AS/NZS 3008.1.1:2017 — Understanding the Cable Sizing Tables (Appendix A vs B)
A deep dive into AS/NZS 3008.1.1:2017 cable sizing tables — installation methods (Table 3), current ratings (Tables 13-14), derating factors (Tables 22, 25, 27, 28), and the critical difference between Appendix A and Appendix B approaches.
AS/NZS 3008.1.1:2017 is the most table-heavy electrical standard I work with. The current rating tables, the derating factor tables, the installation method definitions — they are comprehensive, but they are also easy to misapply if you do not understand how the pieces fit together.
This article walks through the key tables, explains the Appendix A versus Appendix B distinction that most engineers get wrong, and finishes with a complete worked example showing how derating factors stack. If you size cables to AS/NZS 3008, this is the reference you need.
Table 3: Installation Methods
Before you can look up a current rating, you must identify the installation method. Table 3 of AS/NZS 3008.1.1:2017 defines 29 installation methods, each describing a specific physical arrangement of the cable relative to its surroundings.
AS/NZS 3008.1.1:2017, Table 3 — Methods of cable installationThe installation methods that matter most for industrial and commercial work:
| Method | Description | Typical Application |
|---|---|---|
| 1 | Enclosed in thermal insulation | Cables in insulated walls — worst case |
| 3 | Enclosed in conduit in air | Surface-mounted conduit on walls |
| 4 | Enclosed in conduit in masonry | Conduit embedded in concrete or brick |
| 7 | Cable tray — unperforated (touching) | Cables laid flat on solid tray |
| 11 | Cable tray — perforated (spaced) | Cables spaced on ventilated tray |
| 13 | Cable ladder — touching | Cables touching on ladder rack |
| 14 | Cable ladder — spaced | Cables spaced on ladder rack |
| 22 | Free air — multicore, spaced | Suspended multicore cables |
| 25 | Underground — direct buried | Direct burial in soil |
| 26 | Underground — in conduit | Cables pulled through buried conduit |
The installation method determines which current rating table to use and which column within that table applies. Getting the installation method wrong cascades through the entire calculation — every subsequent number will be incorrect.
Method 11 vs Method 13 — A Common Mistake
Method 11 (perforated cable tray, cables spaced) gives substantially higher current ratings than Method 13 (cable ladder, cables touching). Engineers sometimes select Method 11 because the cable is on a perforated tray, but if the cables are touching rather than spaced by at least one cable diameter, the correct method is different. Check the actual installation, not just the tray type.
Tables 13 and 14: Current Ratings
Table 13 provides current-carrying capacities for multicore cables. Table 14 provides capacities for single-core cables. Both tables are indexed by cable size, insulation type (PVC or XLPE/EPR), conductor material (copper or aluminium), and installation method.
AS/NZS 3008.1.1:2017, Table 13 — Sustained current-carrying capacity — multicore cables AS/NZS 3008.1.1:2017, Table 14 — Sustained current-carrying capacity — single-core cablesKey points about these tables:
1. The reference conditions. All current ratings in Tables 13 and 14 assume:
- Ambient air temperature of 40 degrees C (NOT 30 degrees C — this is an Australian standard, designed for Australian conditions)
- Ground temperature of 25 degrees C for buried cables
- Soil thermal resistivity of 1.2 K.m/W for buried cables
- Single circuit (no grouping)
- No thermal insulation contact
2. PVC vs XLPE columns. XLPE (cross-linked polyethylene) and EPR (ethylene propylene rubber) cables have higher current ratings than PVC for the same size because they can operate at higher conductor temperatures — 90 degrees C versus 75 degrees C for PVC. For a 95 mm squared copper cable on a perforated tray (Method 11), the difference is significant:
- PVC insulated: 257 A
- XLPE insulated: 330 A
3. Sustained vs short-time ratings. Tables 13 and 14 give sustained (continuous) ratings. For loads that are genuinely intermittent, Clause 3.3 and associated tables allow higher currents — but only if the duty cycle is well-defined and documented. In practice, most industrial loads should use sustained ratings. The exceptions are motor starting, welding, and similar genuinely cyclic loads.
The Derating Factor Cascade
This is where AS/NZS 3008 calculations become either precise engineering or hand-waving guesswork, depending on whether the engineer applies the derating factors correctly.
The effective current-carrying capacity of a cable is the table value multiplied by all applicable derating factors:
I_derated = I_table x f_ambient x f_grouping x f_soil x f_depth x f_other
Each factor accounts for a specific departure from the reference conditions assumed in Tables 13 and 14.
Table 25: Ambient Temperature Derating
AS/NZS 3008.1.1:2017, Table 25 — Rating factors for ambient air temperatureThe current rating tables assume 40 degrees C ambient for cables in air. If the actual ambient temperature is different, Table 25 provides correction factors.
| Ambient Temp | PVC (75 degrees C max) | XLPE (90 degrees C max) |
|---|---|---|
| 25 degrees C | 1.17 | 1.12 |
| 30 degrees C | 1.10 | 1.08 |
| 35 degrees C | 1.05 | 1.04 |
| 40 degrees C | 1.00 | 1.00 |
| 45 degrees C | 0.94 | 0.96 |
| 50 degrees C | 0.87 | 0.91 |
| 55 degrees C | 0.79 | 0.87 |
| 60 degrees C | 0.71 | 0.82 |
Note that the reference temperature is 40 degrees C — factor = 1.00. Below 40 degrees, the factor is greater than 1.0 (the cable can carry MORE current). This is the opposite of BS 7671 and IEC 60364, which use a 30 degrees C reference. Engineers switching between standards frequently make errors here.
