How cable sizing works

The idea in two minutes

A cable is a pipe for current. Too thin and it gets hot, wastes power and, in a fault, melts before the breaker trips. Too thick and you pay for copper you never use. Sizing finds the thinnest cable that passes four tests.

The calculator runs all four tests on every standard size, picks the smallest that passes, and tells you which test decided it. Often it is not the one you expect: on long runs it is voltage drop, near big transformers it is the short-circuit test.

The complete process

These ten steps are the same in every country; only the tables and limits change. Formulas, clauses and worked examples are in docs/METHOD.md and docs/WORKED_EXAMPLES.md.

1. Load current. 3-phase: I = P ÷ (√3 × 400 V × pf × η). Single phase: I = P ÷ (230 V × pf × η). 1 HP = 0.746 kW of shaft power.
2. Design current Ib. Add spare for growth (25 % for a building supply) and 25 % for a motor branch.
3. Breaker or fuse. The next standard rating at or above Ib: 6, 10, 16, 20, 25, 32, 40, 63, 80, 100, 125 A …
4. Rating factors. Multiply the factors for air or ground temperature, soil, depth and neighbouring cables.
5. Thermal size. The table rating must be at least breaker rating ÷ total factor.
6. Voltage drop. ΔU = √3 × I × L × (R cos φ + X sin φ). Keep it within 3–5 %.
7. Motor starting. A direct-on-line motor draws 6–7 times its current while starting; the dip must not stall it.
8. Short circuit. Cross-section ≥ I × √t ÷ k (k = 143 copper XLPE, 94 aluminium XLPE, 115/76 for PVC).
9. Parallel cables. If no single cable is enough, use two or more identical cables side by side (they heat each other, so each is derated).
10. Earth conductor. Same size up to 16 mm², 16 mm² up to 35 mm², half the phase size above that.

Where the cable runs changes everything

The same 16 mm² copper cable can safely carry anywhere from about 60 A to 100 A depending on how it is installed, because what limits it is how fast heat escapes.

How it is done in Nepal

There is no separate Nepal wiring code. The rules come from the Electricity Regulation 2050 and the National Building Code NBC 207, and NBC 207 points to Indian Standards for the actual numbers. In practice, IS tables from Indian and Nepali manufacturers are the national default. Projects funded by ADB, AIIB or JICA usually follow IEC, so the calculator offers both.

The rules that apply

What NEA networks look like

• LV 230/400 V from 50, 100 and 200 kVA distribution transformers; aerial bundled cable 25–150 mm² from Nepali makers (IS 14255).
• 11 kV overhead: ACSR Dog (100 mm²), Rabbit (50 mm²), Weasel (30 mm²). 33 kV rural lines also use Dog.
• 11 kV underground: 3-core 300 mm² aluminium XLPE armoured has appeared in NEA tenders (rated 28.2 kA for 1 s).
• 132 kV: ACSR Bear; 220 and 400 kV: ACSR Moose; some 132 kV lines re-conductored with high-temperature ACCC.
• NEA's design criteria for distribution projects keep feeder voltage drop under 5 % at the target-year load.

What you must find out yourself

These are not published; ask NEA's distribution centre or your consultant: the fault level at your point of supply (11 kV switchgear in Kathmandu is rated 25 kA), the protection clearing time, and whether the LV network at your site is treated as TT or TN-C-S. With TT (local earth electrode only), an RCD is what disconnects an earth fault fast — fit 30 mA on socket circuits.

Climate across Nepal

Altitude: no cable standard applies an altitude factor; the cooler air at height already raises the rating. Altitude derating above ~1000 m is for equipment — breakers, drives, inverters. Strong sun at altitude is the real risk: cables in direct sun are not covered by any table, so shade them or use a conduit.

How the rest of the world does it

Every code follows the same logic. They differ in the reference temperature, the voltage-drop limit, and how they treat continuous loads.

Sources for these rows are mostly secondary (standard summaries, manufacturer guides); check the current edition for a formal design.

High voltage: 3.3 to 33 kV

The steps are the same, with three differences.

• There is no breaker rating to size for. The cable is sized on the load current, and the protection relay is then set at or below the cable's derated rating.
• The short-circuit test usually decides. 13.1 kA for 1 second needs 139 mm² of aluminium whatever the load is. Ask NEA for the fault level and the clearing time.
• The reference depth is deeper (0.9 m up to 11 kV, 1.05 m for 22–33 kV), and soil and temperature factors differ from LV. The calculator uses the MV tables from the Polycab HT catalogue, with KEI as a cross-check.

