CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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1. Introduction: This document presents a report on cable sizing calculation for the Ras Tanura Intigrated Project. The design calculation aims at appropriate selection of cables considering the installation conditions, ambient temperature, group de-rating and short circuit withstand. The cable sizes and the maximum cable lengths are calculated based on continuous current rating, ensuring voltage drop is within permissible limits and are checked for thermal short circuit capacity. Cable selection charts have been prepared and are annexed for reference.
2. Standards And Specifications : A554-K-PRG-EL-GEN-PHL-005 A554-K-PRG-PE-GEN-PDD-001 G7Z-0001-00 G7C-0302-00 RT7D-0665-00 RT7D-0613-00
Electrical Design Philosophy Project Design Data Electrical Design Engineering Criteria Process Plant Cabling / Wiring Methods. Form Wound Motors. Squirrel Cage Induction Motors NEMA Frame Horizontal and Vertical Totally Enclosed Designs 600 Volts and below.
3. References : XXXX XXXX A554-K-299-EL-DWG-EA1-001
Electrical System Study Report Grounding Philosophy and Calculations Electrical Overall One-Line Diagram (SS-299C001) Riyadh Cable Catalogue Ducab Cable Catalogue
4. Specification Requirements: As per clause 10.1.1 of A554-K-PRG-EL-GEN-PHL-005 “The voltage at any point on the RTIP system will normally remain within the nominal values as stated below, unless abnormal conditions prevail. Under exceptional circumstances, the maximum over voltage limits stated below should not be exceeded for more than 30 minutes.” • •
Nominal range 30 min over voltage
+/+
5% 10%
As per clause 10.2.2 of A554-K-PRG-EL-GEN-PHL-005 “The maximum voltage drop (measured on the distribution bus) under motor starting or restarting conditions should not exceed 10% of nominal voltage. Voltage drop at the motor terminal shall not drop below 85% of the rated motor voltage.” As per clause 26.1.1 of A554-K-PRG-EL-GEN-PHL-005 “All cables shall be routed above ground in continuous tray systems. Underground cable installations shall be by exception and require Owner Electrical SME approval.” As per clause 26.1.2 of A554-K-PRG-EL-GEN-PHL-005 “All low voltage and instrument cables shall be supplied according to IEC standards. Cable jackets shall be flame retardant, UV rated for direct sunlight exposure and suitable use in cable tray. Cable shall be steel wire armoured”.
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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As per clause 26.1.3 of A554-K-PRG-EL-GEN-PHL-005 “The maximum conductor operating temperature under normal conditions shall not exceed 90 degree Centigrade. Under emergency conditions the conductor shall not exceed 130 degree centigrade. The maximum conductor temperature under short circuit conditions shall not exceed 250 degree centigrade based on maximum cable fault duration of 250 ms”.
5. Basis of Design: The system design parameters and the site ambient conditions are listed below. •
Nominal Bus Voltage Levels Medium Voltage Low Voltage
4160 V, 3-phase 480 V, 3-phase,3 Wire
•
System Frequency
:
60 Hz
•
Site ambient air temperature
:
500 C
•
Fault Levels at various voltage levels Medium Voltage Low Voltage
50kA, 1 sec 65kA, 1 sec
•
Zsystem : 0.02 ohm (Typical Value)
•
Allowable voltage drops (starting and running) for LV and MV motors: 15 % and 5% respectively.
•
Minimum conductor sizes at various voltage levels :
Medium Voltage motor feeder
25 mm2
Low Voltage motor feeder
2.5 mm2
•
Riyadh Cable data has been used for MV motor feeders and Ducab Cable data has been used for LV motors.
•
For Motor details of MV motors ABB ANSI motor catalogue has been used, for LV motors Brook Crompton ANSI motor catalogue has been used.
6. Calculation
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
•
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MV Motor Feeder Cable Sizing Calculation The sizing of 4.0 kV motor feeder cables shall take into account the following parameters: a) b) c)
Current carrying capacity of cable. Short circuit withstand capacity. Voltage drop in the cable.
