Cable Calc

[IAmSparkytus!]

Hello

Existing 69kva supply with the board 100m from the incoming supply (within a shopping centre).

Three phase board supplying lighting and power circuits.

Cable ran along a corridor on cable tray/ladder.

Online Cable calculators saying the run is OK for 25mm 4core but when I’ve used the BBB, it’s coming out at over 3% VD.

anyone care to do their own calc for me and let me know what they come up with?

Thanks

 

[westward10]

Your volt drop must take into consideration the final circuits from the distribution board, a rule of thumb is 2% for the submain. I don't need to calculate to know it isn't big enough to comply assuming the protective device is 100A.

 

[IAmSparkytus!]

What figure are you getting? Yes all that’s fitted at the minute is an existing fuse board at the end of an 85m cable which is fed from the 100A fuses (not sure if 88s or 1361s if that matters?)

Additional 15m of cable to be added to get the new board where it needs to be

Switched fused isolator with more 100A fuses inside to be used to extend the cable.

 

[DefyG]

Just done a quick/rough calc on app I have on phone and it shows >4% at FL.
Do you need all the capacity available? If you can reduced it with (say 63A fuses) it might comply?

 

[IAmSparkytus!]

I came out with 3.75% or something I think so definitely too high. Never know whether to take the VD as a percentage of 400V or 415V though which would effect it a bit.

The fuses have already been upgraded by someone else which they clearly shouldn’t have been.

Calculated that 35mm would fine for 100A over the distance but would the SWA suffice as the main earth or would I be best pulling a 35mm earth in as well?

 

[westward10]

Depends on whether the armour has to support main protective bonding and for TN-C-S it would be undersized.

 

[IAmSparkytus!]

35mm earth to run alongside it it is then!

 

[DefyG]

35mm earth to run alongside it it is then!

I haven't bbb to hand but think you could save a few £££'s and install a smaller earthing conductor (maybe 16sq)?? unless that might lead to higher Zs at the load end!