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IEC 60287 Explained: How Cable Ampacity Is Actually Calculated
Most engineers reach for a table when they need to know a cable's ampacity. IEC 60364-5-52, BS 7671, NEC 310 — pick your standard, look up the cross-section, read the number. Fast, easy, done.
But tables are only snapshots. They assume a specific set of reference conditions: 30°C air, 20°C soil, ρ = 1.0 K·m/W, one circuit, flat formation. The moment your installation differs — and it almost always does — the table stops being accurate. That's where IEC 60287 comes in.
IEC 60287 is the calculation standard that sits behind every ampacity table. It's a thermal model of the cable and its surroundings. If you understand its structure, you can calculate ampacity for any installation — not just the one the table assumes.
This guide breaks IEC 60287 down into its building blocks, explains what each part does, and shows how you can use it on real projects.
What Is IEC 60287?
IEC 60287 is an international standard published in three parts:
- IEC 60287-1-1 — General equations for calculating current rating. Defines the ampacity formula, loss factors, and thermal resistances.
- IEC 60287-2-1 — Methods for calculating thermal resistance. Gives the formulas for T1 through T4 based on cable construction and installation.
- IEC 60287-3-1 — Operating conditions: Site reference conditions. Provides standard values for ambient temperature, soil thermal resistivity, and depth of burial.
Together, they provide a complete thermal model of a current-carrying cable. The standard covers AC cables at all voltage levels, and DC cables up to 5 kV (HV/EHV DC cables are covered by separate standards). All common installation methods are included — buried, in air, in duct, and in tray.
Scope note: IEC 60287 is the basis for most national ampacity tables worldwide. BS 7671 tables (4E4B, 4E5A, etc.) are derived from IEC 60287 calculations. NEC tables use a different methodology (Neher-McGrath) but arrive at very similar results. The physics is the same — only the path to the number differs.
The Thermal Circuit — Heat Flows Like Current
The core idea behind IEC 60287 is beautifully simple: a cable can carry as much current as the amount of heat it can get rid of. The standard models this as a thermal circuit — an exact electrical analogue where:
- Heat flow (watts) is analogous to electric current
- Temperature difference between conductor and ambient (°C) is analogous to voltage
- Thermal resistance (K·m/W) is analogous to electrical resistance
The fundamental relationship:
Δθ = W × T
(Temperature rise = Heat generated × Thermal resistance)
(Temperature rise = Heat generated × Thermal resistance)
For a cable, the conductor generates heat (I²R losses). That heat must travel through the insulation, bedding, sheath, and surrounding medium before reaching the ambient environment. Each layer adds thermal resistance. The total temperature rise is the sum of all heat × all resistances along the path.
The Four Thermal Resistances — T1 Through T4
IEC 60287 divides the heat path into four distinct thermal resistances. Each one depends on the cable construction and the installation environment.
T1
Conductor to Sheath
Thermal resistance of the insulation layer between the conductor and the metallic sheath or screen. Depends on insulation material (XLPE, PVC, EPR) and thickness.
Typical range: 0.3–0.8 K·m/W for LV XLPE cables
T2
Bedding Between Sheath & Armour
Thermal resistance of the internal bedding layer that separates the metallic sheath from the armour wires or tape. Only applies to armoured cables.
Typical range: 0.05–0.15 K·m/W
T3
External Sheath / Covering
Thermal resistance of the outer sheath that protects the cable. PVC and LSZH have higher thermal resistance than PE.
Typical range: 0.1–0.3 K·m/W for PVC sheathed cables
T4
External Medium (Air or Soil)
Thermal resistance of the surrounding environment. For buried cables, T4 is calculated from soil thermal resistivity (ρ), cable diameter, and burial depth. For cables in air, it depends on cable diameter and surface emissivity.
Varies with ρ and geometry — typically 1.0–2.5 K·m/W for buried cables (e.g., ~1.35 at ρ = 1.0, 1 m depth); often the largest component
Which one dominates? For low-voltage armoured cables (0.6/1 kV), T4 (the external environment) is often the largest component — 50–70% of the total thermal resistance for buried cables, 30–50% for cables in air. That's why installation method and soil conditions have such a dramatic effect on ampacity. The cable's own insulation (T1) matters less than what's outside the sheath.
The Three Loss Components
IEC 60287 accounts for three sources of heat in a cable. The most obvious one — I²R loss in the conductor — is rarely the only one.
I²R
Conductor Joule Loss
Primary heat source. DC resistance corrected for operating temperature and skin/proximity effect. Typically 85–95% of total heat for LV cables.
λ₁
Sheath / Screen Loss
Eddy currents and circulating currents in the metallic sheath or screen. Significant for single-core cables in AC — can reach 20–30% of conductor loss in large cross-sections.