These cables are sized to account for the minimum thermal rating required by applying the de-rating factors based on the laying conditions and the maximum short circuit withstand capability of the cable has to be chosen greater than 50 kA ( for circuit breaker protected feeders). The maximum feeder length is guided by the voltage drop calculations.
SIZING CRITERION – A Cables feeding a single motor used in a continuous duty application shall have an ampacity of not less than Service Factor times the motor’s full load current rating. Therefore the cable should be chosen such that the normal ampacity meets this criterion.
Normal Ampacity required =
FLC xS .F DF
(A)
Where, FLC
–
Motor Full Load Current.
S.F
–
Motor Service Factor 1.0 (as per RT7D-0665-00 clause 4.1)
D.F
–
De-rating factor (based on cable laying conditions)
The motor full load current can be calculated as follows:
I=
kW 3 xkVxP .Fxefficien cy
(A)
Where, kW
–
Rated kW of the motor.
V
–
Cable Operating Voltage.
P.F
–
Motor Power Factor.
De-rating Factor
a) Ambient air temperature factor (500 C) b)
Grouping factor ( 5 cables in group 3 core 1 layer, 1D separation)
= =
0.82 0.85
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS c)
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Overall de-rating factor
= = =
aXb 0.82 X 0.85 0.70.
Now for a 300 HP motor feeder the following calculation demonstrates the selection of cable cross section based on SIZING CRITERION A: The motor full load current can be calculated as :
I=
kW (A) 3 xkVxP .Fxefficien cy
I=
224 = 3 x 4 x0.84 x.93
41 (A) (224kW is the rating of the motor)
Now the cable ampacity required to feed the above motor is calculated as: Ampacity required =
=
FLC xS .F DF
(A)
41 x1.0 0.70
= 58.57 (A). 1R X 3C X 25mm2 cable has a non de-rated ampacity of 150 (A). This is the minimum cable size available for MV feeders. Based on the calculations above it can be concluded that 1R X 3C X 25mm2 cable can feed 300 HP motor feeder. SIZING CRITERION – B The short circuit withstand capacity of the cable determines the minimum cross section area of the cable which can be used. The minimum cross section of the cable –
A=
Ix t K
Where, A
–
Minimum Area of cross section required for 4.16 kV cable system.
K
–
Insulation coefficient of XLPE cable.
T
–
Total fault clearing time in second.
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS I
–
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Fault current rating of the switchgear.
Total Tripping time of 4.16 kV Circuit Breaker = 200 ms (Typical value) The maximum fault rating of the 4.16kV switchgear is 50 kA. The insulation coefficient of XLPE cables is 143. The admissible current density D in short circuit conditions is given by the formula:
A=
Ix t K
Where A is the minimum Area of cross section required for 4.16 kV cable system. K is the insulation coefficient of XLPE insulated cable = 143. Then the Minimum Cross Section Area of the cable for 4.16 kV system A =
50000 x 0.2 143
= 156.36 mm2 185 mm2 being the nearest available standard cross section of cable available is chosen to be the minimum cross section of the cable. FOR FUSE PROTECTED FEEDERS : The Fuse rupturing time
= 0.005 sec.
The maximum fault rating of the 4.16kV switchgear is 50 kA. The insulation coefficient of XLPE cables is 143. The admissible current density D in short circuit conditions is given by the formula:
A=
Ix t K
Where A is the minimum Area of cross section required for 4.16 kV cable system. K is the insulation coefficient of XLPE insulated cable = 143.
Then the Minimum Area of the cable for 4.16 kV system A =
50000 x 0.005 143 = 24.72 mm2
25 mm2 being the nearest available standard cross section is chosen to be the minimum cross section of cables for fuse protected motor feeders. So it can be concluded that for fuse protected feeders all cable sizes can be used and for breaker operated feeders minimum cable size which can be selected is 185 mm2.
