Example of Cable Calculation

7/27/2019 Example of Cable Calculation

1/6

From Electrical Installation Guide

1 Worked example of cable calculation

2 Calculation using software Ecodial

3 The same calculation using the simplified method recommended in this guide

3.1 Dimensioning circuit C1



3.4 Calculation of short-circuit currents for the selection of circuit-breakers Q1, Q3, Q7 (see Fig. G67)

3.5 The protective conductor

3.6 Protection against indirect-contact hazards

3.7 Voltage drop

(see Fig. G65)

The installation is supplied through a 630 kVA transformer. The process requires a high degree of supply continuity and

part of the installation can be supplied by a 250 kVA standby generator. The global earthing system is TN-S, except for the

most critical loads supplied by an isolation transformer with a downstream IT configuration.

The single-line diagram is shown in Figure G65below. The results of a computer study for the circuit from transformer T1down to the cable C7 is reproduced on Figure G66. This study was carried out with Ecodial software (a Schneider Electric

product).

This is followed by the same calculations carried out by the simplified method described in this guide.

ed example of cable calculation - Electrical Installation Guide http://www.electrical-installation.org/enwiki/Worked_example_o

10/13/2013


2/6

Fig. G65: Example of single-line diagram

General network characteristics Number of poles and protected poles 4P4d

Earthing system TN-S Tripping unit Micrologic 2.3

Neutral distributed No Overload trip Ir (A) 510

Voltage (V) 400 Short-delay trip Im / Isd (A) 5100

Frequency (Hz) 50 Cable C3

Upstream fault level (MVA) 500 Length 20

Resistance of MV network (m) 0.0351 Maximum load current (A) 509

Reactance of MV network (m) 0.351 Type of insulation PVC

Transformer T1 Ambient temperature (C) 30

Rating (kVA) 630 Conductor material Copper

Short-circuit impedance voltage (%) 4 Single-core or multi-core cable Single

Transformer resistance RT (m) 3.472 Installation method F

Transformer reactance XT (m) 10.64 Phase conductor selected csa (mm2) 2 x 95


10/13/2013


3/6

3-phase short-circuit current Ik3

(kA) 21.54 Neutral conductor selected csa (mm2) 2 x 95

Cable C1 PE conductor selected csa (mm2) 1 x 95

Length (m) 5 Cable voltage drop U (%) 0.53

Maximum load current (A) 860 Total voltage drop U (%) 0.65

Type of insulation PVC 3-phase short-circuit current Ik3 (kA) 19.1

Ambient temperature (C) 30 1-phase-to-earth fault current Id (kA) 11.5

Conductor material Copper Switchboard B6

Single-core or multi-core cable Single Reference Linergy 800

Installation method F Rated current (A) 750

Number of layers 1 Circuit-breaker Q7

Phase conductor selected csa (mm2) 2 x 240 Load current (A) 255

Neutral conductor selected csa (mm2) 2 x 240 Type Compact

PE conductor selected csa (mm2) 1 x 120 Reference NSX400F

Voltage drop U (%) 0.122 Rated current (A) 400

3-phase short-circuit current Ik3 (kA) 21.5 Number of poles and protected poles 3P3dCourant de dfaut phase-terre Id (kA) 15.9 Tripping unit Micrologic 2.3

Circuit-breaker Q1 Overload trip Ir (A) 258

Load current (A) 860 Short-delay trip Im / Isd (A) 2576

Type Compact Cable C7

Reference NS1000N Length 5

Rated current (A) 1000 Maximum load current (A) 255

Number of poles and protected poles 4P4d Type of insulation PVC

Tripping unit Micrologic 5.0 Ambient temperature (C) 30

Overload trip Ir (A) 900 Conductor material Copper Short-delay trip Im / Isd (A) 9000 Single-core or multi-core cable Single

Tripping time tm (ms) 50 Installation method F

Switchboard B2 Phase conductor selected csa (mm2) 1 x 95

Reference Linergy 1250 Neutral conductor selected csa (mm2) -

Rated current (A) 1050 PE conductor selected csa (mm2) 1 x 50

Circuit breaker Q3 Cable voltage drop U (%) 0.14

Load current (A) 509 Total voltage drop U (%) 0.79

Type Compact 3-phase short-circuit current Ik3(kA) 18.0

Reference NSX630F 1-phase-to-earth fault current Id (kA) 10.0

Rated current (A) 630

Fig. G66: Partial results of calculation carried out with Ecodial software (Schneider Electric)

Dimensioning circuit C1


10/13/2013


4/6

The MV/LV 630 kVA transformer has a rated no-load voltage of 420 V. Circuit C1 must be suitable for a current of:

per phase

Two single-core PVC-insulated copper cables in parallel will be used for each phase.These cables will be laid on cable

trays according to method F.

