JB/T 10181.4-2000 Calculation of cable current carrying capacity Part 2: Thermal resistance Section 2: Calculation of current carrying capacity reduction factor for cable groups in free air not exposed to direct sunlight
Some standard content:
1 Scope
Machinery Industry Standard of the People's Republic of China
Calculation of the current rating of electric cables Part 2: Thermal resistance
Section 2: A method for calculating reduction factors for groups of cables in free air. protected from solar radiation JB/T10181.4-2000
idt IEC 60287-2-2:1995
This standard applies to any type of cable or cable group laid in a plane, as long as all the cables have the same diameter and the same loss. This standard proposes a calculation method for the reduction of the allowable current carrying capacity when cables are laid adjacent to each other, which is limited to several cases: a) a maximum of 9 cables when laid in a square, see Figure 1; b) a maximum of 6 circuits consisting of three cables arranged in a triangle, including three circuits placed in parallel or two circuits arranged in double layers, see Figure 2.
Caution should be taken when the air flow around the cable is restricted by adjacent objects. Note: Further work is to expand and refine the data and include the influence of dielectric loss. Calculation methods are proposed for the following cases:
A single cable or a circuit laid separately, the current carrying capacity reduction factor of the cable group can be derived from the same type of cable, see 4.1. In the absence of current carrying capacity data, the data provided is used to calculate the allowable current carrying capacity of the cable group using the formulas of JB/T10181.1 and JB/T10181.3, see 4.2.
In order to avoid the reduction of the allowable current carrying capacity, suitable spacing between cables is proposed, see Chapter 5. 2 Reference Standards
The provisions contained in the following standards constitute the provisions of this standard through reference in this standard. When the standard is published, the versions shown are valid. All standards will be revised, and parties using this standard should explore the possibility of using the latest versions of the following standards. JB/T10181.1-2000 Calculation of cable current carrying capacity Part 1: Current carrying capacity formula (100% load factor) and loss calculation Section 1: General provisions
JB/T10181.3-2000 Calculation of cable current carrying capacity Part 2: Thermal resistance Section 1: Calculation of thermal resistance
3 Symbols
The symbols used in this standard are as follows:
Outer diameter of a multi-core cable or a single cable laid in a triangle Approved by the State Machinery Industry Bureau on 2000-04-24 mm
Implementation on 2000-10-01
Reduction number of current carrying capacity of cable group
Cable group current carrying capacity of the hottest cable
JB/T10181.4-2000
When the separation number is set, the current carrying capacity of a single cable or a loop is set. When the separation number is set, the external thermal resistance of a cable is used for calculation I Use the external thermal resistance separation number of the hottest cable in a cable group to set a multi-core cable or a single-core cable arranged in a triangle. When the current carrying capacity is I, the power loss is
The gap between adjacent cables in a group of cables
K·m/W
K·m/W
(Note: This is the distance between the cable surfaces, not the distance between the cable axes as in JB/T10181.1 and JB/T10181.3). The surface heat dissipation coefficient in the air of a single multi-core cable or a single-core cable arranged in a star-triangle is W/m*ks.4
The heat dissipation coefficient of the hottest cable in the cable group
The surface temperature rise factor in free air of a single cable or a single-core cable arranged in a triangle with separation number set is =
△Calculate I, the ambient temperature used is
△0. Conductor temperature used in calculation of 1I,4 Method
Cable surface temperature rise
Conductor temperature rise
4.1 Reduction factor applies to known current carrying capacity W/m*k$4
When the permissible current carrying capacity of a separate cable or loop is known and the reduction factor for a cable group is to be calculated, the reduction factor for the current carrying capacity of the hottest cable in the cable group is:
Then the current carrying capacity of the hottest cable is given by: I=Fg
The surface temperature rise factor k is calculated by:
Note: When calculating I, two parameters W and T, can be obtained, which effectively allows k, to be obtained at the same time as calculating 1,. The value of (T/T,) can be obtained from the ratio of (h/h) by substitution (TT)n+=(h/h)
The substitution starts with (TaJT)=(hlhg). 1- k,
[(Tag/Tat),
Note: The above equation converges quickly, and it is usually sufficient to use (T/T), (hh) to estimate. 25
In addition, when (h/hg) <1.4, it is sufficient to replace (Tg/T,,) with (h/h) in formula (1). For multi-core cable groups and single-core cable groups laid in a triangle, Table 1 gives the ratio of (h,/h.) in Figures 3 to 5. 76
(1)
JB/T10181.4-2000
Note: For cable groups laid in other ways, the value of (hh,) should be determined by experiment. 4.2 Cases where the current carrying capacity needs to be calculated
The current carrying capacity of the hottest cable in the cable group should be calculated using the formula for cables in free air given in JB/T10181.3, but h is required. Replace the heat dissipation coefficient h in JB/T10181.3. The heat dissipation coefficient h, value of the cable group in Table 1 and Figures 3 to 5 is given by the following formula: hg
wherein the parameter h value for a multi-core cable laid separately or a single-core cable arranged in a triangle is given in JB/T10181.3, and the ratio (h,/h,) is found in Table 1 or Figures 3 to 5 of this standard. 4.3 Cable groups with more than one plane layer
In the case where cables are laid in both horizontal and vertical directions, in order to ignore the adjacent thermal effect of side-to-side, the current carrying capacity reduction factor and current carrying capacity of the hottest cable should be calculated by using the corresponding value of the vertical gap (h,lh,) and ensuring that the horizontal gap e between cables is not less than the corresponding value in Table 1.