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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SIZING CRITERION – C The cable impedence causes a voltage drop at the load terminal, the same needs to be arrested within the permissible limits. The following checks are performed towards achieving the above. For motor feeders voltage drop criteria needs to be checked for both running and starting conditions. Voltage drop in a motor feeder can be expressed as follows:
Vd running =
3 xI m ( R cos φr + X sin φr ) xL 1000 x N C
(V)
Vd starting =
3 xI lrc ( R cos φst + X sin φst ) xL 1000 x N C
(V)
Where, Im
–
Rated current of motor.
Ilrc
–
Locked Rotor current of the motor.
cosΦr
–
Motor power factor for running condition.
cosΦst –
Motor starting power factor.
R
–
Cable Resistance Ohm/km
X
–
Cable Reactance Ohm/km
Nc
–
Number of cables running in parallel.
The maximum length of cable which can be used maintaining the prescribed voltage drop limits is calculated as follows:
m running =
m starting
=
x1000 (m)
(
Vd x N C 3 xI m x ( R cos φr + X sin φr )
(
Vd x N C 3 xI lrc x( R cos φst + X sin φst )
)
)
x1000 (m)
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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The permissible running and starting voltage drops are 5% and 15% respectively. When feeding a 300 HP motor with 1R X 3C X 25 mm2 cable the maximum running and starting lengths are:
mrunning
=
(
4000 x5 x 1 3 x 41 x ( 0.927 x 0.84 + 0.146 x0.54 ) x100
)
x1000
= 3283 m.
mstarting
=
(
4000 x15 x 1 3 x 266 .5 x( 0.927 x0.22 + 0.146 x 0.975 ) x100
)
x1000
= 3778 m.
Based on the above calculations it is concluded that a fuse protected 300 HP motor feeder can be fed with a single run of 3 core 25 mm2 cable to a maximum length of 3283 m satisfying the running and the starting voltage drops. As suggested above the same cable would not be suitable for feeding the same motor feeder in the event the feeder is circuit breaker protected.
•
LV Motor Feeder Cable Sizing Calculation
Sizing of 460 volt motor feeder cables shall take into account the following parameters: a) b)
Current carrying capacity of the cable. Voltage drop in the cable.
These cables are sized to account for the minimum thermal rating required by applying the de-rating factors based on the laying conditions and the voltage drop limitations. SIZING CRITERION – A Cables feeding a single motor used in a continuous duty application shall have an ampacity of not less than Service Factor times the motor’s full load current rating. Therefore the cable should be chosen such that the normal ampacity meets this criterion.
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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Normal Ampacity required =
FLC xS .F (A) DF
Where, FLC
–
Motor Full Load Current.
S.F
–
Motor Service Factor 1.15 ( as per RT7D-0613-00 clause 2.2)
D.F
–
De-rating factor (based on cable laying conditions)
The motor full load current can be calculated as follows :
I=
kW (A) 3 xkVxP .Fxefficien cy
Where, kW
–
Rated kW of the motor.
V
–
Cable Operating Voltage.
P.F
–
Motor Power Factor.
De-rating Factor
a) Ambient air temperature factor (50 degree C) b) Grouping factor for multi core cables laid in air c)
= = = = =
Overall de-rating factor
0.82 1 aXb 0.82 X 1 0.82.
Now for a 10 HP motor feeder the following calculation demonstrates the selection of cable cross section based on SIZING CRITERION A: The motor full load current can be calculated as:
I=
kW (A) 3 xkVxP .Fxefficien cy
I=
7 .5 = 3 x 0.46 x0.84 x.895
12.5 (A) (7.5kW is the rating of the motor)
Now the cable ampacity required to feed the above motor is calculated as: Ampacity required =
FLC xS .F DF
(A)
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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=
12 .5 x1.15 0.82
= 17.53 (A). 1RX 4CX 2.5 mm2 cable has a non de-rated ampacity of 33 ampere. This is the minimum cable size available for LV feeders. Based on the calculations above it can be concluded 1RX 4CX 2.5 mm2 cable can feed 10 HP motor feeder.
