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Cable Current Carrying Capacity & Sizing Guide
Complete reference tables for ampacity, voltage drop, derating factors, and cable tray sizing — IEC 60364 / BS 7671 / NEC 392
⚡ Engineering Reference Tool1 Current Carrying Capacity (Ampacity) Tables
All values are for copper conductors, XLPE insulated, 0.6/1 kV. Ratings are in amperes (A) for three-phase AC circuits. Always apply derating factors when installation conditions differ — see Section 3.
1.1 Multi-Core XLPE/SWA/PVC — 3 or 4 Core (Most Common)
| Cross-Section (mm²) | Method C Clipped Direct | Method E Perforated Tray / Free Air | Method D1 Single Duct in Ground | Method D2 Direct Buried |
|---|---|---|---|---|
| 1.5 | 23 | 25 | — | 21 |
| 2.5 | 31 | 33 | — | 30 |
| 4 | 42 | 44 | — | 39 |
| 6 | 53 | 56 | — | 49 |
| 10 | 73 | 78 | — | 65 |
| 16 | 94 | 99 | 71 | 84 |
| 25 | 124 | 131 | 91 | 107 |
| 35 | 154 | 162 | 110 | 129 |
| 50 | 187 | 197 | 130 | 153 |
| 70 | 238 | 251 | 160 | 188 |
| 95 | 289 | 304 | 192 | 226 |
| 120 | 335 | 353 | 218 | 257 |
| 150 | 386 | 406 | 244 | 287 |
| 185 | 441 | 463 | 275 | 324 |
| 240 | 520 | 546 | 319 | 375 |
| 300 | 599 | 628 | 356 | 419 |
| 400 | 673 | 728 | 394 | 464 |
| Reference: BS 7671:2018 Table 4E4A / 4E4B (C/E columns), Table 4E4A Reference Method D (direct buried). D1 (single duct) approx 85% of D2 values — consult BS 7671 for project-specific calculations. Ambient: air 30°C, ground 20°C. Soil thermal resistivity: 2.5 K·m/W. Depth: 0.7 m. | ||||
1.2 Single-Core XLPE — Trefoil / Flat Formation
Single-core cables dissipate heat better than multi-core. Values for copper, XLPE, in free air (40°C ambient per IEC 60287):
| Cross-Section (mm²) | 1-Core Trefoil (A) | 1-Core Flat (A) | 3-Core Cable (A) (for comparison) |
|---|---|---|---|
| 16 | 110 | 120 | 85 |
| 25 | 140 | 155 | 115 |
| 35 | 170 | 185 | 135 |
| 50 | 210 | 225 | 165 |
| 70 | 270 | 285 | 210 |
| 95 | 330 | 345 | 255 |
| 120 | 380 | 400 | 290 |
| 150 | 440 | 460 | 335 |
| 185 | 500 | 520 | 380 |
| 240 | 590 | 610 | 440 |
| 300 | 680 | 700 | 500 |
| 400 | 790 | 820 | 575 |
| Reference: IEC 60287. Ambient air: 40°C (differs from Table 1.1 which uses 30°C — the 3-core comparison column reflects this lower base). For direct comparison with Table 1.1, apply the 30°C temperature factor from Section 3.1. Single-core in flat formation typically carries 5–8% more than trefoil. | |||
1.3 PVC Insulated Cable (for comparison)
PVC has a lower max conductor temperature (70°C vs 90°C for XLPE), so ampacity is typically 15–25% lower. Use this table when working with older PVC cable types or BS 6346 cables:
