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REFERENCE · JULY 2026

IS 3961:2024 Ampacity Chart — Indian Standard Current Carrying Capacity Reference

Q: What is the ampacity of 1.5 sqmm aluminium vs copper?

1.5 sqmm copper: 24 A in air / 18 A in conduit. 1.5 sqmm aluminium: 18 A in air / 14 A in conduit — about 75–78 % of copper. Aluminium at 1.5 sqmm is rare in practice; Indian distributors stock aluminium from 4 sqmm upwards, and most utility designs start at 16 sqmm.

The current-carrying capacity (ampacity) of an Indian cable depends on five variables: (a) conductor material — copper vs aluminium, (b) cross-section in sqmm, (c) conductor class — Class 2 stranded vs Class 5 fine-stranded per IS 8130:2013, (d) installation method — single-core in air, in conduit, in trefoil, multi-core, or buried, and (e) ambient-temperature derating from the 30 °C (in air) / 20 °C (buried) reference. The master tables below give the published IS 3961:2024 values for the full 1.0–400 sqmm range. Voltage drop is tabulated in mV/A/m — the convention every Indian wiring manual uses — so the worked example at the bottom of this page can be copied into any BOQ.

CablePriceIndia.com is a third-party comparison reference. The values are reproduced from IS 3961:2024 (the current revision) and, where the older IS 3961 Part 2 is the only published value, the older edition is cited explicitly. Always verify the standard against the current edition on bis.gov.in before sealing a design.

How IS 3961:2024 ampacity values are derived

The ampacity of a cable is the steady-state current it can carry without exceeding its conductor temperature limit. The standard solves a heat-balance equation: the I²R loss in the conductor must equal the heat that the cable surface can dissipate into the surrounding medium (air, conduit wall, soil) at the assumed ambient temperature.

The three inputs to the calculation are:

Reference ambient temperature. IS 3961:2024 uses 30 °C for cables in air and 20 °C for cables buried direct in soil. Every published value in the master tables below assumes the reference. Field temperatures above the reference require a derating factor — see §6.
Conductor temperature limit (the "hot-spot" cap).

- 70 °C for plain PVC insulation (IS 694:2010 Type A, FR, FR-LSH) - 85 °C for HRFR / heat-resistant PVC - 90 °C for XLPE (IS 7098 Part 1) - 105 °C for elastomeric/EPR / silicone insulations (industrial use)

Conductor resistance. The R in I²R is taken from IS 8130:2013 — the conductor standard. Class 2 (stranded, fixed installation) has lower DC resistance per metre than Class 5 (fine-stranded, flexible) at the same sqmm, because Class 5 packs more, thinner strands and the geometry slightly increases the effective resistance. Class 5 ampacity is therefore 2–4 % lower than Class 2 at the same cross-section.

International cross-reference: IEC 60364-5-52 ("Selection and erection of electrical equipment — Wiring systems") publishes parallel ampacity tables that IS 3961 is broadly aligned with for installation methods A1, B1, C, D and E. Where IS 3961:2024 differs (typically the buried-direct values, which assume Indian soil thermal resistivity of 1.5 K·m/W), the IS value governs Indian designs.

> Reaction to Fire ≠ Fire Survival. FR, FR-LSH, HFFR, HRFR are Reaction to Fire classifications (smoke / halogen / flame-spread behaviour during a fire). They do not change the cable's steady-state ampacity, because the conductor and insulation temperature limits are unchanged. A Fire Survival / Circuit Integrity cable (IEC 60331 / BS 6387) is a different product entirely and has its own derating curves while in-fire — not relevant to normal sizing.

Master Table 1 — Copper conductor ampacity (IS 3961:2024)

Reference 30 °C ambient, single-phase or three-phase a.c. at 50 Hz, plain PVC (70 °C conductor limit). For XLPE (90 °C) multiply ampacity by ≈ 1.20. Values are for Class 2 conductor; Class 5 ampacity is ≈ 3 % lower (see §10 FAQ).

Size sqmm	Single core in air (A)	Single core in conduit (A)	2-core in air (A)	2-core in conduit (A)	3/4-core in air (A)	3/4-core in conduit (A)	Voltage drop mV/A/m
1.0	19	14	17	13	15	11	44
1.5	24	18	22	16	19.5	14	29
2.5	32	25	30	22	27	19.5	18
4	42	33	38	30	36	26	11
6	54	43	49	38	46	33	7.3
10	75	60	68	53	63	46	4.4
16	100	80	91	71	85	62	2.8
25	133	105	121	94	112	82	1.75
35	164	130	150	117	138	101	1.25
50	198	158	181	142	168	122	0.93
70	253	202	231	181	213	156	0.63
95	306	245	280	219	258	188	0.46
120	354	284	324	253	299	218	0.36
150	407	327	372	291	344	250	0.29
185	464	374	425	332	392	286	0.23
240	546	442	500	390	461	336	0.180

Sources: IS 3961:2024 Table 11–14 (in-air), Table 21–24 (conduit), Annex C voltage-drop values; cross-checked against Polycab Technical Brochure 2024-Q4 (FR/FRLS Wires, p. 18–22) and KEI House Wires Catalogue 2024 (p. 12).

