CSA for Cable on Motor with VSD

[LoadMonitor]

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
Load – 18.5kw
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
1. Radial
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.
Can anyone advise?
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?

 

[waross]

Hi LoadMonitor;
I'm a little confused.
Where are you?
I know CSA as Canadian Standards Association.
My CSA uses AWG wire sizes.
Our conductor ampacities are based on rated current of the load. The conductors to the VFD are based on the VFD rated current, not the motor rated current.
We size motor conductors at 125% of rated motor current.
I believe that the NEC folk size motor conductors based on 125% of the current given in a table based on HP.
This allows for a motor to be changed out for a motor of the same HP but greater rated current.
We work to 3% voltage drop in a feeder or branch circuit. Maximum 5% from supply to utilization.

Let us know where in the world you are and what codes you work under and someone familiar with those codes will be glad to help.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

waross, CSA outside of Canada probably means Cross Sectional Area, as most of the IEC compliant locations tend to use it for sizing cable.

I'm guessing that the higher DOL figure is possibly due to either voltage drop to get the motor up and running, or possibly due to higher current during starting. Since a distance hasn't been advised its a little harder to tell if voltage drop is likely to be a factor.

I'd review the numbers outside of the software just to see what the difference is likely to be, but as waross rightly asserted, relevant standards depend on location.

 

[LoadMonitor]

Sorry for the confusion. It is based on UK standards so CSA stands for cross sectional area. The cable run is approx 50mts from the VSD to the motor and the VSD located adjacent to the MCC.

 

[waross]

Thanks Freddy.
Under our CSA we don't consider the starting voltage drop in the motor circuit conductors. In special cases, where a very large motor is to be started on a soft grid or generator, we may consider the voltage drop at the Point of Common Coupling. The calculated voltage drop at the PCC is often reviewed when a motor of more than about 15% or 20% of the generator capacity will be started.
Ironically, if the the motor conductors are so long that the conductor impedance is significant, the conductors act as a primary starting resistance and reduce the voltage drop at the PCC and the generator terminals.
Over-sizing the conductors for DOL starting may be counter productive in reducing the voltage drop at the PCC or generator terminals.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The DOL starter uses a larger cable to reduce the volt-drop during starting when the current is about 6x full load current. I would treat the VFD as a feeder circuit for volt-drop, short circuit and earth loop impedance requirements. The VFD itself should protect the motor and cable.

Odd to see a Type B breaker with a VFD given the high inrush characteristics of the input, although at that size you might have a soft charge of the DC link. Would it be simpler / more economical to change the MCB to a smaller size and thus allow use of the smaller feeder cable?