“Bhai, cable to sahi size ki lagayi hai, phir bhi garam ho rahi hai — kya scene hai?” Ye sawaal sunke mera mann karta hai poochhu — “sahi size” ka matlab kya hai aapke liye? Kyunki 90% electricians ke liye “sahi size” ka matlab hai — “kisi ne bola 4mm² lagao, to laga di.” Paper pe cable heating despite correct size ka problem nahi dikhta — lekin site pe bahut common hai. Me Mohammad Arif — 9+ saal ka experience hai India aur Gulf me. Aaj aapko detail me samjhata hu ki cable size technically correct hone ke baad bhi heating kyun hoti hai — aur kaise fix karo.
- Cable Rating Kaise Decide Hoti Hai
- Reason 1: Grouping Derating
- Reason 2: Ambient Temperature
- Reason 3: Loose Connections & Bad Joints
- Reason 4: Harmonics
- Reason 5: Installation Method
- Reason 6: Voltage Drop Over Long Runs
- Complete Derating Calculation — Real Example
- Prevention — Cable Heating Avoid Karo
- Frequently Asked Questions
Cable Rating Kaise Decide Hoti Hai — Pehle Ye Samjho
Jab manufacturer bolta hai “4mm² cable 32A carry kar sakti hai” — ye IDEAL conditions me hai. Ideal conditions matlab ambient temperature 30°C, cable free air me hai charon taraf hawa ke saath, sirf ek cable hai koi aur paas me nahi, poori length me koi joint nahi, aur cable straight hai koi sharp bend nahi. Ab bolo — aapke site pe ye IDEAL conditions hain? Kabhi nahi. Gulf me ambient temperature 50°C+ ho jaata hai. Cable conduit me hai jahan hawa nahi milti. Paanch cables ek saath hain. Har 10 meter pe joint hai.
In sab conditions ke liye “derating factors” apply karne padte hain — jo cable ki actual safe current carrying capacity KAM karte hain. Cable size calculator use karke sahi size nikalo — paper calculation se kaam nahi chalega.
Reason 1: Grouping Derating — “Bheed Me Garmi”
Jab multiple cables ek saath conduit me, trunking me, ya cable tray me bunched hain — har cable heat generate karti hai (I²R losses). Ye heat dissipate nahi ho paati kyunki adjacent cables bhi garam hain. Result — sab cables ka temperature badhta hai. Ye cable heating despite correct size ka sabse common reason hai.
| Cables in Group | Derating Factor | Effective Capacity (32A cable) |
|---|---|---|
| 1 cable | 1.00 | 32A |
| 2 cables | 0.80 | 25.6A |
| 3 cables | 0.70 | 22.4A |
| 4 cables | 0.65 | 20.8A |
| 6 cables | 0.57 | 18.2A |
| 9 cables | 0.50 | 16.0A |
| 12 cables | 0.45 | 14.4A |
Dekho kya hua — 32A ki cable, 9 cables ke group me dal do to effectively sirf 16A carry kar sakti hai safely. Agar aap 25A load chala rahe ho “kyunki cable 32A ki hai” — cable overheat hogi.
Reason 2: Ambient Temperature — “Gulf Ka Special Gift”
Cable rating 30°C ambient temperature pe calculate hoti hai. Gulf me summer temperature 50°C+ jaata hai. Outdoor cable trays, rooftop installations, aur poorly ventilated electrical rooms me ambient 45–55°C aasaani se hota hai. Ye cable heating despite correct size ka Gulf me sabse bada reason hai.
| Ambient Temperature | Derating Factor | Effective Capacity (32A cable) |
|---|---|---|
| 30°C (reference) | 1.00 | 32A |
| 35°C | 0.94 | 30.1A |
| 40°C | 0.87 | 27.8A |
| 45°C | 0.79 | 25.3A |
| 50°C | 0.71 | 22.7A |
| 55°C | 0.61 | 19.5A |
Gulf me common scenario: Rooftop cable tray — direct sunlight — ambient 55°C + solar radiation. 6 cables ek tray me grouped. Combined derating: 0.61 (temperature) × 0.57 (grouping 6 cables) = 0.35. 32A cable ab sirf 11.2A safely carry kar sakti hai. Paper pe 32A cable thi — site reality me 11.2A cable ban gayi.
Reason 3: Loose Connections & Bad Joints
Ye sabse common aur sabse khatarnaak reason hai — aur ye cable size se bilkul independent hai. Cable size chahe 100mm² bhi ho — agar joint loose hai to heat hogi. Har electrical joint ek resistance hai. Achhe joint ki resistance negligible hoti hai. Bure joint ki resistance significant hoti hai. Current flow hone pe ye resistance heat generate karta hai — P = I²R.
