What cable size (mm²) do I need for a 32 amp load?

For a 32 A load on a 415 V, 3-phase circuit in standard conduit installation (ambient 30°C, single circuit), a 6 mm² copper cable is the typical selection — IS 694 / IEC 60364 ampacity for 6 mm² PVC copper in conduit is approximately 32–36 A. For runs above 30 m, check voltage drop — for a 50 m run at 32 A, a 10 mm² cable may be needed to keep drop below 5%. Always apply derating factors for grouping and ambient temperature.

How do I calculate cable size for a motor?

Step 1: Calculate full-load current: I = P / (√3 × V × PF × η). Step 2: Apply derating for installation method, ambient temperature, and grouping. Step 3: Select cable size where rated ampacity × derating ≥ full-load current. Step 4: Check voltage drop for the run length. Step 5: Verify short-circuit rating. Example: 15 kW motor, 415 V, PF 0.85, η 0.91: I = 15000/(1.732×415×0.85×0.91) ≈ 27 A → select 4 mm² copper (ampacity 32 A in conduit) and verify voltage drop.

What is ampacity and how does installation method affect cable current rating?

Ampacity is the maximum current a cable can carry continuously without exceeding its temperature rating. It depends on conductor size, insulation type (70°C PVC, 90°C XLPE), and installation method. Derating factors per IS 732 / IEC 60364-5-52: ambient above 30°C (factor 0.87–0.94), cables in conduit vs free air (factor 0.75–0.85), cables bunched together (grouping factor 0.5–0.9 depending on count), buried cables (factor 0.8). Apply all applicable factors and compare the derated ampacity with load current.

What voltage drop percentage is acceptable for motor and power cables?

IS 732 and IEC 60364-5-52 recommend: 3% maximum for lighting circuits, 5% maximum for power/motor circuits from the supply origin to the load. For VFDs and sensitive electronic loads, limit to 2–3%. Excessive voltage drop causes: motor overheating (voltage below rated), reduced motor torque (torque ∝ V²), under-voltage tripping of contactors, and higher running current that shortens motor life.

What are the standard cable sizes available in India (IS 694)?

Standard copper conductor sizes per IS 694 (PVC insulated cables up to 1.1 kV): 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm². Typical ampacity (single-core copper, PVC, in air at 30°C): 2.5 mm² ≈ 24 A, 4 mm² ≈ 32 A, 6 mm² ≈ 41 A, 10 mm² ≈ 57 A, 16 mm² ≈ 76 A, 25 mm² ≈ 101 A, 35 mm² ≈ 125 A, 50 mm² ≈ 151 A, 70 mm² ≈ 192 A, 95 mm² ≈ 232 A. Always apply derating for actual installation conditions.

Should I use copper or aluminium cables?

Copper: higher conductivity (better ampacity per mm²), preferred for control cables, motor feeders, and smaller power cables up to 50 mm². Aluminium: 50–60% cheaper per ampere, used for large power cables (70 mm² and above), HT cables, transformer connections, and underground feeders. Per IS 694 and standard Indian practice: copper is standard for motor feeders up to about 240 mm²; aluminium is common for LT and HT main feeders from transformers. Aluminium terminations require anti-oxidant compound.

How does cable size affect energy losses and electricity bills?

An undersized cable has higher resistance → higher I²R losses → more heat generated and wasted. For a 50 A load on 100 m run: 6 mm² cable (R = 3.08 Ω/km, resistance of run = 0.308 Ω each way) → I²R losses = 50² × 0.616 = 1,540 W = 1.54 kW per circuit. Upgrading to 10 mm² (R = 1.83 Ω/km, 0.366 Ω round trip): losses = 50² × 0.366 = 915 W — saving 625 W continuously. At ₹8/kWh, running 8,000 hr/yr: annual saving = 5,000 kWh = ₹40,000. Cable upgrades often pay back in 1–2 years in high-load circuits.

What cable size for a 3-phase 415V, 1 HP (0.75 kW) pump?

A 0.75 kW (1 HP) motor at 415 V, 3-phase, PF 0.82, η 0.80: full-load current = 750/(1.732×415×0.82×0.80) ≈ 1.6 A. Minimum cable size by ampacity: 1 mm² or 1.5 mm² is more than adequate (rated ≈ 15 A in conduit). In practice, 1.5 mm² is the minimum practical cable for motor feeders in India for mechanical protection. If the run is above 50 m, check voltage drop — a 1.5 mm² cable on a 100 m run at 1.6 A gives only about 1.5 V drop, well within the 5% limit.

Cable Size Calculator — Wire Size in mm² for Any Motor, Load & Cable Run

Calculate cable size in mm² from load current, cable length, voltage drop, and installation conditions per IS 694 / IEC 60364.

Calculator

No signup required. Results are indicative—verify for your standards.

Load power (kW)

Voltage (V) — 3-phase

Power factor

Cable run length (m)

Load current: 36.0 A

Minimum cable size: 6 mm² copper (ampacity 38 A)

Voltage drop: 9.6 V (2.3% of 415 V)

Formula

Load current I = P / (√3 × V × PF) for 3-phase. Voltage drop ΔV = (√3 × I × L × R) / 1000 for 3-phase, where R is cable resistance (Ω/km). Select cable so ΔV ≤ allowable (typically 3–5% of supply voltage per IS 732).

Example calculation

22 kW motor, 415 V, PF 0.85, 3-phase: I = 22000/(√3×415×0.85) ≈ 36 A. For 50 m run, ΔV limit 5%: suggested 10 mm² copper cable (R = 1.83 Ω/km, ΔV ≈ 5.8 V = 1.4%). Derate for ambient and grouping to confirm ampacity.

