12

It is essential to consider the specific system and installationconditions to be able to select the right cable.

The following criteria should be taken into account tochose the suitable cable.

Cable LayingDepending on the nature of the cable system (fixed ormobile) a rigid or flexible cable should be selected.The appropriate protection of a cable will bedetermined taking into account the mechanical stressand presence of chemical, oils or hydrocarbons.

Ambient TemperatureThe quality of the material used to manufacture a cableshall be determined according to the maximum andminimum temperatures to which the cable will be submitted.

Nature of ConductorsCopper or aluminium conductors will be used.

• for equal current rating aluminium cross-section =1.28 copper cross-section

• for equal ohmic resistance aluminium cross-section =1.65 copper cross-section

• for copper, sector shaped conductors are availableon request from 70 mm2 and above

Current Ratings The current rating of a cable is the capacity that causesa temperature rise equal to the admissible temperatureof the cable insulation at the surface of the core.

The current rating shall not exceed the admissiblecapacity of a cable taking into account the differentcorrection factors due to the ambient temperature andthe laying conditions. It is dependent of the constructionof the cables, the copper and the material used.

Underground: cables buried into ground (values aregiven according to NF C 15100):

• ground temperature: 20 °C• thermal resistivity: 1 K.m/W• Depth : 0,8 m

Above ground:• air temperature: 30 °C

For other conditions (soil resistivity and temperature),please refer to the below correction factors.

Calculations according to IEC 60364-5-52 can bemade by applying a correction factor to soil resistivity =2.5 K.m/W.

Low Voltage cables

A - Selection of cables

1 - Thermal resistivity of the ground

For buried cables, the current rating shown incharacteristics tables, are given for a soil on whichthermal resistivity of the ground is 1 K.m/W.

For ground on which thermal resistivity of the soil isdifferent, a factor K shall be applied to referencecurrents, according to this table.

B - Correction factors

Nature of soil KThermal resistivity

of the ground K. m/WSubmerged 0.40 1.25

Very humid 0.50 1.21

Humid 0.70 1.13

Normal 0.85 1.05

Dry 1.00 1.00

1.20 0.941.50 0.86

Very dry 2.00 0.762.50 0.703.00 0.65

2 - Ambient temperature

Permissible at 20 °C for underground installation and at30 °C for installation on cable trays.

For other ambient temperatures reference current ratingshall be multiplied by the correction factor X, given bythe formula:

θ : admissible conductor temperature in continuousduty in °C.T : ambient temperature in °C.t : temperature is20 °C for underground cables,30 °C for cables in free air.

Other correction factors for groups circuits and differentlaying conditions can be found in NFC 15 100 orIEC 60 364-5-52.

1 - Short-Circuit Current ( I.c.c. )Short-circuit current and breakdown time shall not causea too high temperature rise which depends on thenature of the insulation compound.

The admissible current density in short-circuit is given bythe formula:

D : admissible current density in amperes by mm2 ofcross-section.K : coefficient depending on the conductor nature, onthe initial temperature at overload moment and on theadmissible temperature at the end of overload.t : duration of short-circuit in seconds.I.c.c. : short-circuit current (A).

Ambient temperature

Correction factors X

Low-voltage cablesInsulation

Underground cables

Cables in free air

PVC XLPE or EPR

10°C 1.10 1.0715°C 1.05 1.0420°C 1.00 1.0025°C 0.95 0.9630°C 0.89 0.9335°C 0.84 0.89

10°C 1.22 1.1515°C 1.17 1.1220°C 1.12 1.0825°C 1.06 1.0430°C 1.00 1.0035°C 0.94 0.9640°C 0.87 0.9145°C 0.79 0.8750°C 0.71 0.8255°C 0.61 0.7660°C 0.50 0.7165°C — 0.6570°C — 0.5875°C — 0.50

Nature of insulation

Values of K

Coefficient KCopper cond. Alu. cond.

PVC 115 76XLPE 143 94EPR 143 —

X= θ — Tθ — t

D= Kt

13

C - Electrical details

2 - Voltage Drop (Du)We recommend voltage drops not to exceed:

• 3 % for lighting wire systems• 5 % for driving force wire systems• 10 % on starting time for motors

14

Oil & Gas IntroductionInsulation

PVC 70 160 163 115 81XLPE 90 250 202 143 101EPR 90 250 202 143 101

PVC 70 160 107 76 54XLPE 90 250 133 94 66

Initial

Copper conductors

Aluminium conductors

Final 0.5

Temperature (°C) Duration of overload (in seconds)

1 2

Temperature/Current density: D in Amperes/sq.mm

Formula• In D.C.

Du= 2 Iι Rc• In single-phase alterning current

Du= 2 Iι (Ra cos.ϕ + Lωsinϕ)• In three-phrase A.C.

Du= ÷3 Iι (Ra cos.ϕ + Lωsinϕ)

Du : voltage dropRc : conductor resistance in D.C. at operating

temperature (Ω/km)Ra : conductor resistance in A.C. at operating

temperature (Ω/km)L : core inductance (H/km)ω : pulsation equal to 2 π f (314 for f= 50 Hz)cos.ϕ : power factorI : Carried intensity in normal operating conditions (A)ι : simple length of cable (km)

Section ≥ I.c.c. (A)D (A/sq.mm)