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Major Factors Affecting Cable Ampacity Francisco de León, Senior Member, IEEE
Abstract —This paper presents a parametric study of the major
II. UNDERGROUND CABLE INSTALLATION
Several installation features were varied to factors affecting cable ampacity calculations. The current effect in the ampacity. In the Appendix the reader carrying capacity (or ampacity) of a cable depends on many of the installation properties and conditions. In this paper the data of cables and installations used to perform the effects on ampacity of conductor size, ambient temperature, studies. The ambient temperature temperature was always always bonding arrangement, duct size, soil thermal resistivity, target temperature has been set to 90 °C for a resistivity and size of backfill (or duct bank) and depth of installation for underground installations are presented. For calculations. The soil thermal resistivity is 1.0 except when indicated. All cases are balanced w cables air the effects on ampacity of the intensity of solar radiation, the spacing from the wall and the grouping of cables load factor. are analyzed. For riser pole installations the effect effect of the solar Conductor Caliber Caliber radiation, wind speed, ventilation and diameter of the duct are A. Varying Conductor shown.
— Ampacity. Cable Rating. Underground Cables. Cables in Air. Air. Cables in Riser Poles. IEC Standards. CYMCAP. Neher-McGrath. Index
Terms
I. INTRODUCTION
A
MPACITY (or current-carrying capacity) of a cable is greatly affected by the installation conditions and material properties. In this paper a parametric study of the major factors affecting ampacity is presented. All simulations were performed using the commercial ampacity program CYMCAP, which works in accordance to the IEC standards; see references [1] to [7]. The IEEE Standard 835-1994 835-1994 [8] gives very similar results to those of the IEC Standards for underground cables. Differences are more noticeable for cables in air. Both the IEC and IEEE Standards are based on the Neher-McGrath Neher-McGrath method published in 1957 1957 [9]. The reader is referred to [10] for a thorough review the theory of ampacity calculations, the historical developments and the differences between the two methods. For underground installations installations the effects on cable ampacity
The size of the cable has been varied from 250 MC MCM. Figure 1 shows the results for single-point a point bonding. 600 500
Single-Point Bonded
] 400 A [ y t i 300 c a p m 200 A
Two-Poi Bonded
100 0 0
250
500
750
1000
12
Conductor Size [MCM]
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Figure 1. Ampacity versus conductor size for two bonding types
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cable reduces in inverse proportion to the conductor area, for ac excitation the skin and proximity effects play an important role. The larger the cross sectional area of the conductor the larger the effects of the induced eddy currents in single-point bonded installations and the circulating currents in two-point bonded installations. Figure 2 shows that directly buried cables have a slightly higher ampacity, about 6%, than cables installed in PVC conduits. The reason is that the PVC has a higher thermal resistivity than the native soil.
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TABLE 1. VARIATION OF AMPACITY FOR TREFOILS WITH DIFFER
Bonding Arrangement Single-Point Two-Point
Cross Bonding
Equal Section Lengths AM = 1.5 / AN = 1.25 AM = 2.0 / AN = 1.5 AM = 3.0 / AN = 2.0
The same effects can be appreciated in fla installations. Figure 4 summarizes the ampacity re flat formation installation shown in the Appendix bonding arrangements were used and the distan cables was varied. 1000 Single P oint = Cross
800
800
Two P oint (trans
y t i 600 c a p m400 A
600 ] A [ y t i c400 a p m A
Two P oint (not transpo
200 You're Reading a Preview
1000 MCM
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B. Varying Soil Thermal Resistivity
The thermal resistivity of the native soil using 4-trefoils (for 500 and 1000 MCM) directly buried cables was varied from 0.4 to 4.0 [ °K-W/m]; this covers the conditions for most installations. The computed ampacities are presented in Figure 3. One can note that the ampacity reduces as the thermal resistivity of the soil increases and seems to follow a hyperbolic function.