| Cross-Section (mm²) | Method C Clipped Direct | Method E Perforated Tray | Direct Buried |
|---|---|---|---|
| 1.5 | 19 | 20 | 23 |
| 2.5 | 26 | 27 | 30 |
| 4 | 35 | 37 | 40 |
| 6 | 44 | 46 | 50 |
| 10 | 61 | 64 | 67 |
| 16 | 78 | 82 | 96 |
| 25 | 103 | 109 | 125 |
| 35 | 128 | 135 | 150 |
| 50 | 155 | 164 | 179 |
| 70 | 198 | 209 | 221 |
| 95 | 241 | 253 | 262 |
| 120 | 279 | 294 | 300 |
| 150 | 321 | 338 | 337 |
| 185 | 367 | 386 | 383 |
| 240 | 433 | 455 | 441 |
| 300 | 499 | 523 | 491 |
| Reference: BS 7671:2018 — Table 4D2A (Methods C/E, clipped direct & tray), Table 4D4A (Method D, direct buried). Ambient: air 30°C, ground 20°C. PVC max conductor temp: 70°C. | |||
2 Voltage Drop Reference
Voltage drop values in mV/A/m (millivolts per ampere per metre). To calculate total voltage drop: VD (V) = mV/A/m × I (A) × L (m) / 1000
2.1 Copper Conductor, XLPE Insulated, 0.6/1 kV — Multi-Core (3-Phase)
| mm² | mV/A/m (3-ph) | mm² | mV/A/m (3-ph) |
|---|---|---|---|
| 1.5 | 27 | 95 | 0.45 |
| 2.5 | 16 | 120 | 0.37 |
| 4 | 10 | 150 | 0.30 |
| 6 | 6.8 | 185 | 0.26 |
| 10 | 4.0 | 240 | 0.21 |
| 16 | 2.5 | 300 | 0.185 |
| 25 | 1.65 | 400 | 0.165 |
| 35 | 1.15 | ||
| 50 | 0.87 | ||
| 70 | 0.60 | ||
| For single-phase (2-core), multiply by approx. 1.15. Source: IEC 60287 calculation method. | |||
📐 Quick rule of thumb: For a 3-phase 400 V supply, BS 7671 allows maximum 5% drop (20 V). For a 150 m run at 200 A with 95 mm² cable: VD = 0.45 × 200 × 150 / 1000 = 13.5 V (3.4%) — within limits.
2.2 Maximum Cable Length for 3% and 5% Voltage Drop
Use this quick lookup to check if a given cable size can serve a given distance. Based on 3-phase 400 V, 0.85 power factor, XLPE cable.
| Load (A) | Cable Size | Max Length @ 3% Drop | Max Length @ 5% Drop |
|---|---|---|---|
| 100 | 50 mm² | 138 m | 230 m |
| 150 | 70 mm² | 133 m | 222 m |
| 200 | 95 mm² | 133 m | 222 m |
| 250 | 120 mm² | 130 m | 216 m |
| 300 | 185 mm² | 154 m | 256 m |
| 400 | 240 mm² | 143 m | 238 m |
| For 3-phase only. Formula: L_max = (VD% × VLL × 10) / (mV/A/m × I). Where VD% is the allowed percentage (e.g., 3). Example: (3 × 400 × 10) / (0.87 × 100) = 12000 / 87 ≈ 138 m (3% drop for 100 A @ 50 mm²). For single-phase, use the 2-core mV/A/m value. | |||
3 Derating Factors — Adjusting for Real Conditions
The ampacity tables above assume ideal conditions. In real installations — hot attics, grouped cables, sandy soil — you must apply correction factors. The corrected rating = tabulated rating × all applicable factors.
