Cable Ampacity Chart: IEC 60364-5-52 B.52.4 vs B.52.5

01The short answer: ampacity does not scale with cross-section

Most sizing mistakes on low-voltage power circuits come from one intuition: that a cable twice as thick carries twice the current. It does not. Across IEC 60364-5-52 Table B.52.5, doubling the cross-section raises the rating by roughly 50 % to 56 % — never 100 %.

Table 1 derives that exponent directly from the standard's own published values. Every pair below uses two sizes that appear in Table B.52.5, so you can check each row yourself rather than taking our word for it.

Exponent = ln(current ratio) ÷ ln(area ratio). Source values: IEC 60364-5-52 Table B.52.5, three loaded conductors, copper, installation Method C (clipped direct), 30 °C air.

Why it happens: the thermal path, not the copper

The reason is visible in the IEC 60287-1-1 rating equation. In its steady-state form for a buried or air-installed cable:

I = √[ ( Δθ − Wd(0.5 T1 + n(T2+T3+T4)) ) ÷ ( R T1 + n R(1+λ1)T2 + n R(1+λ1+λ2)(T3+T4) ) ]

Every term that matters here works against you as the cable grows. R, the conductor resistance, falls roughly in proportion to cross-section, so I rises as about √A. But T1, the thermal resistance between conductor and sheath, has a logarithm in it — T1 = (ρT/2π) G — so it barely improves as the conductor gets fatter. The heat has to travel through more insulation, not less.

What saves you slightly is T4, the resistance from the cable surface to the surrounding medium. A bigger cable has a bigger surface, so it does shed heat better. That is why the observed exponent, 0.62, sits a little above the √A floor of 0.5 rather than below it.

02The base tables: IEC 60364-5-52 Annex B

Annex B of IEC 60364-5-52 is the source most of the world sizes from. The two tables you will use most are Table B.52.4 (PVC insulation, 70 °C conductor) and Table B.52.5 (XLPE or EPR insulation, 90 °C conductor), both for three loaded copper or aluminium conductors. Table 2 below reproduces the copper columns.

Source: IEC 60364-5-52:2009+AMD1:2024 CSV, Annex B, Tables B.52.4 and B.52.5, three loaded conductors, copper. Reference conditions: 30 °C ambient air, 20 °C ground, soil thermal resistivity 2.5 K·m/W, one circuit. Method B1 = multicore cable in conduit on a wall; Method C = clipped direct; Method D2 = directly buried. No grouping or ambient correction applied.

The reference conditions you must not forget

Every number in Table 2 is conditional. Five conditions sit behind them, and if your site differs on any one you must correct:

• 30 °C ambient air for methods A1–C, E–G.
• 20 °C ground temperature for methods D1 and D2.
• Soil thermal resistivity 2.5 K·m/W — a reference, not a typical value.
• One circuit, i.e. no mutual heating from neighbours.
• Three loaded conductors for these two tables. Tables B.52.2 and B.52.3 cover two loaded conductors and give higher values; using them for a three-phase circuit is a common and unsafe shortcut.

Method D1 (in ducts in the ground) is deliberately absent from Table 2 because it needs its own column. Ducting adds an air gap in the thermal path, so D1 ratings sit below D2 for the same size above about 25 mm² — at 300 mm² XLPE the difference is 365 A against 419 A, about 13 %. We cover installation-method comparisons in detail in cable ampacity by installation method.

03Copper or aluminium: what equal ampacity costs you in mm²

The question we are asked most often is not "which is better" but "what size do I buy instead". Table 3 answers it from the same Annex B data, by interpolating the aluminium column to find the cross-section that delivers the same current as each copper size.

Aluminium values log-interpolated from IEC 60364-5-52 Table B.52.5 aluminium column, Method C. "Sizes up" counts steps in the standard size series 1.5 / 2.5 / 4 / 6 / 10 / 16 / 25 / 35 / 50 / 70 / 95 / 120 / 150 / 185 / 240 / 300. At 240 mm² copper the required aluminium section exceeds the largest size tabulated in B.52.5, so we did not extrapolate.

