Cable sizing vs voltage drop: complete engineer's guide

Short answer

Cable sizing confirms that the conductor can carry the design current continuously without exceeding its thermal limit, after all derating factors are applied. Voltage drop confirms that the same conductor delivers a voltage at the load within the project or code limit over the actual route length. The two checks are separate. A cable can satisfy ampacity and still fail voltage drop on a long feeder. A cable can also satisfy voltage drop and still fail ampacity in a hot ceiling void bundled with other circuits.

Run both checks every time. The larger of the two cable sizes wins. Then coordinate the upstream circuit breaker so the protective device disconnects fast enough under short circuit and earth fault, and confirm the short-circuit withstand of the cable itself.

Why the two checks exist

Two independent physical limits govern low-voltage power cables. Ampacity is a thermal limit. When current flows, I²R losses heat the conductor and insulation. If the insulation exceeds its rated continuous temperature, its life degrades quickly and fault behaviour becomes unpredictable. Voltage drop is a performance limit. The impedance of the conductor consumes a share of the source voltage, and if the load sees too little voltage, motors overheat, LED drivers flicker, contactors chatter, and control gear misbehaves.

Codes handle these two limits differently. IEC 60364-5-52 and BS 7671 publish ampacity tables and derating factors. NEC Article 310 gives ampacity tables with separate temperature and adjustment factors, and NEC 210.19 and 215.2 give informative voltage drop targets. Local authority documents in the GCC, UK, EU, USA, and Australia typically reference one of these frameworks and may add stricter project limits.

Ampacity: what the cable can carry

The base current from tables

Every cable table gives a base ampacity for a reference condition, for example a single circuit clipped direct to a wall at 30°C ambient with 70°C or 90°C insulation. Actual installations rarely match the reference condition, so the base value must be corrected by derating factors before it can be compared to the design current.

Derating factors

• Ambient temperature (C_a). A cable in a plant room at 45°C is worse off than one in a corridor at 25°C. Look up the ambient factor for the insulation grade in the applicable table.
• Grouping (C_g). When multiple circuits share the same tray, ladder, conduit, or trench, mutual heating reduces ampacity. Tables list a factor by number of circuits and installation method.
• Installation method. Buried, in conduit, clipped, in free air, in a duct bank, on a tray with covers — each has its own base column.
• Soil thermal resistivity. For buried cables. The default table value assumes a specific resistivity; drier or rockier soil derates further.
• Harmonic content. Non-linear loads push neutral current up and can require a bigger conductor even when phase current looks fine.

The rule you actually use

Design current I_b must not exceed the corrected ampacity I_z. In code form: I_b ≤ I_n ≤ I_z, where I_n is the protective device rating. The corrected ampacity is I_z = I_tab × C_a × C_g × C_i × ..., where each C factor is a multiplier less than or equal to one.

Voltage drop: what actually reaches the load

The three-phase and single-phase formulas

The planning-level equations for voltage drop are:

• Single phase: V_d = 2 × I × L × (R cosφ + X sinφ) / 1000
• Three phase: V_d = √3 × I × L × (R cosφ + X sinφ) / 1000

Where I is the load current in amps, L is one-way route length in metres, R and X are the AC resistance and reactance per unit length at operating temperature (typically mOhm/m), and cosφ is the load power factor. For quick sizing, many designers use the manufacturer millivolt-per-amp-per-metre value from the cable data sheet: V_d = (mV/A/m × I × L) / 1000.

Route length matters more than nameplate current

Voltage drop is proportional to route length. Doubling the run doubles the drop. A 63 A feeder over 15 m might land at 1% drop; the same 63 A feeder over 90 m may exceed 5%. This is why long risers, remote HVAC plant, external car-park lighting, and site pumps often need one or two sizes larger than the ampacity check alone would suggest.

Target limits

Common planning limits are 3% for final lighting circuits, 5% total from source to load, and 2% for sensitive equipment or where the project specification tightens the requirement. GCC authority documents in the UAE, Saudi Arabia, and Qatar frequently mirror the IEC 5% target. NEC provides 3% branch and 5% total as informative recommendations.