λ₂
Armour Loss
Magnetic and eddy-current losses in the armour wires or tape. Applies to armoured cables, especially single-core in AC where it can be substantial.
For a typical multi-core SWA armoured cable in AC, the armour loss (λ₂) adds roughly 5–15% to the total heat. For single-core cables in trefoil, the sheath loss (λ₁) can be much higher. Ignoring these terms can overstate the ampacity by 10–20%.
Common mistake: Using DC resistance instead of AC resistance. Skin effect (yₛ) and proximity effect (yₚ) both increase the effective AC resistance above the DC value. For single-core conductors above about 50 mm² in AC, the combined ratio (1 + yₛ + yₚ) can reach 1.10–1.15 at 50 Hz for a 240 mm² conductor. The I²R loss is 10–15% higher than you'd calculate from DC resistance alone.
The Ampacity Formula — The Full Picture
Putting it all together, the general ampacity formula for a buried AC cable is:
General ampacity (buried AC cable, IEC 60287-1-1):
I =
√ [Δθ − Wd (0.5·T1 + n·(T2 + T3 + T4))] ÷ [R·T1 + n·R(1+λ1)T2 + n·R(1+λ1+λ2)(T3+T4)]
√ [Δθ − Wd (0.5·T1 + n·(T2 + T3 + T4))] ÷ [R·T1 + n·R(1+λ1)T2 + n·R(1+λ1+λ2)(T3+T4)]
Where:
| Symbol | Meaning | Typical Value (120 mm² 4-core, buried) |
|---|---|---|
| Δθ | Max conductor temp rise above ambient | 90°C − 20°C = 70 K |
| Wd | Dielectric loss per unit length | ~0.1 W/m (LV cables, negligible) |
| n | Number of conductors in cable | 4 (for 4-core cable) |
| R | AC resistance per unit length at operating temp | ~0.000195 Ω/m (120 mm² Cu at 90°C) |
| λ1 | Sheath / screen loss factor | ~0.08 (SWA, multi-core) |
| λ2 | Armour loss factor | ~0.05 (SWA, multi-core) |
| T1–T4 | Thermal resistances (see breakdown above) | Sum typically ~1.5–2.5 K·m/W |
It's iterative: R depends on conductor temperature, but conductor temperature depends on I²R. IEC 60287 is solved iteratively — assume a temperature, calculate I, check that the resulting temperature matches the assumption, repeat if not. Most commercial software handles this automatically, but understanding the iteration helps you spot unrealistic results.
Know the boundary: IEC 60287 assumes a 100% continuous load factor — the cable carries steady current until thermal equilibrium is reached. For real-world variable loads (daily peaks, emergency overloads, intermittent renewables), the cable's thermal capacitance (heat stored in insulation, sheath, and soil) allows short-term overloading without exceeding the temperature limit. These transient ratings are covered by IEC 60853 (cyclic and emergency current ratings of cables). If your project involves load cycling or contingency ratings, use IEC 60287 for the base rating and IEC 60853 for the transient check.
Simplified Form for LV Cables
For low-voltage cables where dielectric losses are negligible (Wd ≈ 0) and there's no ferromagnetic pipe (λ2 = 0), the formula simplifies to something you can work through by hand:
Simplified for LV cables:
I = [Δθ ÷ (n·R·Ttot)]0.5
where Ttot = T1/n + (1+λ1)·(T2+T3+T4)
where Ttot = T1/n + (1+λ1)·(T2+T3+T4)
This simplified form reveals the key relationship: ampacity is proportional to the square root of the temperature rise divided by the total thermal resistance. Cut the thermal resistance in half, and ampacity increases by about 40% (√2 ≈ 1.414). Double the resistance, and ampacity drops by about 30%.
How the Calculation Plays Out — A Rough Walk-Through
Rather than bury you in numbers, let me show the relative contribution of each thermal resistance using a 120 mm² 4-core Cu/XLPE/SWA/PVC cable buried at 1 m depth in average soil (ρ = 1.0). The values below are approximate for illustration — the exact BS 7671 standard table result for this configuration is 180 A (see our soil thermal resistivity guide for the full table). A full IEC 60287 calculation with all loss factors included converges to a similar value.
| Component | Thermal Resistance (K·m/W) | Share of Total | Comment |
|---|---|---|---|
| T1 — XLPE insulation | ~0.45 | ~22% | Fixed by cable construction |
| T2 — Bedding | ~0.08 | ~4% | Small — thin layer |
| T3 — PVC outer sheath | ~0.18 | ~9% | Modest contribution |
| T4 — Soil (ρ = 1.0) | ~1.35 | ~65% | Dominant term |
| Total | ~2.06 | 100% |
What this tells you: 65% of the thermal resistance comes from the soil, not the cable. That's why two identical cables can have wildly different ampacities — it's the environment, not the cable, that determines the limit. If you want to increase ampacity, improving the external conditions (thermal backfill, spacing, cooler soil) is far more effective than changing the cable construction.
