What are the two criteria for cable sizing per IS 732?

IS 732 requires every cable to satisfy two simultaneous criteria: (1) Current carrying capacity — the cable must carry the design current without exceeding its maximum operating temperature; and (2) Voltage drop — the voltage drop must not exceed 3% of the supply voltage at full load. The cable size that satisfies both criteria governs.

What derating factors apply to cables in conduit per IS 732?

IS 732 Table 22 provides grouping derating factors: 2 cables in conduit = 0.80; 3 cables = 0.70; 4 cables = 0.65; 5 cables = 0.60; 6 cables = 0.57. Ambient temperature above 30°C also requires derating per IS 732 Table 16. Both factors must be multiplied together.

What is the minimum cable size for a 15A circuit in India?

For a 15A single-phase circuit, IS 732 requires a minimum of 2.5mm² copper cable when clipped direct to surface, or 4mm² if installed in conduit (after applying grouping derating). Always verify the voltage drop for the actual cable length — long runs may require a larger cable to meet the 3% limit.

When does voltage drop govern cable sizing vs current capacity?

Voltage drop typically governs for long cable runs with moderate current — typically runs exceeding 50m for branch circuits. Current carrying capacity typically governs for short, heavily loaded circuits. For feeders over 100m in large buildings, voltage drop almost always dictates a larger cable than thermal capacity alone would require.

How do I size a cable for a three-phase motor?

Step 1: Calculate full-load current (I = kW / (√3 × V × PF × η)). Step 2: Select cable from IS 732 tables with rated current ≥ I × 1.25 (motor starting factor). Step 3: Apply derating factors for installation method and ambient temperature. Step 4: Check voltage drop per IS 732 3% limit. The governing size satisfies both checks.

Cable Sizing as per IS 732 — Current Capacity and Voltage Drop Method

Cable sizing is the most fundamental calculation in LV electrical design. Pick a cable that is too small and it overheats — becoming a fire hazard and a reliability risk. Pick one that is too large and you waste material and cost. IS 732 (Code of Practice for Electrical Wiring Installations) mandates that every cable satisfies two simultaneous criteria: current carrying capacity and voltage drop. Both checks must pass — whichever requires the larger cable governs.

The Two-Criteria Rule of IS 732

Criterion 1 — Current Carrying Capacity (Thermal Limit)

Every cable has a maximum continuous current rating determined by its conductor cross-section, insulation type, and installation method. Exceeding this rating causes the insulation to overheat, degrade, and potentially cause fire. IS 732 Tables 4–9 give rated currents for various cable sizes and installation methods at a base ambient temperature of 30°C.

Criterion 2 — Voltage Drop (3% Limit)

Even if a cable can thermally carry the current, a thin long cable in a long run may cause excessive voltage drop. IS 732 limits this to 3% of the nominal supply voltage on any branch circuit. For 230V single-phase: max 6.9V. For 415V three-phase: max 12.45V.

Step-by-Step Cable Sizing Method

Step 1 — Calculate Design Current

Single Phase: I = P / (V × PF) Three Phase: I = P / (√3 × V × PF × η) For motor circuits, multiply by 1.25 (IS 732 overcurrent protection): I_design = I_FLC × 1.25

Step 2 — Select Cable from IS 732 Tables (Thermal Check)

Select a cable whose derated current capacity ≥ design current. Derate the tabulated current for installation method and ambient temperature:

I_rated_derated = I_table × Ca × Cg Where: I_table = Tabulated current from IS 732 (at 30°C, clipped direct) Ca = Ambient temperature derating factor Cg = Grouping (bunching) derating factor

Step 3 — Apply Derating Factors

Installation Method Factor: IS 732 base tables assume cables clipped direct to surface. For other methods:

Installation Method	Factor vs Clipped Direct
Clipped direct (base)	1.00
In conduit (single cable)	0.90
In trunking (touching)	0.80
Buried in ground (direct)	0.85–1.00 (soil dependent)

Grouping Factor (Cg) — IS 732 Table 22:

