Current Rating Derating Factors

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Low voltage cable derating factors -
Australian and New Zealand conditions

The current rating of a cable is affected by the presence of certain external influences. Under such conditions the current ratings given from the standard AS/NZS 3008.1 shall be corrected by the application of an appropriate factor or factors. Where more than one derating factor applies these shall be multipled to obtain an overall derating factor.
 
You can determine derating factors by hand from the Standards or using software.

Cable Pro Web software has a simple-to-use Wizard for derating factors. Try Now for Free.

Installation conditions that avoid derating

Derating factors may be avoided per Section 3.5.2.2 of AS/NZS 3008.1 including:
(a) Mineral insulated (MIMS) cables without serving unless other types of cables are installed in close proximity or within the same wire enclosure.
(b) Limited length of grouping where length does not exceed and whichever is shorter:
(i) 1 m for conductor sizes < 300 mm2 aluminium or < 150 mm2 for copper; or
(ii) 3 m for conductor sizes 300 mm2 or larger aluminium and 150 mm2 or larger for copper; or
(iii) half the length of the cable.
(c) Cables operating at less than 35 % of their rated current capacity.

Derating factor tables

The derating factor Tables below are from both AS/NZS 3008.1.1 (Australian conditions) and AS/NZS 3008.1.2 (New Zealand conditions). The Tables from both standards are the same except for TABLE 27(1) and 27(2) which differ.

Disclaimer: No responsibility is assumed for the accuracy of the Tables below. Further, there are additional notes to these tables which have not been included.

In Air

Table 22:

Derating Factors for Bunched Circuits

Cable Types:

Installation Conditions:

Single-Core and Multicore

In Air or In Wiring Enclosures

123456789101112131415161718
Item No.Arrangement of cables (see Notes 1 and 2)Derating factors
Number of circuits
123456789101214161820 or more
1Bunched in air1.000.870.750.720.700.67------------------
2Bunched on a surface or enclosed1.000.800.700.650.600.570.540.520.500.480.450.430.410.390.38
3Single layer on wall or floorTouching1.000.850.790.750.730.720.720.710.700.700.700.700.700.700.70
4Spaced (see Notes 5 and 6)1.000.940.900.900.900.900.900.900.900.900.900.900.900.900.90
5Single layer under ceilingTouching0.950.810.720.680.660.640.630.620.610.610.610.610.610.610.61
6Spaced (see Notes 5 and 6)0.950.850.850.850.850.850.850.850.850.850.850.850.850.850.85
Table 23
Table 24
Table 27(1)
Table 27(1) - New Zealand

Buried - General

Table 27(2)
Table 27(2) - New Zealand
Table 29

Buried - Direct

Table 25(1)
Table 25(2)
Table 28(1)

Buried - Enclosures

Table 26(1)
Table 26(2)
Table 28(2)
To purchase standards please visit: https://www.saiglobal.com

Table 22:

Derating Factors for Bunched Circuits

Cable Types:

Installation Conditions:

Single-Core and Multicore

In Air or In Wiring Enclosures

123456789101112131415161718
Item No.Arrangement of cables (see Notes 1 and 2)Derating factors
Number of circuits
123456789101214161820 or more
1Bunched in air1.000.870.750.720.700.67------------------
2Bunched on a surface or enclosed1.000.800.700.650.600.570.540.520.500.480.450.430.410.390.38
3Single layer on wall or floorTouching1.000.850.790.750.730.720.720.710.700.700.700.700.700.700.70
4Spaced (see Notes 5 and 6)1.000.940.900.900.900.900.900.900.900.900.900.900.900.900.90
5Single layer under ceilingTouching0.950.810.720.680.660.640.630.620.610.610.610.610.610.610.61
6Spaced (see Notes 5 and 6)0.950.850.850.850.850.850.850.850.850.850.850.850.850.850.85

Table 23:

Cable Type:

Installation Conditions:

Single-Core

In trays, racks, cleats or other supports in air

12345678
Item No.InstallationNumber of tiers or rows of cable supportsArrangements of cables in a circuit (see Note 2)Derating factors
Number of circuits per tier or row
123
1Unperforated trays(See Note 6)12 or 3 cables in horizontal formation0.950.850.84
220.920.830.79
330.910.820.76
4Perforated trays12 or 3 cables in horizontal formation0.970.890.87
520.940.850.81
630.930.840.79
7Ladder supports, racks and cleats12 or 3 cables in horizontal formation1.000.950.94
820.950.900.88
930.950.890.85
10Vertical perforated trays12 or 3 cables in vertical formation0.940.85--
1120.920.83--
12Unperforated trays12 or 3 cables in horizontal formation0.950.950.94
1320.920.910.87
1430.910.900.85
15Perforated trays12 or 3 cables in horizontal formation0.970.970.96
1620.940.930.89
1730.930.920.86
18Ladder supports12 or 3 cables in horizontal formation1.001.001.00
1920.950.950.93
2030.950.940.90
21Vertical perforated12 or 3 cables in vertical formation1.000.910.89
222