3.1 Ambient Temperature Correction (IEC 60364-5-52 Tables B.52.14 & B.52.15)
| Temperature (°C) | Factor — Cable in Air (XLPE) | Factor — Cable in Air (PVC) | Factor — Direct Buried (XLPE) | Factor — Direct Buried (PVC) |
|---|---|---|---|---|
| 10 | 1.15 | 1.22 | 1.07 | 1.10 |
| 20 | 1.08 | 1.12 | 1.00 | 1.00 |
| 25 | 1.04 | 1.06 | 0.96 | 0.95 |
| 30 (ref) | 1.00 | 1.00 | 0.93 | 0.89 |
| 35 | 0.96 | 0.94 | — | — |
| 40 | 0.91 | 0.87 | 0.85 | 0.77 |
| 45 | 0.87 | 0.79 | — | — |
| 50 | 0.82 | 0.71 | 0.76 | 0.63 |
| 55 | 0.76 | 0.61 | — | — |
| 60 | 0.71 | 0.50 | 0.65 | 0.45 |
| 70 | 0.58 | — | — | — |
| 80 | 0.41 | — | 0.38 | — |
3.2 Grouping / Multi-Circuit Correction
When multiple cables run together, heat builds up. Apply these typical factors (from BS 7671 Table 4C1, method C):
| Number of Circuits / Cables | Factor (Bunched / touching) | Factor (Spaced > 1× cable OD) |
|---|---|---|
| 1 | 1.00 | 1.00 |
| 2 | 0.80 | 0.88 |
| 3 | 0.70 | 0.82 |
| 4 | 0.65 | 0.78 |
| 5 | 0.60 | 0.75 |
| 6 | 0.57 | 0.73 |
| 7 | 0.54 | 0.71 |
| 8 | 0.52 | 0.70 |
| 9 | 0.50 | 0.68 |
3.3 Soil Thermal Resistivity (for Direct Buried Cables)
The standard assumption is 2.5 K·m/W (normal soil). For drier conditions, reduce ampacity:
| Soil Type | Thermal Resistivity | Correction Factor |
|---|---|---|
| Wet / saturated soil | 0.7 K·m/W | 1.18 |
| Damp soil | 1.5 K·m/W | 1.06 |
| Normal (reference) | 2.5 K·m/W | 1.00 |
| Dry soil | 3.0 K·m/W | 0.93 |
| Sandy / very dry soil | 4.0 K·m/W | 0.83 |
⚠️ Common mistake: Most cable sizing errors come from forgetting to apply derating factors. If you skip grouping and temperature correction, a 50 mm² cable rated for 197 A in free air could be limited to just 197 × 0.82 (50°C) × 0.70 (3 circuits grouped) = 113 A — a 43% reduction.
Corrected Ampacity = I_tabulated × k_temp × k_group × k_soil × ... (all applicable factors)
4 Cable Tray Sizing — Fill Ratio & Width Calculation
Too many cables in a tray creates overheating and makes installation difficult. Here's how to size trays correctly using the NEC 392 method (widely used internationally).
4.1 The Basic Formula
Fill % = (Total Cable Cross-Sectional Area ÷ Tray Usable Area) × 100
- Tray Area = Tray Width × Usable Depth (e.g., 300 mm wide × 100 mm deep = 30,000 mm²)
- Cable Area (single) = π × (D/2)² where D = cable outer diameter
- Total Cable Area = sum of all individual cable areas in the tray
4.2 Maximum Fill Limits (NEC 392.22)
| Tray Type | Max Fill Limit | Notes |
|---|---|---|
| Ladder type | 50% | Good ventilation |
| Ventilated trough | 50% | Good ventilation |
| Solid bottom | 40% | Reduced ventilation |
| Wire mesh / basket | 50% | Typically ≤ 50 mm depth |
| Medium voltage (> 2000 V) | 40% | Additional clearance |
| Control / signal cables only | 50% | Ventilated tray |
4.3 Sizing for Multi-Conductor Cables (≤ 600 V)
Case A: All cables #3/0 AWG and smaller → sum cross-sectional areas, select tray width from fill table.
