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.

📅 June 9, 2026 ⚡ By CoreCalx Engineering Team

cable sizing voltage drop NEC

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:

S = (2 × ρ × I × L) / ΔV

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.

Quick reference: For copper at standard operating temperature, the voltage drop per ampere per meter for common cable sizes is approximately: 1.5 mm² = 25 mV/A/m, 2.5 mm² = 15 mV/A/m, 4 mm² = 9.5 mV/A/m, 6 mm² = 6.4 mV/A/m, 10 mm² = 3.8 mV/A/m, 16 mm² = 2.4 mV/A/m.

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:

ΔV = 230 × 0.03 = 6.9 V

Step 2 — Calculate minimum cross-section:

S = (2 × 0.0175 × 16 × 25) / 6.9 = 14.0 / 6.9 = 2.03 mm²

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:

ΔV_actual = 15 × 10⁻³ × 16 × 25 = 6.0 V (2.6%) ✓

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)

Parallel Cable Runs

In high-current applications where a single cable would be impractically large, engineers run multiple cables in parallel. Each cable carries a share of the total current, and the effective cross-section is the sum of all parallel conductors.

For parallel runs to work correctly:

Same cross-section and length: All parallel conductors must be identical in size and length to ensure equal current sharing. If one path has lower resistance, it will carry disproportionately more current and may overheat.
Same installation conditions: Conductors must share the same thermal environment. Running one cable in conduit and another in free air defeats the purpose.
Minimum cable count: NEC Article 310.4 permits parallel conductors for sizes 1/0 AWG and larger. Smaller conductors cannot be paralleled (with exceptions for specific listed applications).
Per-phase grouping: Each phase and neutral must include all parallel conductors. For a 3-phase system with two parallel cables per phase, you need six conductors total (two per phase), not three pairs randomly grouped.

Effective Area = N × S (where N = number of parallel cables, S = cross-section of each cable)

Example: Two parallel 70 mm² copper cables provide an effective cross-section of 140 mm², but with better heat dissipation than a single 150 mm² cable because the surface area is greater relative to the conductor volume.

Derating Factors in Practice

Cable current-carrying capacity is always quoted for a specific reference condition — typically 30°C ambient, single cable in free air, or a defined installation method. In real installations, multiple derating factors apply simultaneously:

Condition	Derating Factor	Reference
2-3 cables bunched	0.80	NEC Table 310.15(C)(1)
4-6 cables bunched	0.70	NEC Table 310.15(C)(1)
Ambient 40°C (vs 30°C ref)	0.91	NEC Table 310.15(B)(1)
Cable in thermal insulation (500mm+)	0.50	IEC 60364-5-52 Table B.52.2

These factors multiply together. A cable running at 40°C in a group of 4 has a combined derating of 0.91 × 0.70 = 0.637, meaning it can carry only 63.7% of its tabulated rating.

NEC Ampacity Quick Reference

The following table shows allowable ampacities for common copper conductor sizes under standard conditions (60°C terminals, 30°C ambient, per NEC Table 310.16):

AWG/kcmil	mm²	60°C Rating (A)	75°C Rating (A)	Typical Application
14 AWG	2.08	15	20	Lighting circuits
12 AWG	3.31	20	25	General purpose outlets
10 AWG	5.26	30	35	Window AC units, dryers
6 AWG	13.3	55	65	Subpanels, ranges
2 AWG	33.6	95	115	Service entrance, feeders

NEC Article 210.19(A)(1) FPN No. 4: Recommends sizing branch-circuit conductors to limit voltage drop to a maximum of 3% at the farthest outlet, and the total drop including feeders to no more than 5%. These are recommendations, not mandatory requirements, but they represent good engineering practice.

Use our Cable Cross-Section Calculator to quickly determine the right cable size, or the Voltage Drop Calculator to check voltage loss on existing runs.

⚡

CoreCalx Engineering Team

Electrical engineers and technical writers dedicated to creating free, accurate engineering calculation tools. Our team has hands-on experience in electrical systems, LED displays, and power distribution.

💬 Have a question about this topic or spot an error? Leave a comment below — we read every one and reply within 24 hours.