Master Table 2 — Aluminium conductor ampacity (IS 3961:2024)

Aluminium has ≈ 1.6× the resistivity of copper, so a given sqmm of aluminium carries ≈ 78 % of the equivalent copper ampacity. Aluminium becomes economical only above 16 sqmm — sizes below that exist on paper but are rare in Indian practice.

Reference 30 °C ambient, plain PVC (70 °C conductor limit). For XLPE multiply by ≈ 1.20.

Size sqmm	Single core in air (A)	Single core in conduit (A)	2-core in air (A)	2-core in conduit (A)	3/4-core in air (A)	3/4-core in conduit (A)	Voltage drop mV/A/m
1.5	18	14	17	12	15	11	47
2.5	25	19	23	17	21	15	29
4	33	26	30	23	28	20	18
6	42	33	38	30	36	26	12
10	58	47	53	41	49	36	7.1
16	78	62	71	55	66	48	4.5
25	103	82	94	73	87	64	2.85
35	127	101	116	91	107	78	2.05
50	154	123	141	110	130	95	1.50
70	197	157	180	141	166	121	1.05
95	238	191	218	170	201	146	0.77
120	276	221	252	197	233	170	0.63
150	318	255	290	227	268	195	0.51
185	362	291	331	259	305	223	0.41
240	425	344	390	304	359	262	0.32
300	486	394	446	348	411	300	0.26
400	564	462	—	—	478	348	0.21

Sources: IS 3961:2024 Table 15–18, Table 25–28; cross-checked against Havells Power Cables Catalogue 2024 (p. 34, aluminium XLPE armoured) and Polycab Aluminium House Wire datasheet 2024.

Derating factors

Table values above are at reference conditions. Field installations almost always require derating. Multiply the table ampacity by all applicable derating factors to get the field ampacity.

6.1 Ambient temperature derating (cables in air, PVC 70 °C)

Ambient °C	Derating factor (PVC)	Derating factor (XLPE 90 °C)
25	1.06	1.04
30 (ref.)	1.00	1.00
35	0.94	0.96
40	0.87	0.91
45	0.79	0.87
50	0.71	0.82
55	0.61	0.76
60	0.50	0.71

Source: IS 3961:2024 Table 31; IEC 60364-5-52 Table B.52.14.

6.2 Grouping derating (multiple cables in a single conduit, tray, or trefoil)

Number of cables / circuits	Derating factor
1	1.00
2	0.80
3	0.70
4	0.65
6	0.57
9	0.50
12	0.45
16+	0.41

Source: IS 3961:2024 Table 32; IEC 60364-5-52 Table B.52.17.

6.3 Soil thermal resistivity derating (buried direct)

Soil thermal resistivity K·m/W	Derating factor
1.0 (wet sand, river soil)	1.13
1.5 (reference, average Indian soil)	1.00
2.0 (clay, dry-season agricultural)	0.91
2.5 (rocky, mixed back-fill)	0.84
3.0 (dry sandy / desert)	0.78

Source: IS 3961:2024 Table 35; NBC 2016 Part 8 §3.4.6 cites 1.5 K·m/W as the design default for plains India.

Voltage drop calculation

Use the mV/A/m column in the master tables. For a balanced three-phase circuit:

> V_drop (V) = √3 × I (A) × L (m) × Z_mV/A/m / 1000

For single-phase, drop the √3:

> V_drop (V) = 2 × I (A) × L (m) × Z_mV/A/m / 1000

(The factor of 2 accounts for line + neutral round-trip; the table's mV/A/m is per single conductor metre.)

The Z_mV/A/m in IS 3961:2024 already includes both the resistive (R) and reactive (X) components at 50 Hz at full conductor temperature. For sizes ≥ 50 sqmm the X component is non-negligible — IS 3961:2024 Annex C splits R and X if a power-factor-corrected calculation is needed.