The pattern is remarkably stable: between 50 and 185 mm² copper, aluminium needs about 1.5 times the cross-section, which works out to two standard sizes up almost every time. Below 16 mm² the ratio widens to about 1.7, which is why small aluminium conductors rarely make sense.

That is the whole trade in one line. Aluminium needs more section, but not as much more as the resistance ratio alone suggests. Whether the larger, lighter, cheaper conductor wins on your project depends on tray fill, bending radius, termination hardware and lugging practice — not on the ampacity table. We go through those in copper versus aluminium in practice and in the total cost of ownership guide.

04Derating: getting from the table to your site

The tabulated value is the starting point, never the answer. Three corrections cover most real projects, and they multiply.

Ambient temperature

For air installations the correction follows directly from the thermal model, and you can compute it rather than look it up:

f = √[ (θc − θa) ÷ (θc − 30) ]

where θc is the maximum conductor temperature and θa your ambient. For XLPE at 40 °C this gives √(50/60) = 0.913, which matches the 0.91 published in IEC 60364-5-52 Table B.52.14. For PVC at 40 °C it gives √(30/40) = 0.866 against a tabulated 0.87. If your ambient is 50 °C, XLPE falls to 0.82 and PVC to 0.71 — the point where PVC stops being viable for a heavily loaded circuit. High-ambient design has its own failure modes, covered in our derating guide for temperature, altitude and grouping.

Grouping (Table B.52.17)

This is the correction people forget, and it is often the largest. Table B.52.17 gives reduction factors by arrangement and circuit count. The spread between arrangements is far wider than most engineers assume.

Source: IEC 60364-5-52 Table B.52.17, to be used with the ratings in Tables B.52.2 to B.52.13. For bunched cables the series continues to 0.45 at 12 circuits, 0.41 at 16 and 0.38 at 20. For the four single-layer arrangements the standard states there is no further reduction beyond nine circuits.

The practical lesson is in the first two rows. Six circuits bunched together lose 43 % of the rating. The same six cables laid in a single layer on a perforated tray lose only 27 %. Spacing is not cosmetic — it is capacity you have already paid for. Buried grouping behaves differently again because soil, not air, is the heat sink; see burial spacing and derating.

Soil thermal resistivity and depth of burial

For buried circuits the 2.5 K·m/W reference is a convention, not a measurement. Dried sand can sit well above 3.0 K·m/W and moisture-retentive clay below 1.0, which moves the rating in opposite directions by more than the usual safety margins. Because this variable dominates buried designs and deserves its own treatment, we keep it out of this page and point you to soil thermal resistivity and buried ampacity.

05Worked example: 180 A on a perforated tray at 40 °C

Here is the full sequence on a realistic circuit, using nothing but the tables above.

The circuit. Three-phase, 400 V, design current I_b = 180 A. Multicore XLPE copper cable, single layer on a perforated horizontal tray together with five other circuits (six in total, touching), ambient air 40 °C.

Step 1 — collect the correction factors. Ambient: XLPE at 40 °C gives 0.91 (Table B.52.14; the √ formula returns 0.913). Grouping: six circuits, single layer on a perforated tray, Table B.52.17 row 4 gives 0.73. No soil factor applies — this route is in air.

Step 2 — combine them. 0.91 × 0.73 = 0.664. This is the single number most sizing errors come from: the corrections multiply, they do not add.

Step 3 — find the required tabulated rating. The installed rating must be at least I_b, so the tabulated value must satisfy I_z × 0.664 ≥ 180 A, i.e. I_z ≥ 271 A.

Step 4 — read the table. From Table 2, Method C, XLPE: 70 mm² gives 229 A, which fails. 95 mm² gives 278 A, which passes with a little margin. The answer is 95 mm².

Step 5 — voltage drop, which is a separate limit. Ampacity tells you the cable will not overheat. It does not tell you the equipment will see usable voltage. For a three-phase run the drop is approximately ΔU = √3 × I × L × (R cosφ + X sinφ), and you need R and X from the datasheet of the specific cable you are buying. We deliberately do not publish a generic resistance table here: values shift with stranding, compaction and armour, and a resistance table that is one row out has caused real specification errors before. Take the numbers from your datasheet, then run them through our cable ampacity calculator or the DC and PV voltage-drop method if the run is long.

