Star delta starter cable sizing

[melvinkoshy]

This is pertaining to cable selection for star-delta starter which is to be connected from a panel. It is understood that we have six terminals from star-delta starter to connect to motor (i.e. two runs of 3-core cables from star-delta starter are to be connected to motor). Should the cable from panel to star-delta starter be greater in size as compared to that from star-delta starter to motor.

A sample calculation is as follows:-

Main Supply Voltage = 400V, 3Phase, 50Hz
Motor Power = 18.5kW [52 hp] (Condenser Pump)
Starting Method = Star Delta
Distance from the panel to starter = 1m
Distance from the starter to motor = 20m

Parameters to be calculated are:-
a. MCCB rating
b. Cable size from main supply panel to starter and from starter to motor

Load current Calculation

Full Load Current = 18.5kW/1.732*400*0.85*0.94 (0.85 = p.f and 0.94 = efficiency)
FLC = 33.42A say 34A
MCCB = 1.5*FLC = 1.5*34A = 51A (choosing next higher rating say 63A)
(whether the MCCB selection is Ok? Or should I go for 2.5 times of FLC.)

Cable size selection

From Panel to starter
MCCB rating =63A
Cable size = 16 sq. mm Aluminium (Cable ampere rating = 62A)

Cable Size from starter to motor
We have 6 cables going from the starter to motor. Each set of cable need to be rated at 58% of the FLC. Therefore
Cable current = 0.58*34 = 19.72A
Cable size = 6 sq. mm Aluminium (Cable ampere rating=43A)

Please check and comment on the calculation

 

[don_resqcapt19]

Take a look at 430.22(C).

 

[kwired]

Take a look at 430.22(C).

And the informational note that follows it.

 

[Julius Right]

First of all 18.5 kW=24.8 hp only.
As don_resqcapt19 and kwired said:
If you intend to follow NEC then NEC 2014/430.22 Single Motor.(C) Wye-Start, Delta-Run Motor.
This article states you have to choose at first the HP in the Table 430.250 Full-Load Current, Three-Phase Alternating-Current Motors. Since NEC does not recognize 400 V, I think, motor nameplate current will be enough-instead of FLC.
So, you have to state the current of conductors on the line side of the controller shall not be less than 125 percent and the conductors between the controller and the motor shall not be less than 72 percent of the motor full-load current rating.
430.32 Continuous-Duty Motors.(1) Separate Overload Device. states:
overload setting 115% up to 130% -if no marking of of service factor or maximum temperature rise.
Table 430.52 Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices states for Inverse Time Breaker -for instance-250% of FLC.
However, in my opinion, you don't need to follow NEC.
For 400 V 50 Hz overcurrent protection IEC 60364-4-43 Electrical installations of buildings Part 4-43: Protection for safety - Protection against overcurrent or: BS7671 CHAPTER 43.
IEC 60364-5-52 for cable ampacity[ or again BS7671] .
See also:
ABB Electrical installation handbook Protection, control and electrical devices
https://library.e.abb.com/public/ae2141fea4bfa9d748257a700024a579/1SDC010002D0206.pdf

 

[Tony S]

Table 4L1A of BS7671 may be more appropriate

 

[Smart $]

...
(whether the MCCB selection is Ok? Or should I go for 2.5 times of FLC.)

Go with 250%.

Cable size = 16 sq. mm Aluminium (Cable ampere rating = 62A)

The minimum cable size is (FLC)×125%, approximately 42A.

Cable Size from starter to motor
We have 6 cables going from the starter to motor. Each set of cable need to be rated at 58% of the FLC. Therefore
Cable current = 0.58*34 = 19.72A
Cable size = 6 sq. mm Aluminium (Cable ampere rating=43A)

72% FLC or 58% minimum cable size panel to starter

• 33.4A×72%=24A
• 42A×58%=24A

FWIW, 58% is the reciprocal of the square root of 3, rounded up.

 

[Tony S]

Bangalore is in India hence IEC not NEC

 

[Smart $]

Bangalore is in India hence IEC not NEC

Point noted mentally earlier. Now noted publicly.... and on that note, my earlier post was NEC.

Now consider the project specification requires NEC compliance (in addition to the IEC requirements)...