NBC 2016 Part 8 §3.4.5 sets the voltage-drop limit at:

≤ 3 % between the consumer terminal and any final sub-circuit (lighting and general power)
≤ 5 % including the supply transformer where the user owns the transformer

IEEMA Technical Practice TP-22 reads the same limits. In practice, designers target ≤ 2 % for motor circuits to protect starting torque.

Worked example — sizing a 3 kW single-phase motor on a 30 m run

Step 1 — Full-load current. I = P / (V × pf) = 3000 / (230 × 0.85) = 15.3 A. Round up to the next standard MCB rating: 16 A. Step 2 — First cable pick from Master Table 1. 2.5 sqmm copper Class 5 (FR), in conduit: ampacity 25 A at 30 °C — passes the thermal check at 16 A. Step 3 — Voltage drop check. 2.5 sqmm copper, 18 mV/A/m, single-phase, 16 A, 30 m: V_drop = 2 × 16 × 30 × 18 / 1000 = 17.3 V ≈ 7.5 % of 230 V.

That fails NBC's 3 % limit by a wide margin. Step up.

Step 4 — Re-check with 4 sqmm. 4 sqmm copper, 11 mV/A/m, 16 A, 30 m: V_drop = 2 × 16 × 30 × 11 / 1000 = 10.6 V ≈ 4.6 %. Still fails. Step 5 — Re-check with 6 sqmm. 6 sqmm copper, 7.3 mV/A/m, 16 A, 30 m: V_drop = 2 × 16 × 30 × 7.3 / 1000 = 7.0 V ≈ 3.0 %. At the NBC limit — acceptable. Final spec: 6 sqmm copper Class 5 FR (or FR-LSH if the run passes through a basement / shaft of an NBC G+4+ residential building, per NBC 2016 Part 4 §A-3.4.13).

The lesson: at long runs, voltage drop — not ampacity — sets the cable size. The 2.5 sqmm passes the heat check at 16 A but fails the volt-drop check by a factor of 2.5×. Always run both.

How to use this chart

Compute the design current. I = load / (V × pf) for motors; sum of branch currents × diversity for distribution boards. Add a 25 % continuous-load factor for circuits feeding a continuously-loaded device (≥ 3 hours, per NBC 2016 §3.4.4).
Pick the column — match installation method (single-core in air, in conduit, multi-core in conduit, buried).
Select a candidate row whose ampacity ≥ design current.
Apply derating factors from §6 (ambient × grouping × soil). If the derated ampacity drops below the design current, step up one size and re-test.
Voltage-drop check against the NBC 3 % limit. Step up further if it fails. The worked example above shows this is often the binding constraint on long runs.
Round up to a standard size, never down.

Frequently asked questions

What is the current-carrying capacity of 2.5 sqmm copper cable?

Per IS 3961:2024, 2.5 sqmm copper Class 2 carries 32 A as a single core in free air, 25 A in conduit, and 19.5 A as part of a 3/4-core cable in conduit — all at 30 °C ambient and 70 °C conductor limit (plain PVC). Class 5 (flexible) is ≈ 3 % lower at 31 A / 24 A / 19 A respectively. For domestic 16 A socket circuits, the in-conduit figure (25 A) is the relevant one.

What is the current-carrying capacity of 4 sqmm cable?

4 sqmm copper: 42 A in air, 33 A in conduit, 26 A in 3/4-core conduit (IS 3961:2024). 4 sqmm aluminium: 33 A / 26 A / 20 A for the same three cases. 4 sqmm copper is the standard size for Indian 25 A circuits — geysers, 1.5 ton split AC, kitchen high-load — provided run length keeps voltage drop under 3 %.

What is the formula for ampacity calculation?

Ampacity is solved iteratively from the heat-balance equation: I² × R(Tc) = (Tc − Ta) / Rthermal, where Tc is the conductor temperature limit (70/85/90 °C), Ta is ambient (30 °C in air), R(Tc) is the AC resistance per IS 8130:2013 corrected to Tc, and Rthermal is the cable-to-medium thermal resistance per IEC 60287. IS 3961:2024 publishes the solved values in tabular form so engineers don't have to repeat the heat-balance solve in the field.

How do I derate cable for high ambient temperature?

Multiply the table ampacity by the factor in §6.1. At 45 °C ambient (typical industrial Indian summer), PVC-insulated cable derates to 0.79 of its 30 °C value — a 2.5 sqmm in-conduit cable rated 25 A becomes effectively 19.7 A. XLPE derates more gracefully (0.87 at 45 °C) because its 90 °C conductor limit gives more headroom.

What is the voltage drop in 2.5 sqmm copper cable per metre?