Common bad joint practices: Twist joint bina proper crimping ke — do cables ke taar khol ke haath se twist kiya aur tape laga di. Wrong ferrule size — 4mm² cable pe 6mm² ferrule laga diya. Aluminium cable pe direct copper terminal — galvanic corrosion hota hai. Overtightened terminal screws — conductor crush hota hai. Ye sab cable heating despite correct size ke direct reasons hain.
| Detection Method | What It Shows | Best For |
|---|---|---|
| Thermal imaging camera | Hot spots at joints | Quick survey of panels |
| Touch test (with caution) | Obvious heat | Very rough indication |
| Voltage drop measurement | Excessive drop across joint | Confirming specific joint issue |
| Visual inspection | Discoloration, melting | Post-damage assessment |
Reason 4: Harmonics — “The Invisible Tax”
Non-linear loads (VFDs, LED drivers, SMPS, UPS) distorted current draw karte hain. Is distorted current me harmonics hoti hain. Skin effect se high frequency currents cable ke conductor ki surface pe concentrate hoti hain — effectively conductor ka usable cross-section area kam hota hai — resistance badhta hai — heat badhti hai.
Neutral overloading (3-phase systems): Normally balanced 3-phase system me neutral current zero ya negligible hota hai. Lekin 3rd harmonic currents teeno phases me additive hain — ye neutral me cancel nahi hote, COMBINE hote hain. Result — neutral cable me current phase cables se ZYADA flow ho sakta hai.
Real Example: Ek IT office — 200 computers, 100 LED panels, 20 printers. Phase cables ki current 40A per phase. 6mm² cable (46A rated) — paper pe theek hai. Lekin 3rd harmonic current each phase me 15A hai. Neutral me: 15A × 3 = 45A. Neutral cable bhi 6mm² hai — almost full capacity pe hai — cable garam. Solution: High harmonic environments me neutral cable ko phase cable se ek size bada rakho. Ya harmonic filter lagao.
Reason 5: Installation Method
Cable ko kaise install kiya gaya hai — ye uski current carrying capacity directly affect karta hai. Same cable — free air me, conduit me, wall me embedded, ya underground — har jagah alag capacity rakhti hai.
| Installation Method | Capacity (vs free air) | Reason |
|---|---|---|
| Free air (clipped direct) | 100% | Maximum cooling |
| Cable tray (touching) | 85–90% | Reduced air circulation |
| PVC Conduit (surface) | 70–80% | Limited heat dissipation |
| PVC Conduit (embedded in wall) | 60–70% | Insulated by wall — no cooling |
| Underground duct | 55–65% | Earth thermal resistance |
Embedded conduit me 4mm² cable ki effective capacity 32A nahi — approximately 20-22A hai. Agar aap 28A load chala rahe ho — cable heating despite correct size hogi. Gulf me common mistake — contractor ek 25mm conduit me 6-7 cables daal deta hai kyunki ek aur conduit lagane me time lagega. Short-term time bachta hai, long-term cables overheat.
Reason 6: Voltage Drop Over Long Runs
Cable lambi hone pe resistance badhta hai. Zyada resistance pe same load ke liye equipment zyada current draw karta hai (P = V × I, voltage kam to current zyada). Ye extra current cable ko aur garam karti hai.
| Cable Length | Voltage Drop (4mm², 25A) | Equipment Effect |
|---|---|---|
| 10m | ~2.7V (1.1%) | Negligible |
| 30m | ~8.1V (3.5%) | Motors run hotter |
| 50m | ~13.5V (5.8%) | Equipment malfunction risk |
| 80m | ~21.6V (9.4%) | Equipment damage risk |
Solution: Long runs ke liye cable size ek step badhao. Voltage drop calculator use karo — 50 meter se zyada runs pe ye calculation zaroori hai. BOQ preparation me bhi cable length aur derating properly consider karna padta hai.
Complete Derating Calculation — Real Example
Scenario: Gulf construction site, rooftop cable tray, 6 power cables grouped, ambient temperature 50°C, cable length 40m, load 20A per circuit. Paper calculation: 20A load → 2.5mm² cable sufficient (27A rated in free air at 30°C).
Reality check with derating: Temperature derating (50°C): ×0.71. Grouping derating (6 cables): ×0.57. Combined derating: 0.71 × 0.57 = 0.40. Required cable capacity: 20A ÷ 0.40 = 50A minimum. Actual cable needed: 10mm² (57A rated).
Prevention — Kaise Cable Heating Avoid Karo
1. Derating tables use karo. BS 7671 Appendix 4 ya IEC 60364-5-52 me detailed tables hain. 2. Conduit filling ratio 40% se zyada mat rakho. Baaki space cooling ke liye chahiye. 3. Proper joints banao. Crimp connectors use karo, correct ferrule size, manufacturer specified torque. 4. Thermal survey karo. Commissioning ke baad aur annually thermal imaging se hot spots check karo. 5. Voltage drop calculate karo. 30 meter se zyada run pe voltage drop calculation mandatory karo. 6. Harmonic loads pe neutral size badhao. IT offices, commercial buildings me neutral cable ko oversized rakho. 7. Cable route plan karo. Random routing mat karo — planned routes me grouping minimize hoti hai.
Frequently Asked Questions
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