: S = √( 932 x 932 x 0.1) / 51 = 5.78mm

CSA of 6mm 4-core armouring = 38mm ... adiabatic compliant.



Many thanks in advance to anyone who has the time to help me with this, still learning it all at the moment, lots to consider!

All the best,

Tom.

 

[Andy78]

Looks a pretty comprehensive job at first glance.

Only thing I would correct would be the tabulated current carrying capacity. You have used table 4E4A for XLPE thermosetting cable at 90 degrees, but unless you can guarantee all connections, terminals, and accessories will be rated for 90 degrees as well as the cable you should be using table 4D4A for 70 degree PVC SWA

 

[westward10]

I doubt the volt drop for the lighting circuits from the sub board actually comply. Rule of thumb is to use 2% for submains which allows some leeway for final circuits.

 

[tomlonsborough]

Looks a pretty comprehensive job at first glance.

Only thing I would correct would be the tabulated current carrying capacity. You have used table 4E4A for XLPE thermosetting cable at 90 degrees, but unless you can guarantee all connections, terminals, and accessories will be rated for 90 degrees as well as the cable you should be using table 4D4A for 70 degree PVC SWA

Hi Andy, a big thanks for the reply, and yes a good point I hadn't considered there.. 6mm PVC should still do the job.

T
[automerge]1574171086[/automerge]

I doubt the volt drop for the lighting circuits from the sub board actually comply. Rule of thumb is to use 2% for submains which allows some leeway for final circuits.

Hi West, thanks for the reply, you're probably right - I'll do some calcs for my own morbid curiosity - but thankfully I had nothing to do with that bit of the design!

T