Engineering notes

Two criteria govern cable selection: (1) Ampacity — the cable must carry the load current continuously without overheating. (2) Voltage drop — must not exceed 3–5% of supply voltage per IS 732 / IEC 60364-5-52. Always check both and select the larger conductor that satisfies both. Also verify short-circuit rating for protection coordination. IS 694 specifies PVC insulated cables for 1.1 kV; IS 7098 Part 1 for XLPE cables. Standard copper conductor sizes (mm²): 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400.

When to use this calculator

Motor cable sizing — select the correct cable size (mm²) before purchasing and installing any motor feeder
MCC and panel design — size feeder and branch cables for motor control centres from the load list
New plant electrical design — size cables for all electrical loads during FEED or detailed engineering
Energy audit — identify undersized cables causing excessive voltage drop and copper losses
Generator connection — size cables from DG set to main switchboard for adequate fault current capacity

Frequently asked questions

What cable size (mm²) do I need for a 32 amp load?: For a 32 A load on a 415 V, 3-phase circuit in standard conduit installation (ambient 30°C, single circuit), a 6 mm² copper cable is the typical selection — IS 694 / IEC 60364 ampacity for 6 mm² PVC copper in conduit is approximately 32–36 A. For runs above 30 m, check voltage drop — for a 50 m run at 32 A, a 10 mm² cable may be needed to keep drop below 5%. Always apply derating factors for grouping and ambient temperature.
How do I calculate cable size for a motor?: Step 1: Calculate full-load current: I = P / (√3 × V × PF × η). Step 2: Apply derating for installation method, ambient temperature, and grouping. Step 3: Select cable size where rated ampacity × derating ≥ full-load current. Step 4: Check voltage drop for the run length. Step 5: Verify short-circuit rating. Example: 15 kW motor, 415 V, PF 0.85, η 0.91: I = 15000/(1.732×415×0.85×0.91) ≈ 27 A → select 4 mm² copper (ampacity 32 A in conduit) and verify voltage drop.
What is ampacity and how does installation method affect cable current rating?: Ampacity is the maximum current a cable can carry continuously without exceeding its temperature rating. It depends on conductor size, insulation type (70°C PVC, 90°C XLPE), and installation method. Derating factors per IS 732 / IEC 60364-5-52: ambient above 30°C (factor 0.87–0.94), cables in conduit vs free air (factor 0.75–0.85), cables bunched together (grouping factor 0.5–0.9 depending on count), buried cables (factor 0.8). Apply all applicable factors and compare the derated ampacity with load current.
What voltage drop percentage is acceptable for motor and power cables?: IS 732 and IEC 60364-5-52 recommend: 3% maximum for lighting circuits, 5% maximum for power/motor circuits from the supply origin to the load. For VFDs and sensitive electronic loads, limit to 2–3%. Excessive voltage drop causes: motor overheating (voltage below rated), reduced motor torque (torque ∝ V²), under-voltage tripping of contactors, and higher running current that shortens motor life.
What are the standard cable sizes available in India (IS 694)?: Standard copper conductor sizes per IS 694 (PVC insulated cables up to 1.1 kV): 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm². Typical ampacity (single-core copper, PVC, in air at 30°C): 2.5 mm² ≈ 24 A, 4 mm² ≈ 32 A, 6 mm² ≈ 41 A, 10 mm² ≈ 57 A, 16 mm² ≈ 76 A, 25 mm² ≈ 101 A, 35 mm² ≈ 125 A, 50 mm² ≈ 151 A, 70 mm² ≈ 192 A, 95 mm² ≈ 232 A. Always apply derating for actual installation conditions.
Should I use copper or aluminium cables?: Copper: higher conductivity (better ampacity per mm²), preferred for control cables, motor feeders, and smaller power cables up to 50 mm². Aluminium: 50–60% cheaper per ampere, used for large power cables (70 mm² and above), HT cables, transformer connections, and underground feeders. Per IS 694 and standard Indian practice: copper is standard for motor feeders up to about 240 mm²; aluminium is common for LT and HT main feeders from transformers. Aluminium terminations require anti-oxidant compound.
How does cable size affect energy losses and electricity bills?: An undersized cable has higher resistance → higher I²R losses → more heat generated and wasted. For a 50 A load on 100 m run: 6 mm² cable (R = 3.08 Ω/km, resistance of run = 0.308 Ω each way) → I²R losses = 50² × 0.616 = 1,540 W = 1.54 kW per circuit. Upgrading to 10 mm² (R = 1.83 Ω/km, 0.366 Ω round trip): losses = 50² × 0.366 = 915 W — saving 625 W continuously. At ₹8/kWh, running 8,000 hr/yr: annual saving = 5,000 kWh = ₹40,000. Cable upgrades often pay back in 1–2 years in high-load circuits.
What cable size for a 3-phase 415V, 1 HP (0.75 kW) pump?: A 0.75 kW (1 HP) motor at 415 V, 3-phase, PF 0.82, η 0.80: full-load current = 750/(1.732×415×0.82×0.80) ≈ 1.6 A. Minimum cable size by ampacity: 1 mm² or 1.5 mm² is more than adequate (rated ≈ 15 A in conduit). In practice, 1.5 mm² is the minimum practical cable for motor feeders in India for mechanical protection. If the run is above 50 m, check voltage drop — a 1.5 mm² cable on a 100 m run at 1.6 A gives only about 1.5 V drop, well within the 5% limit.

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