Inputs for Coordination Calculation
A 440 V 60 Hz switchboard feeds a 4wire distribution board for small loads such as socket outlets. The switchboard has a fault making capacity of 100kA rms. After applying diversity factors to the loads the total load current is 90 A. Moulded case circuit breakers (MCCBs) rated at 16 A and 32 A are to be used for the loads.
The installation will use cables having copper conductors and XLPE insulation. The cable from the switchboard to the distribution board is 20 metres in length.
A typical load cable is 15 metres in length and will carry a current of 29 A at a power factor of 0.85 lagging.
Ignore the presence of induction motors at the switchboard and find the following:
 Rating of the incoming circuit breaker.
 Size of the incoming cable.
 Size of the load cable.
 Check that the MCCB coordination is complete.
The following sequence will be used to calculate the results:
 Choose the upstream MCCB at the switchboard and its settings
 Choose the incoming feeder cable
 Choose the downstream load MCCB and its settings
 Find the upstream fault source impedance
 Find the cutoff, or letthrough, current from the switchboard
 Find the impedance of the incoming cable
 Find the impedance of the load cable
 Find the fault current at the distribution board, point B
 Find the fault current at the beginning of the load cable, point C
 Find the fault current at the end of the load cable, point D
 Check the peak making capacity and peak letthrough capacity of the MCCBs chosen above
 Find the highestIsquaredt value for the upstream MCCB
 Calculate a suitable size for the load cable to satisfy the I^{2}t duty
 Calculate the voltdrop in the load cable
 Select the largest conductor size from the above calculations
 Plot the results (coordination curve)
Let’s dive into solution!
1. Choose the upstream MCCB at the switchboard and its settings
From a manufacturer’s data sheet a 125 A MCCB with an adjustable 100 A thermal release is chosen. The thermal release is set to 90 A to match the total load.
2. Choose the incoming feeder cable
From a manufacturer’s data sheet several cables can be compared for the same ambient conditions and laying arrangements. Their details are:
 50 mm^{2} cable, maximum current 124 A, R = 0.492, X = 0.110 ohms/km.
 70 mm^{2} cable, maximum current 159 A, R = 0.340, X = 0.106 ohms/km.
 95 mm^{2} cable, maximum current 193 A, R = 0.247, X = 0.093 ohms/km.
3. Choose the downstream load MCCB and its settings
From a manufacturer’s data sheet a 32 A MCCB with an adjustable 32 A thermal release is chosen. The thermal release is set to 29 A to match its load.
4. Find the upstream fault source impedance
For a prospective symmetrical fault current of 100 kA rms the upstream fault source impedance Z_{up} is:
5. Find the cutoff, or letthrough, current from the switchboard
From a manufacturer’s data sheet a 125 A MCCB has a letthrough current I_{p} of 25 kA peak for a prospective fault current I_{s} of 100 kArms.
6. Find the impedance of the incoming cable
The impedance Z_{c1} of the incoming cable is:
7. Find the impedance of the load cable
The impedance Z_{c2} of the incoming cable is:
From a manufacturer’s data sheet several cables can be compared for the same ambient conditions and laying arrangements. Their details are:
 6 mm^{2} cable, maximum current 33.8 A, R = 3.91, X = 0.130 ohms/km.
 10 mm^{2} cable, maximum current 46.7 A, R = 2.31, X = 0.126 ohms/km.
The impedance Z_{c2} of the load cable is:
8. Find the fault current at the distribution board, point B
From a manufacturer’s data sheet the contact impedance data for low voltage MCCBs are:
MCCB (Rating in Amps) 
Resistance (in Ohms) 
Reactanse (in Ohms at 60Hz) 
16  0.01  neglect 
20  0.008  neglect 
25  0.0065  neglect 
32  0.005  0.000009 
50  0.0027  0.000016 
63  0.002  0.000025 
80  0.0014  0.000042 
100  0.0011  0.00007 
125  0.0008  0.0001 
160  0.00055  0.00015 
200  0.0004  0.0002 
250  0.00029  0.00027 
320  0.0002  0.0004 
Hence the upstream MCCB impedance Z_{m1} is 0.0008 + j 0.0001 ohms. Therefore the fault impedance Z_{fb} is:
The fault making current I_{fb} is:
Where V_{p} is the linetoneutral voltage. Locate the point R for 26,195 A on the prospective curve in Figure 1.
9. Find the fault current at the beginning of the load cable, point C
Hence the downstream MCCB impedance Z_{m2} is 0.005+j0.000009 ohms. Add this to Z_{fb} to give the fault impedance Z_{fc} as:
The fault making current I_{fc} is:
10. Find the fault current at the end of the load cable, point D
Add Z_{c2} to Z_{fc} to give the fault impedance Z_{fd} as:
The fault making current I_{fd} is:
11. Check the peak making capacity and peak letthrough capacity of the MCCBs chosen above
The following manufacturer’s data are typical for 125 A and 32 A MCCBs:
MCCB Rating  Making capacity  Letthrough capacity kA_{peak} (cutoff) 

kA_{rms}  kA_{peak}  
32 A  95  209 ***  6.0 
125 A  132  290 ***  25.0 
*** Approximate values of the doubling factor taken to be 2.2
Hence the peak making capacity of the 32 A MCCB is well in excess of the letthrough peak current of the 125 A MCCB.
12. Find the highest I^{2}t value for the upstream MCCB
Locate two points P and Q on the curve of the upstream MCCB as follows,
Point  Current in p.u.  Current in Amps  Time in seconds  I^{2}t 
P  14  406  6  989016.0 
Q  602  17,450  0.0016  487204.0 
Hence I^{2}t at P exceeds that at Q.
13. Calculate a suitable size for the load cable to satisfy the I^{2}t duty
For XLPE cables the ‘k factor’ for the I^{2}t is 143. The crosssectional area A is:
14. Calculate the voltdrop in the load cable
The usual limit to voltdrop in threephase cables feeding static loads is 2.5% at full load.
Where, I_{flc} = 29 A, L = 15 m and φ = 54.5495 degrees. For a 6 mm^{2} cable the voltdrop is found to be:
15. Select the largest conductor size from the above calculations
Comparing the conductor sizes found in 13. and 14. gives the larger as 10 mm^{2}, and this size should be used. Revise the calculation of the fault current I_{fd}. The impedance Z_{c2} of the load cable is:
Add Z_{c2} to Z_{fc} to give the fault impedance Z_{fd} as:
The fault making current I_{fd} is:
16. Plot the results
The results are plotted in Figure 1.
Refrence // Switchgear and Motor Control Centres – Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry by Alan L. Sheldrake (Download here)
Ismail
Hello, just a small question, in the introduction the main cable size is given as 20m so why was it used as 25m to calculate the impedance in number 6 ?