Guide · Electrical design

LT panel cable sizing checklist for Indian projects

By Indravadan Vala, MEP consultant (15+ years) and founder of AECVeda · 4 November 2026

Every LT panel schedule comes down to the same few checks for each feeder. Skip one, and a cable that looks fine on paper overheats in a crowded tray, or cannot survive the fault level at the panel. Use this checklist for every outgoing feeder, from the main LT panel to sub-distribution boards.

This is a design aid, not a substitute for your project specification, the applicable Indian Standards and manufacturers' data. Have every design checked by a qualified electrical engineer.

1. Start from the fault level at the panel

You need it for both the breakers and the cables, so work it out first. For a transformer fed from a strong grid, a quick estimate at the transformer terminals is:

Full-load current, I_FL = kVA × 1000 ÷ (√3 × 415)
Fault current ≈ I_FL × 100 ÷ %Z

For a 500 kVA transformer with 4.5% impedance: I_FL ≈ 696 A, so the fault level is about 15.5 kA. Upstream impedance and the busduct or cable to the panel reduce this slightly; a detailed calculation should include them.

2. Work out the design current of each feeder

Three-phase: I_b = P ÷ (√3 × 415 × pf × η)
Single-phase: I_b = P ÷ (230 × pf × η)

P is in watts, pf is the power factor and η the efficiency (use 1 for non-motor loads). Apply the demand or diversity factor you have agreed for the load, and keep single-phase loads balanced across R, Y and B.

3. Choose the breaker

• Rating: I_n ≥ I_b. Many designers add a margin for continuous loads (for example 1.25 × I_b) and pick the next standard rating.
• Type: MCBs for small final circuits; MCCBs where the rating or the fault level is beyond MCB range; ACBs for incomers and large feeders.
• Breaking capacity: the breaker's rated short-circuit breaking capacity must be at least the fault level where it is installed. In the example above, a 10 kA MCB is not enough at 15.5 kA.
• Poles: 4-pole where the neutral must be switched (for example on incomers with a DG changeover), as your specification requires.

4. Size the cable for current, after derating

Take the base current rating from the cable maker's or standard tables for the cable type and laying method, then multiply by the correction factors for site conditions:

• Ambient air or ground temperature (check the base temperature of the table you are using)
• Grouping: the number of circuits touching on a tray or ladder, and the number of trays stacked
• For buried cables: depth of laying, soil thermal resistivity and spacing between cables

The derated rating I_z must be at least the breaker rating: I_b ≤ I_n ≤ I_z. Grouping is the factor most often forgotten. Six touching circuits on one tray can cut the rating by roughly a quarter or more.

5. Check the voltage drop

Three-phase VD% = √3 × I_b × L × (R cos φ + X sin φ) × 100 ÷ 415

R and X are the cable's resistance and reactance in ohms per km at operating temperature, and L is the route length in km. Add the drop on the incoming link and on upstream panels: the limit applies from the source to the load. Use the limits in your project specification. Many Indian projects allow about 3% on a feeder and about 5% from the transformer to the farthest load. Long single-phase runs, such as a guard room or street lighting, are usually where the voltage drop decides the size.

6. Check short-circuit withstand

The cable must survive the fault until the breaker clears it. The adiabatic check is:

S_min = √(I²t) ÷ k

• For MCBs and MCCBs, take the let-through energy I²t from the manufacturer's curves at the prospective fault current.
• For breakers with a time delay (for example ACBs), use I²t = I_k² × t.
• For XLPE insulation (90 °C to 250 °C), k is about 143 for copper and 94 for aluminium.

Small cables close to a high-fault panel often fail this check even when they pass on current and voltage drop.

7. Size the earth conductor

A common table method (from IEC 60364-5-54) for a protective conductor of the same material as the phase conductor of size S:

Your earthing scheme and specification may ask for more, for example two earth conductors for three-phase feeders.

8. Don't forget the neutral and motors

• Reduced neutral (3.5-core): fine for balanced linear loads. For UPS, VFD, LED lighting and IT loads with heavy harmonics, use a full-size neutral (4-core) or derate.
• Motors: direct-on-line starting draws several times full-load current. Check the starting voltage drop and the breaker's tripping curve against the motor's starting time.

9. Record the governing check

For each feeder, note which check decided the size: current, voltage drop or short circuit. It makes design reviews faster and tells you what to look at first when a load or a length changes.

The checklist

• Fault level at the panel worked out first
• Design current from load, power factor, efficiency and demand factor
• Breaker rating ≥ design current; breaking capacity ≥ fault level
• Cable rating after all derating factors ≥ breaker rating
• Voltage drop within the feeder and cumulative limits
• Short-circuit withstand: S ≥ √(I²t) ÷ k
• Earth conductor sized; neutral suitable for harmonics
• Motor starting checked; governing check recorded

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