NBR 5410 ampacity correction (derating) factors
The complete set of NBR 5410 ampacity correction factors — ambient and soil temperature, circuit grouping by physical arrangement, multiple layers, soil thermal resistivity, burial depth and the 4-loaded-conductor factor — as applied by the LOGOS cable-sizing engine.
Why the table ampacity is never the final answer
The NBR 5410 ampacity tables assume a reference world: 30 °C air (or 20 °C soil), one circuit alone, soil at 2.5 K·m/W, burial at 0.70 m. Every real installation deviates from at least one of these, and the deviation is corrected by multiplying factors:
Iz' = Iz(table) × Ft × Fa (× Fs × Fp when buried)
The corrected Iz′ is what must exceed the design current Ib and coordinate with the protective device rating. Skipping a factor does not fail loudly — it ships a cable that runs hot for years.
The factors in this page
- Ft — temperature (Table 40). Two bases, often confused: 30 °C for lines in air, 20 °C for buried lines. Applying the air column to a buried duct at 30 °C soil flatters the result by ~8% — in the unsafe direction.
- Fa — grouping (Table 42). Depends on the physical arrangement, not just the circuit count: a bundle in conduit derates far more than a single layer on a perforated tray.
- Multilayer (Table 43). Stacked layers on trays derate drastically — two layers of two circuits already fall to 0.68 against 0.88 of a single layer.
- Fs — soil resistivity (Table 41). The tables assume a conservative 2.5 K·m/W; measured wet soil near 1.0 releases up to +50% on directly buried cables.
- Fp — depth. Below the reference 0.70 m, roughly −1% per extra 10 cm (down to ~2 m).
- 4 loaded conductors. With 3rd-harmonic content above 33% the neutral counts as loaded and a 0.86 factor applies.
A worked micro-example
A 70 mm² XLPE copper cable, method E (perforated tray), 40 °C ambient, 4 circuits in a single layer: Iz table = 246 A; Ft = 0.91; Fa = 0.77. Iz′ = 246 × 0.91 × 0.77 ≈ 172 A — 30% below the table value. This is the difference between a cable that meets Ib = 180 A on paper and one that actually does.
Temperature factor Ft — in air, base 30 °C (NBR 5410 Table 40)
| Ambient (°C) | EPR/XLPE 90 °C | PVC 70 °C |
|---|---|---|
| 10 | 1.15 | 1.22 |
| 15 | 1.12 | 1.17 |
| 20 | 1.08 | 1.12 |
| 25 | 1.04 | 1.06 |
| 30 | 1 | 1 |
| 35 | 0.96 | 0.94 |
| 40 | 0.91 | 0.87 |
| 45 | 0.87 | 0.79 |
| 50 | 0.82 | 0.71 |
| 55 | 0.76 | 0.61 |
| 60 | 0.71 | 0.5 |
| 65 | 0.65 | — |
| 70 | 0.58 | — |
Temperature factor Ft — buried lines, base 20 °C soil (NBR 5410 Table 40)
| Soil temperature (°C) | EPR/XLPE 90 °C | PVC 70 °C |
|---|---|---|
| 10 | 1.07 | 1.1 |
| 15 | 1.04 | 1.05 |
| 20 | 1 | 1 |
| 25 | 0.96 | 0.95 |
| 30 | 0.93 | 0.89 |
| 35 | 0.89 | 0.84 |
| 40 | 0.85 | 0.77 |
| 45 | 0.8 | 0.71 |
| 50 | 0.76 | 0.63 |
| 55 | 0.71 | 0.55 |
| 60 | 0.65 | 0.45 |
| 65 | 0.6 | — |
| 70 | 0.53 | — |
| 75 | 0.46 | — |
| 80 | 0.38 | — |
Grouping factor Fa by number of circuits and arrangement (NBR 5410 Table 42)
| Circuits | Bundle / enclosed conduit | Single layer on wall (method C) | Single layer, perforated tray (E/F) |
|---|---|---|---|
| 1 | 1 | 1 | 1 |
| 2 | 0.8 | 0.85 | 0.88 |
| 3 | 0.7 | 0.79 | 0.82 |
| 4 | 0.65 | 0.75 | 0.77 |
| 5 | 0.6 | 0.73 | 0.75 |
| 6 | 0.57 | 0.72 | 0.73 |
| 7 | 0.54 | 0.72 | 0.73 |
| 8 | 0.52 | 0.71 | 0.72 |
| 9 | 0.5 | 0.7 | 0.72 |
| 12 | 0.45 | — | — |
| 16 | 0.41 | — | — |
| 20 | 0.38 | — | — |
Above the last tabulated count, keep the last factor. Layers beyond one: see the multilayer table below.
Multilayer grouping factor (NBR 5410 Table 43) — methods C, E, F
| Layers | 2 circ./layer | 3 circ./layer | 4 circ./layer | 6 circ./layer | 9 circ./layer |
|---|---|---|---|---|---|
| 2 | 0.68 | 0.62 | 0.6 | 0.58 | 0.56 |
| 3 | 0.62 | 0.57 | 0.55 | 0.53 | 0.51 |
| 4 | 0.6 | 0.55 | 0.52 | 0.51 | 0.49 |
| 6 | 0.58 | 0.53 | 0.51 | 0.49 | 0.48 |
| 9 | 0.56 | 0.51 | 0.49 | 0.48 | 0.46 |
Soil thermal resistivity factor (NBR 5410 Table 41) — base 2.5 K·m/W
| Resistivity (K·m/W) | Cables in buried duct | Directly buried cables |
|---|---|---|
| 0.5 | 1.28 | 1.88 |
| 0.7 | 1.2 | 1.62 |
| 1 | 1.18 | 1.5 |
| 1.5 | 1.1 | 1.28 |
| 2 | 1.05 | 1.12 |
| 2.5 | 1 | 1 |
| 3 | 0.96 | 0.9 |
Standards & methods
- ABNT NBR 5410 (Tables 40–43, 6.2.5)
- IEC 60364-5-52
Frequently asked questions
In which order do I apply the factors?
They multiply — order does not matter: Iz′ = Iz(table) × Ft × Fa, plus soil resistivity and depth factors for buried lines. The corrected Iz′ must be ≥ the design current Ib (and coordinate with the protection device).
Which grouping column applies to cables on a perforated tray?
The single-layer perforated-tray column (methods E/F). Using the bundle/conduit column (0.57 at 6 circuits vs the correct 0.73) over-derates by ~28% and buys copper you do not need. If the cables are stacked in more than one layer, switch to the multilayer table — it is much more severe.
My buried line is deeper than 0.70 m. Does it matter?
Yes, slightly: NBR 5410 tables assume 0.70 m. As a practical rule, capacity drops about 1% per additional 10 cm, valid down to about 2 m. At 1.5 m that is ≈ 8% — often the difference of one section step on long feeders.
When does soil thermal resistivity help instead of hurt?
The tables assume a conservative 2.5 K·m/W. Wet, compacted soil around 1.0 K·m/W raises the capacity of a directly buried cable by up to 50% — the single most powerful lever in buried-line design. It requires a measured value, not a guess.
Do the factors apply to the neutral too?
The factors correct the circuit as a whole. What changes with the neutral is the count of loaded conductors: with 3rd-harmonic content above 33% the neutral becomes a loaded conductor and an additional 0.86 factor applies on the 3-conductor ampacity.