Each conductor will therefore carry 433A. Figure G21a indicates that for 3 loaded conductors with PVC isolation, the

required c.s.a. is 240mm.

The resistance and the inductive reactance, for the two conductors in parallel, and for a length of 5 metres, are:

(cable resistance: 22.5 m.mm2/m)

X = 0,08 x 5 = 0,4 m (cable reactance: 0.08 m/m)


Circuit C3 supplies two 150kW loads with cos = 0.85, so the total load current is:

Two single-core PVC-insulated copper cables in parallel will be used for each phase. These cables will be laid on cable

trays according to method F.


required c.s.a. is 95mm2.

The resistance and the inductive reactance, for the two conductors in parallel, and for a length of 20 metres, are:


Circuit C7 supplies one 150kW load with cos = 0.85, so the total load current is:

One single-core PVC-insulated copper cable will be used for each phase. The cables will be laid on cable trays according

to method F.


required c.s.a. is 95mm2.

The resistance and the inductive reactance for a length of 20 metres is:

(cable resistance: 22.5 m.mm2/m)

(cable reactance: 0.08 m/m)

Calculation of short-circuit currents for the selection of circuit-breakers Q1, Q3, Q7 (see Fig. G67)


10/13/2013


5/6

Circuit components R (m) X (m) Z (m) Ikmax (kA)

Upstream MV network, 500MVA fault level (see Fig. G34) 0,035 0,351

Transformer 630kVA, 4% (see Fig. G35) 2.9 10.8

Cable C1 0.23 0.4

Sub-total 3.16 11.55 11.97 20.2

Cable C3 2.37 1.6

Sub-total 5.53 13.15 14.26 17

Cable C7 1.18 0.4

Sub-total 6.71 13.55 15.12 16

Fig. G67: Example of short-circuit current evaluation

The protective conductor

When using the adiabatic method, the minimum c.s.a. for the protective earth conductor (PE) can be calculated by the

formula given in Figure G58: For circuit C1, I = 20.2kA and k = 143.

t is the maximum operating time of the MV protection, e.g. 0.5s

This gives:

A single 120 mm2 conductor is therefore largely sufficient, provided that it also satisfies the requirements for indirect

contact protection (i.e. that its impedance is sufficiently low).

Generally, for circuits with phase conductor c.s.a. Sph 50 mm2, the PE conductor minimum c.s.a. will be Sph / 2. Then,

for circuit C3, the PE conductor will be 95mm2, and for circuit C7, the PE conductor will be 50mm2.

Protection against indirect-contact hazards

For circuit C3 of Figure G65, Figures F41 and F40, or the formula given page F25 may be used for a 3-phase 4-wire

circuit.

The maximum permitted length of the circuit is given by:

(The value in the denominator 630 x 11 is the maximum current level at which the instantaneous short-circuit magnetic tripof the 630 A circuit-breaker operates).

The length of 20 metres is therefore fully protected by instantaneous over-current devices.

Voltage drop

The voltage drop is calculated using the data given in Figure G28, for balanced three-phase circuits, motor power normal

service (cos = 0.8).

The results are summarized on figure G68:


10/13/2013


6/6

c.s.a.

C1 C3 C7

2 x 240mm 2 x 95mm 1 x 95mm

U per conductor

(V/A/km) see Fig. G28

0.21 0.42 0.42

Load current (A) 866 509 255

Length (m) 5 20 5

Voltage drop (V) 0.45 2.1 0.53

Voltage drop (%) 0.11 0.53 0.13

Fig. G68: Voltage drop introduced by the different cables

The total voltage drop at the end of cable C7 is then: 0.77%.

Retrieved from "http://www.electrical-installation.org/enwiki/Worked_example_of_cable_calculation"

Category: Chapter - Sizing and protection of conductors

This page was last modified on 13 August 2013, at 13:59.


Example of Cable Calculation

Documents

Transcript of Example of Cable Calculation