Earthed (E) grades suit systems where an earth fault is cleared in about a minute; unearthed (UE) grades are for systems that may run with an earth fault for longer. Single-core cables must have aluminium (non-magnetic) armour. The copper screen must carry the earth-fault current; check it in the datasheet.

Overhead and transmission lines

Bare overhead conductors (ACSR and AAAC) are not rated from tables like cables. Their rating comes from a heat balance: the sun and the current heat the conductor, the wind and radiation cool it. The calculator uses the IEEE 738-2012 method, the same one behind India's CEA transmission tables, and reproduces 156 published ratings within 0.6 %.

• Altitude matters here, unlike for cables. At Kathmandu's 1400 m the air is about 15 % thinner than in the Terai, so wind removes less heat and the same conductor carries less.
• Long 11 kV lines run out of voltage before they run out of current. Line reactance (about 0.35–0.4 Ω/km) dominates; 3 MVA over 15 km cannot be carried at 11 kV within 5 %. That is why NEA builds 33 kV.
• At 66 and 132 kV a minimum conductor applies (about Dog and Panther): thinner conductors cause corona losses and lack strength, even if they pass thermally.
• 220 and 400 kV use bundles and a different limit. Twin or quad Moose (450 mm spacing) lower the reactance and corona. At these voltages the substations hold the voltage with reactive compensation, so the planning limit is the line's loadability: surge impedance loading × the St. Clair factor for its length (about 3 up to 80 km, 2 at 160 km, 1.3 at 320 km), calculated with the long-line model.
• Losses are shown. When they exceed 3 % of the load, the next conductor that cuts them by a third is suggested; the saved energy often pays for it.

NEA's usual conductors: 11 kV Weasel, Rabbit, Dog; 33 kV Dog, Panther, Zebra; 132 kV Bear, Panther; 220 kV twin Moose or HTLS; 400 kV quad Moose (Dhalkebar–Muzaffarpur). Phase spacing and pole spans are typical values — check the NEA drawings.

Buying and installing

The calculator now lists everything to buy with its rating: breaker (MCB or MCCB, trip curve, poles, breaking capacity), RCD type and sensitivity, isolator (Rule 59) and changeover switch (Rule 57), surge protector, motor starter with overload setting and back-up fuse, power-factor capacitor, lugs, glands sized from the cable's diameter, conduit or buried pipe, earth electrodes, and for HV the RMU or breaker, HV fuses, arresters, CTs and the transformer's LV breaker.

At the shop

• The printing on the sheath must show the maker, the standard (IS 694, IS 1554, IS 7098), the size in mm², the number of cores and the voltage grade (1.1 kV, or 450/750 V for house wire).
• Look for the ISI/NS mark. Avoid unmarked cable and "copper-clad aluminium" sold as copper — scrape a strand: it must be copper all the way through.
• Count strands and check the weight on the label against the catalogue; undersized "2.5 mm²" wire is common.
• Buy about 10 % more than the measured route length.

Laying it

• Underground LV: at least 0.75 m deep (0.9 m for 11 kV, 1.05 m for 33 kV), on 75–100 mm of sand, covered with sand, then bricks or a cover slab, then warning tape 300 mm above.
• Use pipe under roads, driveways and walls. Seal the pipe ends.
• Do not bend tighter than 12 × the cable diameter (multicore) or 15–20 × for HV.
• Terminate with glands and crimped lugs of the right size and material; use bimetallic lugs between aluminium cable and copper terminals.

Before switching on

• Insulation test with a 500 V (LV) or 1000 V tester: phase-to-phase and phase-to-earth, at least 1 MΩ; new cable is normally hundreds of MΩ.
• Check continuity of the earth conductor and the polarity at the far end.
• Test the RCD with its button and, if possible, a tester. Measure the voltage at the far end under load and compare with the calculator.

Limits and sources

The calculator covers continuous ratings, voltage drop, motor starting, short circuit, parallel runs and earth conductor size. It flags but does not calculate: earth-fault loop impedance and disconnection time, harmonic (third-harmonic) neutral loading, cables in direct sun, and HV screen earth-fault capacity.

Data: your original workbook (IS 1554-1 / IS 7098-1 / IS 3961, with 12 transcription errors corrected), Polycab and KEI HT XLPE catalogues (IS 7098-2), Polycab and Gemscab 1.1 kV reactances, IEC 60364-5-52:2009 Annex B (via the identical GOST R 50571.5.52-2011), Electricity Regulation 2050, NBC 207:2003. Full source lists with links: research/world_practice.md, research/lv_reference.json, research/hv_xlpe_raw.json.

Items marked not verified could not be confirmed from a primary source and are left as user inputs in the calculator.