Cable calculation

Hi All, 

I am very rusty on cable calculations as I haven’t really needed to do it very often and kind of need a bit of reassurance. 
 

I need to supply a 3 phase sub board, 45 meter run in a duct from one switch room to another.

main supply is 100Amp 

Sub board is a 3 phase 100Amp main switch 

I was thinking I would need 35mm SWA 4 core with 20mm separate earth. Does that sound correct. 
 

Any help is appreciated.

20mm earth? Good luck with that one

far too little info provided

how do you get to 35mm?

just curious

I used a cable calculator online. What would you suggest then?? 

What is Zdb?

will Ze at far end be low enough to lift the OCP on a fault?

ambient temp?

is duct surface or buried?

mind you a contractor on site has just run a 50m 4c 10 mm SWA S brain with a 63A MCB and he thinks it's ok 

To be honest I don’t know the Ze

it is in a duct under ground with 5 other SWA 

Ambient temp 30

10mm sounds like he is penny pinching! 
 

63 amps I'd have said. Minimum 16mm 4 core possibly 25mm 

Hi m8,

it’s 100Amp main switch

a sub contractor installed a 16mm SWA for this already but it has been flagged that it is too small. 
 

they now want the cable calculation. This is what I am struggling with. 
 

cheers

Sponge bob big pants said:

Hi m8,

it’s 100Amp main switch

a sub contractor installed a 16mm SWA for this already but it has been flagged that it is too small. 
 

they now want the cable calculation. This is what I am struggling with. 
 

cheers

Click to expand...

I was replying to kerching 

I would have said 16mm would not have been big enough h to start with from. Day 1 as I'd have said min 25mm ideally 35mm due to possible. Load at 100 Amps I would have to do a calculation but I'm out at the moment 

ok sorry bud

Badger said:

63 amps I'd have said. Minimum 16mm 4 core possibly 25mm 

Click to expand...

My thoughts exactly...can't get him to understand that he has used thermosetting ratings BUT his terminals are not 90° so he needs to derate to PVC ratings....anyway, not my monkeys, not my circus

Badger said:

I was replying to kerching 

I would have said 16mm would not have been big enough h to start with from. Day 1 as I'd have said min 25mm ideally 35mm due to possible. Load at 100 Amps I would have to do a calculation but I'm out at the moment 

Click to expand...

It’s the calculation part I’m not confident with. I would think a 35mm 4 core would be fine. If you can help me out with the calculation when you have time that would be great.

kerching said:

My thoughts exactly...can't get him to understand that he has used thermosetting ratings BUT his terminals are not 90° so he needs to derate to PVC ratings....anyway, not my monkeys, not my circus

Click to expand...

I wouldn't have put 63 amps onto a 10mm in any case I'd have upped it from a safety sizing POV to be on the safe side at that level of current and protection.

Hi Badger

Badger said:

I was replying to kerching 

I would have said 16mm would not have been big enough h to start with from. Day 1 as I'd have said min 25mm ideally 35mm due to possible. Load at 100 Amps I would have to do a calculation but I'm out at the moment 

Click to expand...

Would you have any time to help me out with this calculation?? Like I said I don’t have the Ze but just an idea would be helpful. Cheers

Load?

Earthing arrangement?

PFC?

Ze/Ra?

Distribution or final circuit?

Any lighting?

Installation method?

Sidney said:

Load?

Earthing arrangement?

PFC?

Ze/Ra?

Distribution or final circuit?

Any lighting?

Installation method?

Click to expand...

The load is 100Amps

just now the board is on a TT 

but I can run an earth in with the new SWA 

Ze 0.08

its in a duct buried 

45 meter run

all that we are putting onto this board is 6 car charging points 

6 x 32Amps

Sponge bob big pants said:

The load is 100Amps

just now the board is on a TT 

but I can run an earth in with the new SWA 

Ze 0.08

its in a duct buried 

45 meter run

all that we are putting onto this board is 6 car charging points 

6 x 32Amps

Click to expand...

Sorry it’s 4 chargers

4 x 32amps

http://jarsoftelectrical.co.uk/

Using 50mm CSA 70c SWA I get VD of 3.6 using the resistive load column.

R1+R2 of 0.034 ohms giving Ze of 0.114.

PFC 4.95Ka.

Adiabatic has 1.06mm for copper and 2.40mm for steel armour for a 5 second disconnection time.

Thanks for that Sidney, it’s appreciated.

I got to say I didn’t think it would be a 50mm SWA. Don’t know if I would get that through the duct

35mm is only good for 98 amps buried direct for three phase.