5 Spacing values to avoid reduction in current carrying capacity
For cable groups of various laying methods, in order to avoid reducing the current carrying capacity of individual cables or circuits installed separately, the minimum spacing between the outer surfaces of adjacent cables is given in the second column of Table 1. When choosing the minimum value, it is taken into account that it is impossible to maintain this spacing exactly in practical engineering. Suitable brackets should be provided to ensure the required spacing.
If it is not possible to maintain a minimum spacing greater than or equal to the corresponding minimum spacing value in the second column of Table 1 for the entire cable, one of the methods in Chapter 6 should be used.
6 Method for deriving the reduction factor of the current carrying capacity of the cable group If it is not possible to maintain an appropriate value of not less than the value given in the second column of Table 1 for the entire cable, the reduction factor should be determined by the following method: For horizontal spacing, assuming that the cables are in contact with each other or with a vertical surface, use one of the methods given in Chapter 4 to calculate the reduction factor using the corresponding (h/hg) value given in the fourth column of Table 1. For vertical spacings of two pairs, the current carrying capacity reduction factor due to cable aggregation shall be determined according to the required spacing: a) When the spacing is less than the corresponding value given in the second column of Table 1 but can be maintained not less than the minimum value in the third column of Table 1, the reduction factor is calculated using one of the methods in Chapter 4, and the (h,/h) value used shall be obtained from the corresponding formula in the fourth column of Table 1 or from one of the curves in Figures 3 to 5:
b) When the spacing cannot be maintained not less than the minimum value given in the third column of Table 1, the cables shall be assumed to be in contact with each other. The reduction factor is calculated using one of the methods in Chapter 4 using the corresponding values of (h,h.) given in the fourth column of Table 1. Note: The formula in Table 1 and the curves in Figures 3 to 5 are only valid for the spacing range indicated in the note to the table, and extrapolation is not allowed. 77
Parallel:
Two multi-core
Three multi-core
Two sets of triangles
Arrange single-core cables
Three sets of triangles
Arrange single-core cables
JB/T10181.4-2000
Table 1 Data for calculating the reduction factor of the current carrying capacity of cable groups Cable arrangement
One on top of the other:
Two multi-cores
Three multi-cores
Two sets of triangles
Arrange single-core cables
Close to the vertical plane
Or the horizontal plane below the cable
Ignore the adjacent thermal effect
If (efD,) ≥
The adjacent thermal effect cannot be ignored
If (elD,)<www.bzxz.net
The h,h, formula in the fourth column of this table and the average value of the curve (hh.) in Figures 3-5 cannot be used\), 2)
1.085(e/D,)-0.128 or
1.19(e/D,)-0.135 or
1.106(e/D,)-0.078 or
1)e/D,<0.5 or greater than the corresponding value in the second column, 2) Applicable to the average h,/h of cables with a diameter of 13~76mm. For a specific cable diameter of a multi-core cable, whether it is extrapolated inside or outside this range, a more accurate value can be calculated according to Table 5 of JB/T10181.3-2000. a)
Figure 1 Typical triangular arrangement of multi-core cables a), b), c), d), e)e)) is the maximum number of cable groups to which the data applies 78
JB/T10181.4-2000
Figure 2 Typical triangular arrangement a), b), c)c) is the maximum number of cable groups to which the data applies o
Values of two cables on a vertical plane (h/h,) Figure 3
Figure 4 Values of three cables on a vertical plane (h/h,) c)
JB/T10181.4-2000
Figure 5 Values of two triangularly arranged cable groups on a vertical plane (h/h,) 80
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