06NEC Table 310.16 for North American projects

If you are sizing to NFPA 70 rather than IEC, the tables are not interchangeable. NEC Table 310.16 works in AWG and kcmil, assumes 30 °C ambient and not more than three current-carrying conductors, and publishes three columns by insulation temperature rating.

Source: NFPA 70 (NEC) Table 310.16, copper conductors, 30 °C ambient, not more than three current-carrying conductors in raceway, cable or earth. One reference lists 150 A for 1 AWG at 90 °C where the ICC reproduction and two others list 145 A; we have used 145 A. Verify against the edition adopted in your jurisdiction.

Because NEC sizing pulls in termination ratings, continuous-load factors and conductor-count adjustment, it warrants its own treatment. See what THHN wire is under UL 83 for the conductor-type side of that decision, and what changed between NEC 2023 and NEC 2026 if your project straddles both code cycles.

07Why catalogue charts run higher than the standard tables

If you have compared an IEC table against a manufacturer's ampacity chart and found the manufacturer's numbers higher, you have not found an error. You have found the difference between a generic model and a measured construction. But you should know how large that difference is before you mix sources.

We checked the chart that previously appeared on this page — compiled from manufacturer selection data — against IEC 60364-5-52 Table B.52.5, Method C, XLPE copper:

Catalogue column: values as published on this page between 2026-06-17 and 2026-09-10, sourced from manufacturer selection tables. IEC column: Table B.52.5, three loaded copper conductors, Method C.

The gap ranges from 3 % to 12 %, and it is widest — 10 % to 12 % — exactly in the 25 to 120 mm² band where most distribution feeders are sized. Both sets of numbers can be legitimate: a catalogue value reflects a specific, tested construction, and a well-made cable with a compacted conductor and good bedding genuinely dissipates heat better than the generic model behind Annex B.

08Pre-installation verification checklist

Before the cable goes on the drum, these are the checks that catch the expensive mistakes. They assume you have already selected a size using the method in section 05.

1. Confirm which loaded-conductor count you used. Tables B.52.4 and B.52.5 are for three loaded conductors. If you have a four-core cable with a lightly loaded neutral, or a single-phase circuit, you are on a different table.
2. Re-derive the ambient factor for the hottest credible hour, not the annual mean. A cable sized for a 30 °C mean and operated at 45 °C in a roof void is a cable running about 15 % over its intended temperature rise.
3. Count the circuits again on site. Grouping factors in Table B.52.17 step down sharply between two and six circuits. The design drawing and the installed tray rarely agree.
4. Check the resistance on the actual datasheet against IEC 60228 Class 2 maxima for that size, at 20 °C, and confirm the figure is a maximum rather than a typical. This is the cheapest way to catch an undersized conductor.
5. Verify the voltage drop with your own R and X, not a rule of thumb, and confirm the limit your specification actually imposes.
6. Confirm the termination temperature rating and, under NEC, that you are not sizing from a column the lugs do not support.
7. Check tray fill if cables are on tray. Physical fill drives spacing, spacing drives grouping, and grouping drives ampacity — they are one calculation, not three. Our tray sizing and fill-ratio guide works through it.
8. Read the marking on the cable sheath and confirm it matches what you specified. If you are not fluent in the code letters, how to read cable markings decodes SWA, AWA, XLPE and LSZH.

09What this article does and does not claim

We have reproduced IEC 60364-5-52 Annex B values for Tables B.52.4, B.52.5 and B.52.17, derived the scaling exponent from those tables rather than asserting it, and given a reproducible aluminium equivalence. The NEC section is included for North American work and is deliberately kept separate, because the two systems do not convert.

We have not published a conductor resistance or reactance table, because the values are construction-specific and a misaligned row is a genuine hazard. We have not stated a universal voltage-drop percentage, because IEC 60364-5-52 refers that to the relevant national annex. And we have not given cost figures, because cable pricing moves with copper and a static number would be wrong before it is read.

Where a value in the source material conflicted — the 1 AWG 90 °C column being the clearest example — we have said so and told you which figure we adopted. Where we could not verify a claim, including a "400 mm² versus 185 mm²" comparison that appeared on an earlier version of this page, we removed it rather than defend it.