FAQ — Technical Questions from Engineers
Why does IEC 60287 use an iterative calculation instead of a direct formula?
Because conductor resistance changes with temperature. Copper's resistivity increases by about 0.39% per °C (α = 0.00393 K⁻¹). At 90°C, a copper conductor has about 27% higher resistance than at 20°C (the standard IEC reference temperature) — which means higher I²R losses, which means more heat. You can't solve for I without knowing R, and you can't know R without knowing the conductor temperature, which depends on I. The iteration converges quickly — typically 3–5 cycles — but you can't skip it. Some simplified methods assume a fixed R at maximum temperature, which is conservative but overstates the losses slightly.
Can IEC 60287 handle mixed cable types in the same trench or tray?
Not directly. The standard gives formulas for a single cable or a group of identical cables. For mixed cable types (different sizes, different loadings) in close proximity, you need either: (a) an equivalent heating approach — calculate the total heat output of all cables and use an effective ρ for the group — or (b) finite element analysis (FEA) software, which models the actual thermal field. For most practical purposes, the conservative approach is to derate for the worst-case cable in the group. For critical installations with mixed cable types, I'd recommend FEA.
How do harmonic currents affect the IEC 60287 calculation?
Harmonics increase the effective RMS current — and the I²R losses — beyond what the fundamental 50/60 Hz component suggests. The impact is twofold: (1) skin effect increases with frequency (√f), so higher-order harmonics see much higher resistance; (2) eddy-current losses in sheaths and armour also increase disproportionately. IEC 60287 doesn't have a built-in harmonic correction, but you can approximate the effect by calculating the harmonic loss factor (FHL) and applying it to the AC resistance. For VFD applications with significant harmonic content (THD > 15%), the effective ampacity can be 10–20% lower than the sinusoidal calculation.
Is IEC 60287 applicable to 1500 V DC solar cables?
Yes, with some simplifications. For DC cables, there's no skin effect, no proximity effect, and no sheath/armour circulating currents (λ1 = 0). The dielectric loss (Wd) is zero at DC. The formula reduces to the simplified version above. However, solar cables are often installed in free air (on cable trays or clipped to structures) with significant solar radiation heating. IEC 60287's in-air calculation includes a solar absorption term — essential for outdoor PV installations. Make sure to account for solar gain on dark-coloured cables (LSZH, PVC) in your Δθ budget.
What's the practical difference between IEC 60287 and the Neher-McGrath method used in NEC?
Both solve the same thermal problem — they just use different terminology and conventions. Neher-McGrath (the basis for NEC ampacity tables) expresses external thermal resistance as a combination of "earth thermal resistance" and "cable surface thermal resistance" rather than the IEC's T4. The numerical results are generally within 5–10% of each other for equivalent conditions. The main practical difference is in how they handle grouping factors and buried ductbanks — Neher-McGrath has more detailed provisions for complex ductbank configurations, while IEC 60287 is more systematic for direct-burial and in-air installations. If you work across both standards, it's worth running both calculations on critical circuits to check agreement.
Pre-Calculation Checklist
Before you run an IEC 60287 calculation, confirm you have these inputs ready:
- Cable construction data: conductor material and cross-section, insulation type and thickness, armour type, sheath material and thickness
- Conductor temperature limit: 70°C for PVC, 90°C for XLPE/EPR, 105°C for some special cables
- Ambient conditions: air temperature (for in-air) or soil temperature (for buried); include solar heating if applicable
- Installation method: buried depth, in-air spacing, duct configuration, grouping arrangement
- Soil thermal resistivity: measured or conservatively estimated — don't default to 1.0 without checking
- Load characteristics: AC or DC, fundamental frequency, harmonic content if significant
- Circuit configuration: single circuit or grouped; for AC, trefoil or flat formation; sheath bonding arrangement
Related reading: IEC 60287 calculations are the starting point for understanding derating. See our practical guides on temperature, altitude & grouping derating, soil thermal resistivity effects, and the ampacity comparison by installation method. For complete ampacity tables, see our current-carrying capacity chart guide.
Every Sorivo cable can be supplied with a detailed IEC 60287 calculation sheet showing the rated ampacity for your specific installation conditions. Email us at sale@sorivocable.com with your project parameters — we'll run the numbers and send you the full calculation.
