Cable Sizing & Voltage Drop: engineer's guide
Cable sizing checks that the conductor can carry the design current after all derating. Voltage drop checks that the load still sees adequate voltage over the actual route length. Both are independent, both must pass.
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What this calculator does
This calculator runs both the ampacity check and the voltage drop check side by side. It applies ambient, grouping, and installation-method derating to the base ampacity, then applies the impedance-based voltage drop formula to the same trial cable, and reports the larger required conductor.
Formulas used
- Design current, 3-phase:
Ib = P / (√3 × VLL × cosφ × η) - Ampacity rule:
Ib ≤ In ≤ Iz, where Iz = Itab × Ca × Cg × Ci - Voltage drop, 3-phase:
Vd = √3 × I × L × (R cosφ + X sinφ) / 1000 - Voltage drop, 1-phase:
Vd = 2 × I × L × (R cosφ + X sinφ) / 1000
Variables
- Ib = design current, amps
- In = protective device rating, amps
- Iz = corrected ampacity of the cable, amps
- Itab = base ampacity from the code table
- Ca, Cg, Ci = ambient, grouping, and installation-method derating factors (≤ 1)
- L = one-way route length in metres
- R, X = AC resistance and reactance per unit length at operating temperature (mΩ/m)
- cosφ = load power factor
Worked example
Scenario: 40 kW three-phase motor, 400 V, cosφ 0.85, efficiency 0.9, 90 m route on a tray shared with two other loaded circuits, 40°C ambient, XLPE insulation.
- Ib = 40,000 / (√3 × 400 × 0.85 × 0.9) ≈ 75.5 A. Use 80 A design basis for a motor.
- Trial 25 mm² four-core XLPE. Base ampacity on tray ≈ 111 A.
- Ca at 40°C for XLPE ≈ 0.91. Cg for 3 grouped circuits ≈ 0.82. Iz = 111 × 0.91 × 0.82 ≈ 82.8 A. Ampacity satisfied.
- Voltage drop: mV/A/m ≈ 1.5 for 25 mm² XLPE. Vd = 1.5 × 75.5 × 90 / 1000 ≈ 10.2 V ≈ 2.55% of 400 V. Under 5%, voltage drop satisfied.
- Select an 80 A protective device with adjustable magnetic pickup; confirm short-circuit withstand of 25 mm² against the prospective fault current at the origin.
Common mistakes to avoid
- Sizing on motor nameplate current without a starting margin
- Skipping grouping and ambient derating below 0.7
- Using single-phase drop formula on a three-phase feeder (or vice versa)
- Ignoring the far-end fault disconnection time check
- Forgetting neutral upsizing under high third-harmonic loads
Frequently asked questions
Is 3% or 5% the correct voltage drop target?
Both. 3% on final circuits, 5% total from source to load is the common convention. Project specifications may tighten it.
Do I include harmonics for a small office?
Not usually. For data centres, LED-heavy retail, or VFD HVAC, yes — run the neutral sizing check per IEC 60364-5-52 Annex E.
Do I use conductor resistance at 20°C or operating temperature?
Operating temperature. Code tables already publish mV/A/m at the maximum insulation temperature.
Which drives sizing more, ampacity or voltage drop?
Short routes are usually ampacity-limited; long routes are usually voltage-drop-limited. Always check both.
Standards and references
IEC 60364-5-52, BS 7671, NEC Article 210/215/310
Related tools on this site
Open the calculator that pairs with this guide: Cable Sizing & Voltage Drop.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.