Table 22: Grouping Derating
AS/NZS 3008.1.1:2017, Table 22 — Rating factors for groups of more than one circuit or multicore cableWhen multiple cables or circuits are installed in proximity, they heat each other. Table 22 provides derating factors based on the number of circuits and the installation arrangement.
| Number of Circuits | Single Layer on Tray (Touching) | Single Layer on Tray (Spaced) | Trefoil |
|---|---|---|---|
| 1 | 1.00 | 1.00 | 1.00 |
| 2 | 0.80 | 0.88 | 0.82 |
| 3 | 0.70 | 0.82 | 0.73 |
| 4 | 0.65 | 0.77 | 0.68 |
| 6 | 0.57 | 0.72 | 0.61 |
| 9 | 0.50 | 0.66 | 0.54 |
The grouping factor is the single largest derating factor in most industrial installations. Six circuits touching on a cable tray reduces each cable's capacity to 57% of the single-circuit value. That means a cable rated at 330 A individually is only good for 188 A when grouped with five other circuits.
Grouping Is the Factor Most Often Underestimated
In industrial cable trays carrying dozens of circuits, the grouping derating alone can halve the cable capacity. I have audited installations where the designer used single-circuit ratings for cables on a tray with 12 other circuits. The cables were operating at 170% of their derated capacity. They survived only because the loads were less than the design current — a matter of luck, not engineering.
Table 27: Soil Thermal Resistivity
AS/NZS 3008.1.1:2017, Table 27 — Rating factors for soil thermal resistivityFor buried cables, the thermal resistivity of the surrounding soil determines how effectively heat dissipates. The reference value is 1.2 K.m/W. Dry sandy soil in outback Australia can be 2.0 K.m/W or higher. Moist clay is around 0.7 K.m/W.
| Soil Resistivity (K.m/W) | Correction Factor |
|---|---|
| 0.7 | 1.10 |
| 1.0 | 1.05 |
| 1.2 | 1.00 |
| 1.5 | 0.94 |
| 2.0 | 0.85 |
| 2.5 | 0.78 |
| 3.0 | 0.73 |
Table 28: Depth of Burial
AS/NZS 3008.1.1:2017, Table 28 — Rating factors for depth of layingDeeper burial means less heat dissipation. The reference depth is 0.5 m. Cables buried deeper — as required by some site specifications or road crossing requirements — must be derated.
| Depth (m) | Correction Factor |
|---|---|
| 0.5 | 1.00 |
| 0.8 | 0.97 |
| 1.0 | 0.95 |
| 1.25 | 0.93 |
| 1.5 | 0.91 |
| 2.0 | 0.87 |
Appendix A vs Appendix B: The Distinction Most Engineers Miss
AS/NZS 3008.1.1:2017 contains two appendices that provide alternative approaches to cable sizing, and the difference between them is fundamental.
AS/NZS 3008.1.1:2017, Appendix A — Cable current-carrying capacity — installed conditions AS/NZS 3008.1.1:2017, Appendix B — Cable current-carrying capacity — uninstalled conditionsAppendix A provides current ratings for cables under installed conditions — the ratings account for the specific installation method, including the thermal effects of the cable support system (tray, ladder, conduit, etc.). This is the approach used in Tables 13 and 14 in the body of the standard.
Appendix B provides current ratings for cables under uninstalled conditions — the ratings represent the cable in isolation, independent of how it will be installed. These ratings are derived from IEC 60287 thermal calculations for the cable alone.
When to Use Which
-
Appendix A (installed conditions): Use for all normal design work. The tables in the body of the standard (Tables 13, 14) are Appendix A values. They already account for the thermal effects of the installation method.
-
Appendix B (uninstalled conditions): Use when the installation method does not match any of the 29 methods in Table 3, or when you need to perform a first-principles thermal analysis. Appendix B ratings are also used by cable manufacturers as the basis for their published current ratings.
The critical mistake: some engineers use Appendix B (uninstalled) ratings for installed cable sizing because they are higher than Appendix A ratings. This is incorrect and unsafe — the Appendix B ratings do not account for the thermal effects of the cable's actual installation environment.
Appendix B Ratings Are Always Higher
Appendix B current ratings are higher than Appendix A ratings for the same cable because they do not include the thermal impedance of the installation method. A cable rated at 350 A under Appendix B (uninstalled, free air) might only be 310 A under Appendix A, Method 14 (cable ladder, spaced), and 270 A under Method 13 (cable ladder, touching). Using the 350 A figure for a cable on a ladder is non-compliant and potentially dangerous.