No. of Cables Grouped	Cg (touching, enclosed)
1	1.00
2	0.80
3	0.70
4	0.65
6	0.57
9	0.50

Ambient Temperature Factor (Ca) — IS 732 Table 16 (for 70°C PVC cables):

Ambient Temp (°C)	Ca
25	1.03
30 (base)	1.00
35	0.94
40	0.87
45	0.79
50	0.71

Step 4 — Check Voltage Drop

Single Phase: Vd = (2 × L × I × ρ) / A Three Phase: Vd = (√3 × L × I × ρ) / A ρ (resistivity): Copper PVC at 70°C: 0.0196 Ω·mm²/m Copper XLPE at 90°C: 0.0206 Ω·mm²/m Aluminium PVC at 70°C: 0.0320 Ω·mm²/m Check: Vd/V ≤ 3% If Vd exceeds 3%, select next larger cable size and repeat.

Fully Worked Example

Problem: Size a cable for a 7.5 kW, 415V three-phase motor located 65m from the MCC. The cable runs in a conduit with 2 other circuits (3 circuits total). Ambient temperature is 40°C. Cable type: Copper PVC (4-core).

Step 1 — Full Load Current: I_FLC = 7500 / (1.732 × 415 × 0.86) = 7500 / 617.9 = 12.1 A I_design = 12.1 × 1.25 = 15.1 A (for motor starting protection) Step 2 — Select initial cable from IS 732 Table (clipped direct): 4mm² copper: 32A rated (base) Step 3 — Apply derating: Ca (40°C) = 0.87 Cg (3 circuits in conduit) = 0.70 I_derated = 32 × 0.87 × 0.70 = 19.5 A 19.5 A > 15.1 A ✓ Thermal check PASSES for 4mm² Step 4 — Voltage drop check (4mm² copper, 65m, 12.1A): Vd = (√3 × 65 × 12.1 × 0.0196) / 4 = (1.732 × 65 × 12.1 × 0.0196) / 4 = 26.56 / 4 = 6.64 V Vd% = (6.64 / 415) × 100 = 1.60% ✓ PASSES 3% limit Result: 4mm² copper PVC cable is suitable. - Thermal check: 19.5A derated capacity > 15.1A ✓ - Voltage drop: 1.60% < 3% ✓

Use our Cable Size Calculator to check both IS 732 criteria simultaneously for any single-phase or three-phase circuit. Also use the Voltage Drop Calculator to quickly verify drop on any proposed cable size.

IS 732 Cable Current Capacity — Quick Reference (Copper PVC, Clipped Direct, 30°C)

Cable Size (mm²)	Single Phase (A)	Three Phase (A)
1.5	17.5	15.5
2.5	24	21
4	32	28
6	41	36
10	57	50
16	76	68
25	101	89
35	125	110
50	151	134

Common Cable Sizing Mistakes

Applying only the thermal check: Many engineers size cables for current capacity alone without checking voltage drop. For runs over 40–50m, voltage drop almost always requires a larger cable.
Not applying all derating factors simultaneously: Ambient temperature AND grouping factors must both be applied. In a plant room at 45°C with 6 cables in a trunking, the combined derating can be 0.79 × 0.57 = 0.45 — less than half the base rating.
Using the same cable for both the motor feeder and the control circuit: Control cables (4-core 1.5mm²) must be separate from power cables. Never run control and power in the same conduit per IS 732.
Ignoring the protective conductor (earth) size: IS 732 Table 54-7 specifies minimum earth conductor size based on the phase conductor size. For 16mm² phase conductor, the minimum earth conductor is 16mm²; for 50mm²+, the earth must be 25mm².

Conclusion

Cable sizing per IS 732 is a two-step process: first confirm thermal adequacy with correct derating, then verify that voltage drop at the design current stays within 3%. Both checks must pass. The larger resulting cable size governs. This systematic approach ensures your electrical installation is safe, code-compliant, and performs reliably throughout its service life.

Use the MEPMate Cable Size Calculator to automate both checks for any LV circuit in seconds.