Electrical Design

How to Size a Cable Correctly: Voltage Drop & Current Capacity

Undersized cable means hot terminations and nuisance trips. Oversized cable means wasted money. Here is the field-proven method for landing in between — design current, derating, and the voltage-drop check that actually bites on long runs.

Why sizing goes wrong

Most cable failures are not cable failures. A circuit trips weeks after installation, a junction runs warm to the touch, or a motor on a long feed runs sluggish. The common thread is almost always the same: the conductor was picked to carry the load but nobody checked voltage drop and derating. Carrying current is necessary, but it is not sufficient.

Step 1 — Establish the design current

Start from the load, not the breaker. For a motor, use the full-load current and then apply the motor branch-circuit multiplier. For a feeder serving several loads, sum the connected loads and apply a reasonable demand factor for loads that do not run simultaneously.

A common field shortcut is to size the cable to the breaker rating rather than the load. That over-sizes the conductor (safer, but wasteful) and can hide a genuinely overloaded branch. Size to the real design current, then make sure the breaker protects the cable.

Step 2 — Pick a base conductor from current-carrying capacity

Reference the appropriate ampacity table for your standard — NEC Table 310.16 (US), BS 7671 Appendix 4 (UK), or the IEC 60364 ampacity tables. Choose the column that matches your installation method: in free air, in conduit, in cable tray, buried direct, or in a thermally insulated wall. The same conductor carries far less current buried in insulation than in open tray.

Step 3 — Apply derating factors

Ampacity tables assume ideal conditions. Real installations derate. The three that matter most:

• Ambient temperature — cable in a hot roof space or next to a boiler carries less.
• Grouping / bundling — several cables bunched together share heat; apply the group correction factor.
• Length — handled separately by the voltage-drop check below.

Pick the smallest conductor whose derated ampacity still covers the design current. Do the math with the table values; do not eyeball "one size up."

Step 4 — The voltage-drop check (where jobs get lost)

This is the step that separates a good install from a failing one. For a single-phase circuit:

For a three-phase circuit:

where I is the design current in amps, L the one-way run length in metres, and Rcable the resistance per kilometre (Ω/km) at operating temperature. Multiply the result by 100 for a percentage and compare against your limit — typically 5% on general circuits, or 3% for lighting where flicker is noticed first.

Long runs are where undersizing bites. A 30 m run is usually fine on the ampacity pick; a 150 m run to a gate motor or a remote pump will often fail voltage drop and need to step up one or two sizes even though it carries current fine.

Step 5 — Short-circuit thermal check

For anything close to a high-fault transformer or a long sub-main, verify the conductor can survive a bolted fault until the protective device clears it. The adiabatic check:

where S is conductor cross-section (mm²), Ifault the prospective fault current, t the disconnection time, and k the conductor constant (copper ≈ 115 for PVC-insulated). If the fault current is high and the breaker slow, the cable may need to step up.

Common mistakes to avoid

• Sizing to the breaker instead of the real load.
• Forgetting the installation-method column and using free-air ampacity for a buried run.
• Skipping the voltage-drop check on long feeders.
• Ignoring grouping — five cables in one tray derate a lot.
• Using the fault current instead of the design current when computing drop.

Doing it by hand vs. in a tool

Doing all five steps by hand for a single cable is fine. Doing it for a whole panel, across forty circuits, with different tray groups and run lengths — that is exactly where a spreadsheet or a dedicated tool pays for itself in a morning.

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