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Sta nding Volt
Unlock full access with a free0trial. 0
200 500 MCM
Download With Free Trial
0 0
1
2
3
4
5
0.1
0.2 0.3 0.4 Distance between phases [m]
Figure 4. Ampacity versus distance for different bonding arra formation installation)
Soil Thermal Resistivity [°K-W/m]
Figure 3. Ampacity as a function of soil thermal resistivity
C. Varying Bonding and Transposition
Figure 1 shows that two-point bonded cables have a smaller ampacity than single-point bonded cables. This is due to the
The ampacity for single-point bonding and cross the highest and increases with the separation of p is due toSign a reduction in on thethis induced up to vote title heating betw While cross bonding cables is more expensive, Useful Not useful bonded cable installations produce standing volt ungrounded terminal. Those voltages increase separation (see the bottom curve in Figure 4).
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Appendix. When only one trefoil circuit is present the computed ampacity is 650 A. When a second trefoil is added the ampacity reduces to 575 A while adding a third trefoil reduces the ampacity to 512 A. When the last (fourth) circuit is added the ampacity becomes only 464 A; this is about 70% of the case with only one cable. Further reductions are expected as the number of cables heating each other increases. Frequently, there is the need to account for the heating (or cooling) induced from neighboring heat sources (sinks) such as steam or water pipes running parallel to the cable installation. It is not possible, however, to give rules of thumb or to perform parametric studies because the installation possibilities are infinite.
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1200 1000
] A [ 800 y t i c 600 a p m 400 A
Single-Point Bonding
Two-Point Bon
200 0 0
E. Varying the Conduit Size
The diameter of a PVC conduit buried in native soil was varied from a very tight fit to very large size; see Figure 5. The plot of Figure 6 shows that the ampacity increases slightly as the diameter of the conduit increases. For steel conduits the slope is even smaller than for PVC conduits.
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250
500
750
1000
1250
Conductor Size [MCM]
Figure 7. Ampacity as a function of conduit diameter
B. Varying the Solar Radiation Intensity
The effects of the variation of the intensi radiation on cable ampacity are shown in Figure appreciate, as expected, that the ampacity of the ca as the intensity of solar radiation increases. The several surfaces having different coefficient is also compared in the figure. As You're Readingabsorption a Preview absorption coefficient increases a larger ampaci obtained a given solar radiation intensity. Unlock full access with a freefor trial. The solar radiation intensity, for not shaded i depends on the geographical location of the Figure 5. Smallest versus largest conduit – 160 mm & 500 mm Download With Free Trial (latitude and altitude) and the day of the year and 1000 day. The surface absorption coefficient depe Steel material type and color of the cable's external 1000 MCM 800 surface exposed to the sun). ] A [ y t i c a p m A
600 1200
400
500 MCM PVC
200 0
1000 ] A [ y t i c
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described in the Appendix, was used to compute the ampacities.
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duct is equal to the minimum circumference that e trefoil formation. As the diameter of the ducts in ampacity reduces reaching a minimum and then slo
1200 1000
] A [ 800 y t i c 600 a p m400 A
200 0 Figure 9. Standardized arrangements for cables in air
0
TABLE 2. AMPACITY FOR CABLES IN AIR
Arrangement Number 1 2 3 4 7 8 9 10
Ampacity [A] 1033 838 714 772 772 947 910 544
0.1
0.2 0.3 0.4 Separation [m]
Figure 10. Ampacity as function of separation
1000 ] 800 A [ y600 t i c a p400 m A
You're Reading a Preview 200
Comparing the ampacity of cases 1 with 9 and 3 with 10 with a free 0 trial. Unlock full access one can see that cable installations near the wall show a 0 0.1 0.2 0.3 0.4 substantially smaller ampacity than those separated from the Separation [m] Download wall. For the single-phase the obtained reduction is 12%,With Free Trial while for the trefoil the reduction is 24%. One can also note Figure 11. Ampacity as function of separation from cases 4 and 7 that there is no influence when installing the cables vertically or horizontally. Grouping the cables has the effect, as expected, of reducing ampacity; compare cases Figure 13 shows the variation of ampacity with 1, 2 and 4. coefficient of solar absorption of the external su installation (cable duct). Sign up to or vote on thisOne titlenotices that th D. Groups of Cables reduces almost linearly with an increase of Useful Not useful The effect of the separation between cables for groups of coefficient of solar absorption. Figure 14 shows t cables was analyzed using the 1000 MCM cable (single-point of ampacity as a function of the intensity of sola The ampacity reduces in a quasi-linear fashion f bonded) described in the Appendix. Figure 10 shows the
