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.
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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
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Item No. | Arrangement of cables (see Notes 1 and 2) | Derating factors | |||||||||||||||
| Number of circuits | |||||||||||||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 12 | 14 | 16 | 18 | 20 or more | |||
| 1 | Bunched in air | 1.00 | 0.87 | 0.75 | 0.72 | 0.70 | 0.67 | -- | -- | -- | -- | -- | -- | -- | -- | -- | |
| 2 | Bunched on a surface or enclosed | 1.00 | 0.80 | 0.70 | 0.65 | 0.60 | 0.57 | 0.54 | 0.52 | 0.50 | 0.48 | 0.45 | 0.43 | 0.41 | 0.39 | 0.38 | |
| 3 | Single layer on wall or floor | Touching | 1.00 | 0.85 | 0.79 | 0.75 | 0.73 | 0.72 | 0.72 | 0.71 | 0.70 | 0.70 | 0.70 | 0.70 | 0.70 | 0.70 | 0.70 |
| 4 | Spaced (see Notes 5 and 6) | 1.00 | 0.94 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | |
| 5 | Single layer under ceiling | Touching | 0.95 | 0.81 | 0.72 | 0.68 | 0.66 | 0.64 | 0.63 | 0.62 | 0.61 | 0.61 | 0.61 | 0.61 | 0.61 | 0.61 | 0.61 |
| 6 | Spaced (see Notes 5 and 6) | 0.95 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | |
Buried - General
Buried - Direct
Buried - Enclosures
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Table 22:
Derating Factors for Bunched Circuits
Cable Types:
Installation Conditions:
Single-Core and Multicore
In Air or In Wiring Enclosures
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Item No. | Arrangement of cables (see Notes 1 and 2) | Derating factors | |||||||||||||||
| Number of circuits | |||||||||||||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 12 | 14 | 16 | 18 | 20 or more | |||
| 1 | Bunched in air | 1.00 | 0.87 | 0.75 | 0.72 | 0.70 | 0.67 | -- | -- | -- | -- | -- | -- | -- | -- | -- | |
| 2 | Bunched on a surface or enclosed | 1.00 | 0.80 | 0.70 | 0.65 | 0.60 | 0.57 | 0.54 | 0.52 | 0.50 | 0.48 | 0.45 | 0.43 | 0.41 | 0.39 | 0.38 | |
| 3 | Single layer on wall or floor | Touching | 1.00 | 0.85 | 0.79 | 0.75 | 0.73 | 0.72 | 0.72 | 0.71 | 0.70 | 0.70 | 0.70 | 0.70 | 0.70 | 0.70 | 0.70 |
| 4 | Spaced (see Notes 5 and 6) | 1.00 | 0.94 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | 0.90 | |
| 5 | Single layer under ceiling | Touching | 0.95 | 0.81 | 0.72 | 0.68 | 0.66 | 0.64 | 0.63 | 0.62 | 0.61 | 0.61 | 0.61 | 0.61 | 0.61 | 0.61 | 0.61 |
| 6 | Spaced (see Notes 5 and 6) | 0.95 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | |
Table 23:
Cable Type:
Installation Conditions:
Single-Core
In trays, racks, cleats or other supports in air
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|
| Item No. | Installation | Number of tiers or rows of cable supports | Arrangements of cables in a circuit (see Note 2) | Derating factors | |||
| Number of circuits per tier or row | |||||||
| 1 | 2 | 3 | |||||
| 1 | Unperforated trays | (See Note 6) | 1 | 2 or 3 cables in horizontal formation | 0.95 | 0.85 | 0.84 |
| 2 | 2 | 0.92 | 0.83 | 0.79 | |||
| 3 | 3 | 0.91 | 0.82 | 0.76 | |||
| 4 | Perforated trays | 1 | 2 or 3 cables in horizontal formation | 0.97 | 0.89 | 0.87 | |
| 5 | 2 | 0.94 | 0.85 | 0.81 | |||
| 6 | 3 | 0.93 | 0.84 | 0.79 | |||
| 7 | Ladder supports, racks and cleats | 1 | 2 or 3 cables in horizontal formation | 1.00 | 0.95 | 0.94 | |
| 8 | 2 | 0.95 | 0.90 | 0.88 | |||
| 9 | 3 | 0.95 | 0.89 | 0.85 | |||
| 10 | Vertical perforated trays | 1 | 2 or 3 cables in vertical formation | 0.94 | 0.85 | -- | |
| 11 | 2 | 0.92 | 0.83 | -- | |||
| 12 | Unperforated trays | 1 | 2 or 3 cables in horizontal formation | 0.95 | 0.95 | 0.94 | |
| 13 | 2 | 0.92 | 0.91 | 0.87 | |||
| 14 | 3 | 0.91 | 0.90 | 0.85 | |||
| 15 | Perforated trays | 1 | 2 or 3 cables in horizontal formation | 0.97 | 0.97 | 0.96 | |
| 16 | 2 | 0.94 | 0.93 | 0.89 | |||
| 17 | 3 | 0.93 | 0.92 | 0.86 | |||
| 18 | Ladder supports | 1 | 2 or 3 cables in horizontal formation | 1.00 | 1.00 | 1.00 | |
| 19 | 2 | 0.95 | 0.95 | 0.93 | |||
| 20 | 3 | 0.95 | 0.94 | 0.90 | |||
| 21 | Vertical perforated | 1 | 2 or 3 cables in vertical formation | 1.00 | 0.91 | 0.89 | |
| 22 | 2 | ||||||