Worked Example: Complete Derating Cascade
Let me walk through a real calculation — the kind I do routinely on mine site projects.
Given:
- Cable: 95 mm squared, 4-core, copper, XLPE insulated
- Installation: perforated cable tray, cables touching (Method 11 with touching — treated as Method 13)
- Number of circuits on tray: 6 (including this one)
- Ambient temperature: 50 degrees C (processing plant in tropical Australia)
- No burial (in air), no thermal insulation
Step 1: Base current rating from Table 13
95 mm squared, 4-core Cu, XLPE, Method 13 (cable ladder, touching):
I_table = 279 A
AS/NZS 3008.1.1:2017, Table 13, Column for Method 13, XLPE — Base current ratingStep 2: Ambient temperature derating (Table 25)
Ambient = 50 degrees C, XLPE (90 degrees C max conductor temp):
f_ambient = 0.91
AS/NZS 3008.1.1:2017, Table 25 — 50 degrees C ambient, XLPEStep 3: Grouping derating (Table 22)
6 circuits, single layer, touching:
f_grouping = 0.57
AS/NZS 3008.1.1:2017, Table 22 — 6 circuits, touching, single layerStep 4: Calculate derated current
I_derated = I_table x f_ambient x f_grouping
I_derated = 279 x 0.91 x 0.57
I_derated = 279 x 0.5187
I_derated = 144.7 A
Step 5: Verify adequacy
The cable that is rated at 279 A in isolation, on a single circuit at 40 degrees C, is only good for 145 A in the actual installation conditions — a reduction of 48%.
If the design current is 150 A, this cable is undersized. You need to go up to 120 mm squared or reduce the number of circuits on the tray.
The Derating Stack Can Cut Capacity in Half
In this example, two derating factors (ambient temperature and grouping) reduced the cable capacity from 279 A to 145 A — a 48% reduction. For buried cables with additional soil resistivity and depth factors, the reduction can exceed 60%. Always calculate the fully derated capacity before selecting the cable. Never use the base table value as the design capacity for grouped or high-temperature installations.
Common Mistakes to Avoid
1. Using 30 degrees C reference ambient. AS/NZS 3008 uses 40 degrees C, not 30. An engineer accustomed to BS 7671 or IEC 60364 who applies a 40 degrees C derating factor from the wrong standard's tables will double-penalise the cable.
2. Counting circuits incorrectly for grouping. Table 22 counts circuits, not cables. A three-phase circuit using three single-core cables counts as ONE circuit. Two three-phase circuits on the same tray = 2 circuits for grouping purposes, requiring 6 single-core cables but a grouping factor for 2 circuits.
3. Mixing Appendix A and B values. Use Appendix A (installed conditions) tables for design. Only use Appendix B when no Appendix A method matches.
4. Ignoring the tray fill limit. Even with grouping derating applied, AS/NZS 3008 requires that cables on trays do not exceed the tray fill percentage limits. Grouping derating and tray fill are separate requirements — satisfying one does not eliminate the other.
5. Forgetting the 0.8 factor for neutral with harmonics. Clause 3.5.2 requires that when the neutral conductor carries significant third-harmonic currents (common with LED lighting and VFD loads), the neutral current must be considered as a separate heat source. This effectively adds another derating factor that many engineers overlook.
AS/NZS 3008.1.1:2017 is a thorough standard. Used correctly, it produces safe, cost-effective cable installations. Used carelessly — wrong table, wrong method, missing derating factor — it produces cables that overheat, insulation that degrades prematurely, and installations that fail audits. Know the tables. Apply them correctly. Document the calculation.
Related Resources
- Cable Sizing: The Complete Guide — Methodology overview across all supported standards
- Cable Grouping Derating: The Factor That Catches Everyone — Why grouping is the most underestimated derating factor
- Cable Operating Temperature vs Maximum Rating — Understanding what conductor temperature means
Try the Cable Sizing Calculator
Free online tool — no signup required
Try the Cable Sizing Calculator
Free online tool — no signup required
Try the Voltage Drop Calculator
Free online tool — no signup required
Lead Electrical & Instrumentation Engineer
18+ years of experience in electrical engineering at large-scale mining operations. Specializing in power systems design, cable sizing, and protection coordination across BS 7671, IEC 60364, NEC, and AS/NZS standards.
Related Articles
Your First Cable Sizing Calculation: A Tutorial for Junior Engineers
Step-by-step cable sizing tutorial for junior engineers. Walk through a real 22kW motor circuit from design current to voltage drop verification, with every number shown and every step explained.
Neutral Sizing: The Hidden Third Harmonic Problem Nobody Calculates
In balanced 3-phase systems, engineers assume neutral current is zero. With LED lighting and SMPS loads, 3rd harmonic currents ADD in the neutral — reaching 173% of phase current. Here's the calculation most designs miss.
Short Circuit Ratings: The Cable Nobody Checks (Until It Fails)
Most engineers size cables for ampacity and voltage drop. The adiabatic equation (k²S² ≥ I²t) is often forgotten. Here's a real scenario where a properly-sized 4mm² cable fails at 16kA — and how to check yours.