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V. CONCLUSIONS
1400 1200
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A. Underground Cable Installations
]1000 A [ y t i 800 c a p 600 m A 400
Vented Ends
Not Vented
200 0 0
0.2
0.4
0.6
0.8
1
Surface Absorption Coefficient Figure 13. Ampacity as function of the surface absorption coefficient of solar radiation
1400 1200 ] 1000 A [ y 800 t i c a p 600 m A 400
Vented Ends
NotVented
200
The three major factors affecting ampacity in u cable installation are: cable caliber, soil thermal re bonding method. Doubling the conductor cross-se does not double the ampacity; see Figures 1 and thermal resistivity plays a very important role in th of an installation. Keeping all other conditions un large variation on the soil thermal resistivity ca ampacity in more than 50% (Figure 3). Depen particularities of the installation, bonding typ account for up 50% of the ampacity (Figure 4). B. Cables in Air
For cables in air the three major factors affe ampacity are: conductor size, the cable groupi distance to the wall. Doubling the conductor cro area does not double the ampacity, but the "reduc is smaller than that of underground cables. Amp sensitive to the bonding type and somehow depen intensity of solar radiation especially for large v absorption coefficient of solar radiation. Howeve is very much dependent on the distance from the wall and on cable groping; see table 2 and Figures C. Cables in Riser Poles
0 0
500
1000
You're Reading aThe Preview ampacity of cables in riser poles greatly dep
1500
2 Unlock
Intensity of Solar Radiation [W/m ] Figure 14. Ampacity as function of intensity of solar radiation
2000
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Figure 15 shows a plot of ampacity versus wind speed. The ampacity increases with an increase of wind speed. However, the ampacity increase is larger at the lower end. Thus increasing the wind speed form 0 to 5 m/s has a large effect than increasing it from 15 o 20 m/s. The length of the riser pole was varied from 1 to 20 meters and the ampacity did not show any significant variation (results are not shown). 800
diameter of the guard, the intensity of solar radiat surface coefficient of solar absorption (Figures 14). It is somehow dependent on the wind speed (
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[1]
[2]
[3]
[4]
VI. REFERENCES
Electric Cables – Calculation of the current rating – Part 1 equations (100% load factor) and calculation of losses General. IEC Standard 287-1-1 (1994-12). Electric Cables – Calculation of the current rating – Part 1 equations (100% load factor) and calculation of losses – Se formation eddy current loss factors for two circuits in flat Sign up to vote on this title 287-1-2 (1993-11). Electric Cables the current rating – P Useful– Calculation Not of useful resistance – Section 1: Calculation of the thermal resistance 287-2-1 (1994-12). Electric Cables – Calculation of the current rating – P
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VII. APPENDIX: CABLE AND INSTALLATION DATA Ambient temp = 25 C
Figure 16 describes the 15 kV, 1000 MCM, concentric neutral cable used in this paper. The cables with different calibers used in the parametric study have the same layers with different sizes; see Table 3. Figure 17 illustrates the installation used in this paper with four trefoil formations installed in a 2 ×2 duct bank. The directly buried case is shown in Figure 18 and the flat formation is shown in Figure 19.
Native Soil = 1.00 C-m/W
-6 Figure 16. Construction and dimensions of the concentric neutral cable used for most simulation
-4
-2
0
2
Figure 18. Four directly buried trefoils (distances in feet)
Ambient temp = 25 C 2
Voltage = 15.0 kV Cond. area = 0.7854 inch (1000 KCMIL)
Figure 16. Construction and dimensions of the concentric neutral cable used for most simulations
TABLE 3. CONDUCTOR SIZES
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External Diameter of Layer [inch] Unlock full access with a free trial. Native Soil = 1.00 C-m/W
Download With Free -3 Trial -2
-1
0
1
2
Figure 19. Flat formation for the parametric study of cable separ in feet)
Ambient temp = 25 C
0 1 2
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VIII. BIOGRAPHY
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Francisco de León (S’86, M’92 born in Mexico City in 1959. H
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