CSA of cable for Motor with VSD

1. Sep 3, 2016

I normally use ELEK Cable Pro to calculate my cable sizes and it works very well however I have a project at the moment which has a circuit which I need help calculating the correct size cable for;

The circuit is for an 18.5kw 400v motor. I have used the following

V- 400v

Efficiency – 90%

PF – 0.95

Voltage Drop – 5%

Cable Length – 50mtr

Earthing – TN-C-S

Correction factor – 0.9

Protective device type B 40A

Installation – In cable duct

Cable Type – 4 core armoured XLPE

My problem is that the software gives the option of 5 different circuit arrangements

2. Sub Mains

3. Ring

4. Motor DOL

5. Motor Star-Delta

My motor is using a variable speed drive, which isn’t an option. If I use the software and select DOL then it tells me 16mm cable but any of the other options gives me 10mm.

In my experience I would think 6mm would be ok if the MCB had been a Type C 32A but I didn’t supply the panel and it is a 40A B type which I am stuck with.

I would think if I select Motor star-delta in the software and use a 10mm cable then my cable report would be okay and I would be covered?

2. Sep 5, 2016

1)If the MCB it is for overload protection the Iz[cable carrying capacity]of the cable has not to be more than MCB In[rated].So, the cable has to withstand at least 40 A.

In my opinion, the installation method [BS 7671 Table 4A2 pos.70] has to be D.

According to Table 4E4A column 7 for 20oC Earth and 2.5 K.m/W resistivity 4*10 mm^2 XLPE insulated copper conductors Iz=58 A. If the correction factor-for other Earth resistivity and/or other temperature [more than 20oC] is 0.9 then Iz= 52.2 A.

2) According to Table 4E4B column 4 the voltage drop will be 4 [mV/A/m].

If the motor starting method will be D.O.L. the Istart/In= 6-8 and the voltage drop for 4*10 copper will be 9.5-12.5%.

However, if VSD-variable speed drive- is actually VFD[variable frequency drive] then Istart/In =<1.5.then the voltage drop will be only 2.34%.

3. Sep 6, 2016