Electrical Cable Sizing IS 1554 — Worked Example India

Electrical

Electrical Cable Sizing IS 1554 — Worked Example

Cable sizing is one of the most fundamental electrical engineering calculations, yet it is frequently done incorrectly in Indian projects — resulting in undersized cables that overheat and create fire risk, or oversized cables that waste money. This article walks through the complete IS 1554 cable sizing methodology with a full worked example.

1. IS 1554 — Overview

IS 1554 (Specification for PVC insulated cables for working voltages up to and including 1100 V) is the primary Indian standard for LV power cables. IS 1554 Part 1 covers single-core cables; Part 2 covers multicore cables. For cable sizing, the relevant parameters from IS 1554 are the current-carrying capacity tables and the derating factor methodology.

2. Cable Sizing Steps

Step	Parameter	Method
1	Design current (Ib)	Calculate load current from power, voltage, power factor
2	Protection device rating (In)	In ≥ Ib — select standard fuse or MCB/MCCB size
3	Current rating required (It)	It ≥ In — after applying derating factors
4	Select cable (Iz)	Iz ≥ It — select from IS 1554 tables
5	Voltage drop check	Calculate voltage drop — must be ≤ 2.5% for power circuits
6	Short circuit check	Verify cable withstands fault current until protection clears

3. Derating Factors

The current ratings in IS 1554 are based on standard conditions: 30°C ambient air, single circuit, installed in free air. Real installations deviate from these conditions — derating factors must be applied.

Ambient Temperature Derating (Ca)

Ambient Temperature (°C)	Derating Factor Ca (PVC insulation)
25	1.06
30 (reference)	1.00
35	0.94
40	0.87
45	0.79
50	0.71

Note: In Indian electrical rooms and cable trenches, ambient can reach 45–50°C in summer — always check actual conditions and apply appropriate Ca.

Grouping Derating (Cg)

Number of Cables / Circuits	Derating Factor Cg (touching)
1 (reference)	1.00
2	0.80
3	0.70
4	0.65
5	0.60
6	0.57
7–9	0.52
10–12	0.50

Installation Method Derating

Installation Method	Typical Derating vs Free Air
Free air (clipped or on ladder)	1.00 (reference)
On cable tray, single layer	0.95
In conduit (in wall, PVC)	0.77
Buried direct in ground	Depends on soil thermal resistivity — see IS 1554
In cable duct (underground)	0.75

4. Worked Example

Cable from MDB to an air-handling unit (AHU) with the following parameters:

Input Parameter	Value
AHU motor power	15 kW
Supply voltage	415V 3-phase 4-wire
Power factor	0.85
Efficiency	92%
Ambient temperature in cable route	40°C
Installation method	Cable tray — 3 circuits touching
Cable length	35 m
Cable type	IS 1554 PVC armoured multicore

Step 1 — Design Current

Ib = P / (√3 × V × PF × η) = 15,000 / (1.732 × 415 × 0.85 × 0.92) = 26.7 A

Step 2 — Protection Device

Select 32A MCB (In = 32A ≥ Ib = 26.7A) ✓

Step 3 — Required Current Rating After Derating

Combined derating factor = Ca × Cg = 0.87 (40°C) × 0.70 (3 circuits) = 0.609

Required It = In / combined derating = 32 / 0.609 = 52.5 A

Step 4 — Cable Selection

From IS 1554 tables: 10 mm² 4-core PVC armoured cable has Iz = 57 A in free air. After derating: 57 × 0.609 = 34.7 A — insufficient.

Try 16 mm² 4-core: Iz = 76 A in free air. After derating: 76 × 0.609 = 46.3 A — still insufficient.

Try 25 mm² 4-core: Iz = 99 A in free air. After derating: 99 × 0.609 = 60.3 A ≥ 52.5 A ✓

Preliminary cable selection: 25 mm² 4-core IS 1554 PVC armoured cable.

Step 5 — Voltage Drop Check

Voltage drop formula (3-phase): ΔV = √3 × I × L × (R cosφ + X sinφ) / 1000

For 25 mm² cable: R = 0.927 mΩ/m, X ≈ 0.08 mΩ/m

ΔV = 1.732 × 26.7 × 35 × (0.927 × 0.85 + 0.08 × 0.527) / 1000 = 1.25 V

% Voltage drop = 1.25 / 415 × 100 = 0.30% — well within 2.5% limit ✓

Result

Select: 25 mm² 4-core PVC armoured cable to IS 1554, with 32A MCB protection. Cable is satisfactory for current rating and voltage drop.

5. Summary Checklist

Calculate design current (Ib) accurately — include power factor and efficiency
Select protection device In ≥ Ib
Determine all applicable derating factors — ambient temp + grouping + installation method
Required It = In ÷ combined derating factor
Select cable with Iz ≥ It from IS 1554 current rating tables
Verify voltage drop ≤ 2.5% for power, ≤ 1% for lighting
Check fault current withstand — especially for long cables or large fault currents

// from the field — engineer’s notebook

From the Field — Where IS 1554 Cable Sizing Trips Up on Indian Sites

IS 1554-1 is a clear standard, but the cable schedule on real Indian projects gets messy where the standard meets reality. Three recurring traps on our hotel and commercial portfolio: (1) cable derating tables from IS 3961 are correctly applied for ambient temperature (40°C in most of India, 45°C+ in Rajasthan and inland Maharashtra) but routinely ignored for grouping in cable trays — three 4-core cables on a single tray drop to 0.85 of the IS 3961 base ampacity, and beyond six cables you are below 0.72; (2) the voltage drop check against IE Rules 2003 (≤ 3% at sub-DB / ≤ 5% combined feeder + final) is the binding constraint for runs over 80 metres, not ampacity, and our internal rule of thumb is to size for VD on any feeder above 60 m; (3) fault-level withstand per IS 1554-1 Clause 12 must be checked against the upstream protection’s let-through I²t — a 70 sqmm Cu cable rated for 8,540 A short-circuit at 1 second is a comfortable headline number but barely survives a 36 kA fault with a 0.2-second protection clearing time.

The MEPVAULT cable-sizing template embeds all three checks (ampacity, VD, I²t) in a single spreadsheet and flags the binding constraint per cable. Newer engineers should sense-check at least these three before committing the cable schedule to BOQ.

Electrical Cable Sizing IS 1554 — Worked Example India