Complete Cable Sizing Guide: How to Choose the Right Wire
Learn how to size cables correctly using voltage drop calculations, current-carrying capacity tables, and NEC guidelines.
Why Cable Sizing Matters
Undersized cables overheat, waste energy through excessive voltage drop, and create fire hazards. Oversized cables waste money and make installation difficult. Getting the right size is one of the most fundamental skills in electrical engineering, yet it is also one of the most commonly mishandled tasks on site.
A properly sized cable must satisfy three conditions simultaneously: it must carry the full load current without exceeding its temperature rating, it must keep voltage drop within acceptable limits, and it must withstand prospective fault current long enough for protective devices to operate.
The Fundamental Cable Sizing Formula
The minimum cross-sectional area required to limit voltage drop is calculated as:
Where:
- S = cross-sectional area in mm²
- ρ = resistivity of conductor material (0.0175 Ω·mm²/m for copper at 70°C)
- I = load current in amps
- L = one-way cable length in meters
- ΔV = permissible voltage drop in volts
The factor of 2 accounts for the outgoing and return conductors in a single-phase circuit. For three-phase balanced loads, replace 2 with √3 and use line-to-line voltage.
Worked Example: 16A Circuit at 230V Over 25m
Let us size a cable for a 16A socket outlet at 230V single-phase, 25 meters from the distribution board. The NEC recommends a maximum voltage drop of 3% for branch circuits.
Step 1 — Determine permissible voltage drop:
Step 2 — Calculate minimum cross-section:
Step 3 — Select the next standard size up: 2.5 mm².
Step 4 — Verify current-carrying capacity. A 2.5 mm² copper PVC cable in conduit can carry approximately 20–27A depending on installation method, which exceeds our 16A load. Good.
Step 5 — Verify actual voltage drop: using 15 mV/A/m for 2.5 mm² cable:
This is within the 3% limit, so 2.5 mm² is adequate.
Standard Cable Sizes Reference
The following table shows common metric and AWG cable sizes with their approximate current ratings for copper conductors in free air:
| mm² | AWG | Typical Rating (A) | Common Use |
|---|---|---|---|
| 1.5 | 16 | 10–16 | Lighting circuits |
| 2.5 | 14 | 16–20 | General sockets |
| 4.0 | 12 | 25–32 | High-power sockets |
| 6.0 | 10 | 32–40 | Cookers, showers |
| 10 | 8 | 40–50 | Sub-mains |
| 16 | 6 | 50–63 | Sub-mains, EV chargers |
| 25 | 4 | 63–80 | Main feeds |
Common Mistakes in Cable Sizing
- Ignoring voltage drop on long runs: Current capacity alone is not enough. A 1.5 mm² cable may carry 16A, but over 40 meters the voltage drop exceeds 4%.
- Using one-way length instead of round-trip: The formula requires one-way length (the factor of 2 handles the return). But some engineers accidentally double-count.
- Not derating for installation method: Cables in thermal insulation, conduit, or groups carry significantly less current than the same cable in free air.
- Forgetting temperature correction: In hot environments (attics, tropical climates), cable ratings must be reduced using correction factors from NEC Table 310.15(B)(2)(a).
- Ignoring power factor on long runs: For inductive loads on long cable runs, the impedance (not just resistance) determines voltage drop.
When to Upgrade Cable Size
Always go one size up when any of these conditions apply:
- The cable runs through thermal insulation for more than 500mm
- The circuit may be extended in the future
- The installation is in a high ambient temperature environment (above 30°C)
- Multiple circuits are bunched together in a single conduit or trunk
- The load is a motor with high starting current (use 1.25× rated current minimum)
Use our Cable Cross-Section Calculator to quickly determine the right cable size for your specific installation parameters.