18 mV/A/m for 2.5 sqmm copper at 50 Hz, full conductor temperature, per IS 3961:2024 Annex C. For a 16 A load that's 0.288 V dropped per metre per single conductor — or about 0.58 V per metre on a single-phase circuit (line + neutral). On a 30 m run at 16 A, the drop is ~17 V or 7.5 % at 230 V; the 3 % NBC limit caps the run length at ~12 m at 16 A.

What size cable do I need for a 32 A circuit?

For a 32 A continuous load (e.g. a 7 kW geyser bank or 2.5 ton AC) in conduit at 30 °C ambient, 6 sqmm copper (43 A in conduit) is the minimum thermal pick; 4 sqmm (33 A) is borderline but only at short runs. Voltage drop usually pushes the answer to 6 sqmm even when 4 sqmm passes thermally. For aluminium: 10 sqmm (47 A in conduit) is the equivalent.

What size cable for an air conditioner?

Compute full-load current from the AC's nameplate (kW or BTU/hr ÷ 3.412 ÷ EER ÷ 230 ≈ amps). Typical Indian splits: 1 ton ≈ 5–6 A → 2.5 sqmm; 1.5 ton ≈ 7–9 A → 2.5 sqmm short run / 4 sqmm long; 2 ton ≈ 10–12 A → 4 sqmm; 2.5–3 ton ≈ 13–18 A → 6 sqmm. Always size for the starting current — typically 5–7× FLA — by checking the breaker, not the wire. NBC 2016 Part 8 §3.5 mandates a dedicated circuit per AC ≥ 1.5 ton.

What is the difference between Class 2 and Class 5 ampacity?

Class 2 (per IS 8130:2013) is stranded for fixed installation — a 2.5 sqmm Class 2 has 7 strands of 0.67 mm, a tighter geometry, and DC resistance ≤ 7.41 Ω/km. Class 5 is fine-stranded for flexibility — 2.5 sqmm has ~50 strands of ~0.25 mm and DC resistance ≤ 7.98 Ω/km. The higher resistance of Class 5 (≈ 8 % at 2.5 sqmm) translates to ≈ 3 % lower ampacity for the same heat-balance limit. House wires and flexible cables are almost always Class 5 in India; armoured power cables are Class 2.

Does FR-LSH have a different ampacity than FR?

No. FR and FR-LSH have the same conductor (Class 5 copper, IS 8130:2013) and the same insulation temperature limit (70 °C, plain PVC base). The LSH compound (low smoke, halogen content < 15 % per IS 694:2010) changes only the Reaction to Fire behaviour — smoke emission and halogen release during a fire — not the steady-state thermal envelope. Same applies to HFFR (HCl < 0.5 %, IS 17048:2018) versus FR. The exception is HRFR and HR-FR-LSH, which use a heat-resistant insulation rated 85 °C — those carry ≈ 8–10 % more ampacity than 70 °C-rated FR at the same sqmm.

What is the ampacity of 1.5 sqmm aluminium vs copper?

1.5 sqmm copper: 24 A in air / 18 A in conduit. 1.5 sqmm aluminium: 18 A in air / 14 A in conduit — about 75–78 % of copper. Aluminium at 1.5 sqmm is rare in practice; Indian distributors stock aluminium from 4 sqmm upwards, and most utility designs start at 16 sqmm.

How does cable installation method (conduit vs in air) affect ampacity?

Heat dissipates faster from a cable hung in free moving air than from one buried in a conduit, because the conduit wall blocks convection and adds thermal resistance. IS 3961:2024 publishes ≈ 20–25 % lower ampacity for "in conduit" vs "single core in free air" at the same sqmm. Multi-core cables in conduit lose another ≈ 10 % because three conductors share the same heat-dissipation surface. Buried direct in 1.5 K·m/W soil sits between the two: lower than in-air, higher than in-conduit-in-wall.

What is IS 3961:2024 (or Part 2)?

IS 3961 is the Bureau of Indian Standards code titled "Recommended current ratings for cables." It has been issued in parts for different cable types: Part 1 (paper-insulated lead-sheathed), Part 2 (PVC insulated, the most-cited part for building wire), Part 3 (rubber-insulated), Part 4 (XLPE), Part 5 (aerial bunched). The 2024 revision consolidates Parts 2 and 4 into a single document covering modern PVC and XLPE cables to IS 694:2010 and IS 7098 Part 1, and updates derating factors for current Indian climate data. Many design references and manufacturer brochures still cite IS 3961 Part 2 (the older edition) — its values are within ± 2 % of IS 3961:2024 for sizes up to 240 sqmm. Always verify the edition you are citing on bis.gov.in.

METHODOLOGY

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