Case B: Mixed sizes with cables ≥ #4/0 AWG → calculate width in two parts:
- Large cables: Sum of their outside diameters = required width portion
- Small cables: Total area → convert to equivalent width: Width = (Area × 6) / 7
- Total width: Part A + Part B → round up to standard tray size
4.4 Step-by-Step Example
Example: 6 cables of 4-core 120 mm² SWA (OD ≈ 45 mm) on a ventilated ladder tray
Step 1: Cable area per cable = π × (45/2)² = π × 22.5² ≈ 1,590 mm²
Step 2: Total cable area = 6 × 1,590 = 9,540 mm²
Step 3: Max fill = 50%. Minimum tray area = 9,540 / 0.50 = 19,080 mm²
Step 4: Using a standard 100 mm deep tray: Min width = 19,080 / 100 ≈ 191 mm → select 300 mm tray (next standard size, allows room for expansion)
✅ 300 mm ventilated ladder tray meets NEC 392 requirements with 20% spare capacity
💡 Pro tip: Always add 20–50% spare capacity for future cables. A tray that's full on day one will need to be ripped out when the plant expands. Most experienced engineers design for 30–40% fill to leave room.
5 Worked Examples — Putting It All Together
Theory is one thing — let me walk you through two real scenarios that show how the tables work together.
📐 Example 1: 50 mm² 3-core SWA, tray installed, 150 A load, 40°C ambient, 200 m run
Step 1 — Base ampacity: From Table 1.1, 50 mm² on perforated tray = 197 A
Step 2 — Temperature correction: 40°C in air, XLPE factor = 0.91
Step 3 — Grouping: Assume 3 circuits bunched, factor = 0.70
Step 4 — Corrected ampacity: 197 × 0.91 × 0.70 = 125 A
Step 5 — Check vs load: 125 A < 150 A → ❌ Not adequate. Step up to 70 mm² (251 A base → 251 × 0.91 × 0.70 = 160 A ✅)
Step 6 — Voltage drop check: 70 mm² = 0.60 mV/A/m. VD = 0.60 × 150 × 200 / 1000 = 18.0 V (4.5%) → within 5% limit ✅
Final selection: 70 mm² 3-core XLPE/SWA/PVC on 300 mm ventilated tray ✅
📐 Example 2: 120 mm² direct buried, group of 4 circuits, sandy soil, 30°C ground, 180 A load
Step 1 — Base ampacity: From Table 1.1 (BS 7671), 120 mm² direct buried = 257 A
Step 2 — Ground temp correction: 30°C, XLPE factor = 0.93
Step 3 — Soil resistivity: Sandy/very dry soil, factor = 0.83
Step 4 — Grouping (buried, touching): 4 circuits, factor ≈ 0.65
Step 5 — Depth correction: If buried deeper than 0.7 m, apply additional factor. Assume standard depth for this case.
Step 6 — Corrected ampacity: 257 × 0.93 × 0.83 × 0.65 = 129 A → ❌ 129 A < 180 A
Step 7 — Try 185 mm²: 324 × 0.93 × 0.83 × 0.65 = 163 A → still under. Try 240 mm²: 375 × 0.93 × 0.83 × 0.65 = 188 A ✅
Final selection: 240 mm² 3-core XLPE/SWA/PVC direct buried (or split into two smaller cables) ✅
⚠️ What this tells us: A 120 mm² cable rated for 257 A per BS 7671 is derated to just 129 A in this real-world scenario — a 50% reduction. Never size from base ratings alone. Always run the derating calculation.
6 Why Cable Quality Matters for Ampacity
The tables in this guide assume a properly manufactured cable. In reality, the ratings you achieve depend heavily on build quality. Here's how Sorivo's cables compare to generic economy-grade products:
| Feature | Market Generic / Economy | Sorivo Premium Grade |
|---|---|---|
| Conductor | Bare copper with possible impurities — higher resistance means higher I²R loss and more heat | Plain annealed copper (IEC 60228 Class 2) — strict purity control, consistent DC resistance |
| XLPE Insulation | Variable cross-linking degree — may soften at rated 90°C, reducing safe ampacity | Type GP8 per BS 7655-1.3 — verified cross-linking, full 90°C rating, consistent wall thickness |
| Armour | Under-gauge galvanised wires — reduced mechanical protection and tensile strength | BS-specified wire diameter — full galvanising, meets all mechanical load requirements |
| Sheath | Recycled PVC compound — may soften or deform at high ambient temperatures | Virgin PVC (Type TM1 per BS 7655-4.1) or virgin LSZH (Type LTS3 per BS 7655) — full thermal rating, UV stable |
| Fire Performance | Self-declared — may not achieve advertised flame retardance in real conditions | Third-party tested to BS EN 60332, BS EN 61034, BS EN 60754 — verified performance |
| Traceability | None — no batch records, impossible to verify actual conductor size or material | Metre-marked sheath, batch traceable — full material certification available on request |
| Warranty | 1–5 years | 25-year design life |
7 Pre-Installation Ampacity Checklist
Use this checklist before finalising your cable size. Missing even one factor can lead to an undersized cable and expensive rework.
