Calculating the voltage drop across a PVC insulated cable- 35mm copper conductor

[Anita]

I am adding a new Air handler panel to a building and its load is 5421KVA. How do I go about getting the the start up current when the phase voltage is 240V?

On obtaining the Start up current, now how do I calculate the voltage drop when the cable that I am thinking about using is the 35mm sq ac three phase cable (1.10mV). The length of cable is 149 feet or 45.41m.

Can someone please indicate a step by step process which I can follow to calculate the voltage drop on this cable please.........

 

[J@mes]

5421KVA? which is about 4330KW, or 4.3 million watts.

Are you sure?

 

[Anita]

sorry about that I am new at this.... I think its 5421VA.....does that make more sense now?

 

[J@mes]

I would expect that the manufacturer technical department would be able to give you the exact figures for the start up current on the air handler unit.

To be honest, your figures aren't making much sense. If it is now 5421VA, then that is 4.33KW and about 6A on 3P 400v or 18A on 1P 230v. this would require a cable of about 4mm in conductor size (or 1.5mm for 3 phase) with a volt drop of around 7v on the 3 phase and 9v on the 1 phase. Why it would require 3 phase and a 35mm conductor I don't know, especially with a circuit length of 46 meters.

I'm hoping someone is going to come along and agree with me, just to let me know I haven't A) misread what you are saying or B) gone completely mental.

sorry!

 

[Anita]

The info below shows the values I was refering to. These figures were calculated from the sum of all the motor loads on the Air hander Panel. From here you can see what i am talking about with regards to the KVA.


connected KVA totals: 5412
Demand factor: 1.00
Total connected KVA: 5412
Total Adjusted KVA: 5412
Single phase Voltage: 240
Total Panel adjusted amperage: 23

 

[Guest123]

Using the info you have given, the worst case scenario would be a load of 23A @ 230V over a rounded up distance of 50M.

Selecting a 6mm PVC SWA (the CCC is more than ample for any reference method) which has a VD of 7.3mV/A/m the sum is 23 x 50 x 11 / 1000 = 8.39V.

The max permissable is 5% of 230V = 11.5V, so a 6mm complies.

 

[Dave_]

You should finally confirm the cable complies regarding thermal constraints.

Using that equation that i seem to constantly bang on about, the adiabatic.

6mm Has a Resistance of 6.16 milliohms per metre including cpc

circuit length 50m therefore 6.16 x 50 x 1.2 (assuming cable to BS5467) = 0.37 ohms

taking Ze as worst case TNCS 0.35 + 0.37 = 0.72 ohms (Zs)

Then calculate fault current

if = Uo/Zs 230/0.72 = 319A

Using Time Current graphs in Regs book

Assuming a 25 A type C CB To 60898

At 319A it would disconnect in 0.1s thus complying with 0.4s maximum disconnection time

Using adiabatic S= √(I²×t)/k Where S is the size in mm² of the cpc

S = √319²x 0.1 / 100 (K values p 129 regs)

S = 1.08mm² therefore 6mm² is fine

 

[Dave_]

spot the mistake