SIZING CRITERION – B The cable impedence causes a voltage drop at the load terminal, the same needs to be arrested within the permissible limits. The following checks are performed towards achieving the above. For motor feeders voltage drop criteria needs to be checked for both running and starting conditions. Voltage drop in a motor feeder can be expressed as follows:
Vd running =
3 xI m ( R cos φr + X sin φr ) xL 1000 x N C
(V)
Vd starting =
3 xI lrc ( R cos φst + X sin φst ) xL 1000 x N C
(V)
Where, Im
–
Rated current of motor.
Ilrc
–
Locked Rotor current of the motor.
cosΦr
–
Motor power factor for running condition.
cosΦst –
Motor starting power factor.
R
–
Cable Resistance Ohm/km
X
–
Cable Reactance Ohm/km
Nc
–
Number of cables running in parallel.
The maximum length of cable which can be used maintaining the prescribed voltage drop limits is calculated as follows:
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CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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m running =
m starting
=
x1000 (m)
(
Vd x N C 3 xI m x ( R cos φr + X sin φr )
(
Vd x N C 3 xI lrc x( R cos φst + X sin φst )
)
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)
x1000 (m)
The permissible running and starting voltage drops are 5% and 15% respectively. When feeding a 10 HP motor fed with 1RX 4CX 2.5 mm2 cable the maximum running and stating lengths are:
m running =
(
460 x 1x5 3 x12 .50 x( 9.45 x0.84 + 0.121 x0.54 ) x100
)
x1000 m
=132.87 m
m starting =
(
460 x 1x15 3 x81 x( 9.45 x 0.54 + 0.121 x0.84 ) x100
)
x1000 m
= 94.5 m Based on the comparison made between the length based on voltage drop, the maximum cable length is taken as the shorter of the two. So based on the calculations above it can be concluded that a 10 HP motor can be fed by a 1RX 4CX 2.5 mm2 cable up to a maximum length of 94.5 m.
LV cable short circuit capabilities: The cable short circuit current capabilities is based upon the adiabatic equation given by T=( k2 X s2)/I2 Where cable withstand capability is given by (k2 X s2) = A2 Where k is the cable insulation thermal constant (143 for XLPE insulated cables) and s is the cable cross section in mm2.
CABLE SIZING CALCULATION AND REPORT FOR MV AND LV MOTOR FEEDERS
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The cable of chosen cross-section and the short circuit capacity is checked to determine if it is capable of withstanding the let-through energy of the connected protective device. Now for feeding a 15 HP motor with a 2.5 mm2 it can be checked that the cable withstand for 0.005 sec is 5.06 kA, while the fuse let through current stands at 5.7 kA. So the only restriction is 2.5 mm2 cable can feed motors up to 10 HP.
7. Cable Selection Chart : Cables are selected by locating the motor kW rating under the relevant Data column, then by viewing horizontally, the first record displayed that exceeds the known cable route length is selected. Viewing up this column indicates the minimum allowable cable size. The following formula is used within the spreadsheet. The cable current carrying capacity after applying a de-rating factor is compared with the Motor FLC. (In case of motors, this is normally set to Service Factor times the motor full load current) ICABLE x F > ITHS ICABLE F ITHS
Cable current rating De-rating factor Thermal relay setting
If the duty exceeds the cable de-rated capability then a =T= is displayed in the chart. For Breaker fed MV motors a =T= is also displayed even if the short circuit capability of the cable is exceeded. For motor feeders, the spreadsheet determines whether the starting conditions or the running conditions determine the maximum permissible cable, this is denoted by a {s} or {r} alongside the figures on the sheet. For the motor feeders, the criterion which determines the maximum permissible cable length is displayed by the side as (=T=) when the thermal limit of the cable is exceeded and a larger cable is required, (=L=) when the cable exceeds the specified maximum length Also {S} when voltage drop at starting dictates the length (for motors). {R} when voltage drop at running condition determines the length (for motors).
