Question 1

What is the current carrying capacity of 2.5mm² cable per IS 3961?

Accepted Answer

Per IS 3961 Part 2 Table 4 at 40°C ambient: 2.5mm² copper PVC insulated cable carries 22A (single phase clipped direct) or 19A (three phase clipped direct). In conduit: 18A (1φ) or 16A (3φ). Apply derating factors for bunching or elevated ambient temperature.

Question 2

What is the voltage drop formula for cable sizing per IS 3961?

Accepted Answer

Voltage drop (V) = Voltage drop factor (mV/A/m from IS 3961 table) × Current (A) × Run length (m) ÷ 1000. Maximum permissible voltage drop per IS 3961 is 5% from the origin of the installation to the load point. For single phase: VD = 2ρL/A. For three phase: VD = √3 × ρ × L / A.

Question 3

What derating factor applies for cable in conduit per IS 3961?

Accepted Answer

IS 3961 Part 2 specifies a derating factor of 0.80 for PVC cables installed in conduit or trunking. For cables bunched in a tray (7-9 cables): derate to 0.70. Multiple derating factors are multiplied together.

Question 4

What is the maximum operating temperature for IS 694:2010 FR cable?

Accepted Answer

IS 694:2010 PVC insulated FR cable has a maximum conductor operating temperature of 70°C. Emergency short-circuit temperature: 160°C. IS 3961 ampacity tables are based on 40°C ambient temperature and 70°C conductor temperature.

Size (mm²)	Clipped Direct (A)		In Conduit (A)		Voltage Drop (mV/A/m)
Size (mm²)	1φ / 2-wire	3φ / 3-wire	1φ / 2-wire	3φ / 3-wire	Single phase	Three phase
1 mm²	13	11	11	9	42.800	37.065
1.5 mm²	16	14	14	12	29.600	25.634
2.5 mm²	22	19	18	16	18.440	15.969
4 mm²	29	25	24	21	11.740	10.167
6 mm²	37	32	31	27	7.820	6.772
10 mm²	50	44	42	36	4.680	4.053
16 mm²	66	57	56	49	2.940	2.546
25 mm²	87	75	73	63	1.878	1.626
35 mm²	107	93	90	77	1.342	1.162
50 mm²	133	116	112	96	0.944	0.818
70 mm²	169	147	143	122	0.674	0.584
95 mm²	205	179	174	150	0.494	0.428
120 mm²	236	207	201	173	0.392	0.339
150 mm²	272	238	234	201	0.316	0.274
185 mm²	312	273	268	232	0.256	0.222
240 mm²	369	323	321	279	0.197	0.171

Installation Method	Derating Factor	Notes
Open air / clipped direct	1.00	No derating. Full rated current.
In conduit or trunking	0.80	20% derating. Most common installation.
Bunched — 2 cables in tray	0.85	15% derating.
Bunched — 3 cables in tray	0.79	21% derating.
Bunched — 4–6 cables in tray	0.75	25% derating.
Bunched — 7–9 cables in tray	0.70	30% derating.
Bunched — 10–12 cables in tray	0.67	33% derating.
Buried direct in ground (20°C soil)	0.95	IS 3961 Part 3.
Ambient 30°C (correction factor)	1.07	Better than 40°C design base.
Ambient 45°C (correction factor)	0.94	Derating for hot climates.
Ambient 50°C (correction factor)	0.87	Common in Indian industrial sites.

Load	3 kW motor, 415V 3-phase, PF = 0.85
Step 1: Full Load Current	FLA = 3000 ÷ (√3 × 415 × 0.85) = 4.92 A
Step 2: Design Current (with 1.25× starting)	Design = 4.92 × 1.25 = 6.15 A
Step 3: Apply derating (conduit 0.80)	Required capacity = 6.15 ÷ 0.80 = 7.69 A
Step 4: Select cable from IS 3961	1.5mm² in conduit = 12A ≥ 7.69A ✓
Step 5: Verify voltage drop (30m run)	VD = √3 × 4.92 × 30 × 14.8 mV/A/m ÷ 1000 = 3.77V = 0.91% ✓ (<5%)
Result	1.5mm² FR IS 694:2010 in conduit — compliant per IS 3961

IS 3961 Cable Ampacity & Voltage Drop Reference

IS 3961 Part 2 — Current Carrying Capacity & Voltage Drop

Derating Factors — IS 3961 Part 2

Worked Example — Cable Sizing per IS 3961

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