✅ 10-Point Cable Sizing Verification
- Base ampacity selected from correct table (XLPE vs PVC, multi-core vs single-core)
- Ambient temperature correction applied — check actual site conditions, not assumed
- Grouping derating applied for all cables in the same tray, duct, or trench
- Soil thermal resistivity verified with site soil report (for direct burial runs)
- Depth of burial correction applied if deeper than standard 0.7 m
- Voltage drop checked at full load current — not just ampacity compliance
- Cable tray fill ratio within 50% limit (or 40% for solid bottom)
- Spare capacity allowed for future expansion (recommend 20-50%)
- Termination temperature rating confirmed — gland and lug ratings match cable
- Manufacturer's test certificate requested to verify actual conductor resistance
8 Frequently Asked Questions
Which installation method gives the highest ampacity — clipped direct or perforated tray?
Perforated tray (Method E) typically gives slightly higher ratings for multi-core cables because air circulates around all surfaces. Clipped direct (Method C) has one surface against the wall, which traps some heat. The difference is small — typically 3–6% — so you can usually use either table. For unperforated solid bottom tray, use Method C values.
How do I convert ampacity between 30°C and 40°C ambient?
Use the temperature correction factors from Section 3.1. For XLPE: at 30°C the factor is 1.00 (reference), at 40°C it's 0.91. So a cable rated 200 A at 30°C is rated 200 × 0.91 = 182 A at 40°C. The factor works both ways — for a 20°C environment, multiply by 1.08 (uprating).
What's the difference between XLPE and PVC ampacity for the same cable size?
XLPE carries about 20–30% more current than PVC for the same cable size because it operates at 90°C vs PVC's 70°C. For example, a 50 mm² multi-core cable in free air: XLPE = 197 A, PVC = 164 A. The XLPE cable also has higher short-circuit capacity (250°C vs 160°C for PVC).
Does SWA armour affect current carrying capacity?
Yes but only slightly. The steel wire armour (SWA) has negligible electrical effect on the conductor ampacity, but the air gaps and bedding layers around the armour create slightly more thermal insulation than an unarmoured cable. SWA cables typically have about 2–5% lower ampacity than equivalent unarmoured cables, though most standard tables already account for this.
What's the fastest way to estimate cable size for a given load?
A rough rule of thumb for XLPE copper, 3-phase: 1 mm² carries approximately 2–3 A in free air. For 150 A, you'd start looking at 50–70 mm². This is a quick sanity check only — always verify with the full derating calculation. And remember, voltage drop often governs on longer runs even if ampacity says the cable is big enough.
Can I run single-core and multi-core cables in the same tray?
Yes, but with caution. Single-core cables carrying AC create magnetic fields that can induce eddy currents in steel tray sections — use aluminium or fibreglass trays for large single-core runs. For mixed installations, sum the cross-sectional areas of all cables and apply the appropriate fill limit (50% for ventilated trays). Also check NEC 392.22(B)(1) for the mixed-size calculation method covered in Section 4.3.
Need help sizing cables for your project? Sorivo's engineering team provides free cable sizing calculations and ampacity reports for your specific installation conditions — ambient temperature, grouping, soil type, and voltage drop requirements.
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