Worked example: 40 kW motor at 90 m

Consider a 40 kW three-phase motor, 400 V, 50 Hz, power factor 0.85, efficiency 0.9, route length 90 m in a cable tray shared with two other loaded circuits, ambient 40°C, XLPE 90°C insulation.

1. Design current I_b. I_b = 40,000 / (√3 × 400 × 0.85 × 0.9) ≈ 75.5 A. Add a starting margin per motor rules; use 80 A as the design basis.
2. Trial conductor. Choose 25 mm² four-core XLPE with base ampacity around 111 A on tray.
3. Derate. Ambient factor at 40°C for XLPE ≈ 0.91. Grouping factor for 3 circuits on a tray ≈ 0.82. Corrected ampacity ≈ 111 × 0.91 × 0.82 ≈ 82.8 A. Above I_b, so ampacity is acceptable.
4. Voltage drop. For 25 mm² XLPE the manufacturer mV/A/m for three-phase is around 1.5. V_d = (1.5 × 75.5 × 90) / 1000 ≈ 10.2 V, which is 2.55% of 400 V. Below 5%, acceptable.
5. Breaker. Choose an 80 A MCCB with adjustable magnetic pickup and check the disconnection time under earth fault at the far end using the cable loop impedance. Confirm short-circuit withstand of the 25 mm² conductor against the prospective fault current at the origin.

If the same motor were placed 250 m away instead of 90 m, voltage drop would climb to about 7%, and the designer would move up to 35 mm² or 50 mm² on the voltage drop constraint alone.

Common mistakes to avoid

• Sizing on nameplate only. Motor starting currents, harmonic loads, and future spare capacity should be part of the design current, not an afterthought.
• Skipping derating. The base table value is optimistic. Ambient and grouping factors below 0.7 are common in real plant rooms and trays.
• Using one-way length for both phases. Single-phase formulas take ×2 for the return path. Three-phase does not.
• Confusing R with impedance. At larger cable sizes the reactance X is not negligible. Use the manufacturer mV/A/m data or the combined (R cosφ + X sinφ) form for accuracy above 35 mm².
• Ignoring the far-end fault check. Ampacity and voltage drop do not guarantee that the breaker will trip within the required time under an earth fault at the load end.
• Forgetting neutral sizing under harmonics. Data centres, LED drivers, and VFDs push third-harmonic neutral current. IEC 60364-5-52 Annex E gives adjustment factors.

How this connects to the calculators on this site

The Cable Sizing & Voltage Drop Planner runs both checks side by side. The Ampacity Derating Calculator applies the C-factor product for ambient, grouping, and installation method. The Breaker Size Calculator proposes the protective device rating and frame. Use them in that order and cross-check against your project specification and local code.

Frequently asked questions

Is 3% or 5% the correct voltage drop target?

Both. The typical convention is 3% on the final circuit and 5% cumulative from source to load. Project specifications may override this. When they do, the specification wins.

Do I include harmonics for a small office?

For lighting-and-power in a small office with modern LED and standard receptacles, planning-level harmonic content is usually low enough to skip. For a data centre, LED-heavy retail store, or any VFD-fed HVAC system, run the neutral sizing check.

Do I use conductor resistance at 20°C or at operating temperature?

At operating temperature. IEC and BS 7671 tables already publish the mV/A/m values at the maximum operating temperature of the insulation, so use those directly rather than the 20°C DC resistance from a data sheet.

How does route length affect the sizing?

Ampacity is independent of length. Voltage drop is directly proportional to length. Long routes are almost always voltage-drop limited, short routes are almost always ampacity limited.

Use these tools next

• Cable Sizing & Voltage Drop Planner
• Breaker Size Calculator
• Ampacity Derating Calculator
• Short Circuit Current Calculator
• Electrical Worked Example PDF

Verification note

This guide is planning support only. Final design must be verified against project specifications, authority requirements, official standards (IEC 60364-5-52, BS 7671, NEC 310/215/210, or the applicable local code), and manufacturer data. Consult a licensed electrical engineer for construction-issue drawings.