🔗 Voltage Drop Calculator
Size a cable by ampacity and voltage drop in one pass. Pick current, length and conductor, and get the minimum cross-section that stays within your voltage-drop limit.
⚙️ Circuit
📊 Result
For electricians and engineers sizing LV feeders and final circuits.
How it is calculated
· Resistance: R = ρ ÷ A (ρ copper 0.0225, aluminum 0.036 Ω·mm²/m).
· Voltage drop: VD = √3·I·L·R (3-phase) or VD = 2·I·L·R (1-phase).
· Voltage drop %: VD% = VD ÷ U × 100 (U = 400 V or 230 V).
Referenced standards
| Standard | Scope |
|---|---|
| IEC 60364-5-52 | Wiring systems — current-carrying capacities and voltage drop |
Worked example — 100 A over 50 m → 25 mm² copper
1. 100 A, 50 m, 3-phase copper, 3% limit.
2. Ampacity first: 25 mm² copper carries 101 A ≥ 100 A.
3. R = 0.0225 ÷ 25 = 0.0009 Ω/m → VD = √3 × 100 × 50 × 0.0009 = 7.8 V = 1.95%.
4. 25 mm² satisfies both ampacity and the 3% drop limit.
FAQ
What is an acceptable voltage drop in a low-voltage circuit?
IEC 60364-5-52 recommends total voltage drop within 5% (distribution plus final circuit), and 3% for lighting. In practice feeders are sized for 2-3% steady-state drop, leaving margin for motor starting. The calculator computes drop for single and three-phase circuits and recommends a cross-section that meets the limit.
How do I calculate voltage drop for a 3-phase circuit?
Voltage drop = sqrt(3) x I x L x (R cos(phi) + X sin(phi)). For a 100 A, 50 m, 35 mm2 copper feeder (R ~0.524 ohm/km), drop is about 1.73 x 100 x 0.05 x 0.524 = 4.5 V, or 1.1% of 400 V. The calculator computes drop in volts and percent and flags the recommended section.
Copper or aluminum for a feeder cable?
Aluminium is lighter and cheaper (30-50% less) but needs a larger cross-section for the same drop because its resistivity is ~1.6x copper. For a 100 A feeder, 35 mm2 copper roughly equals 50-70 mm2 aluminium. Copper wins in ducts and short runs; aluminium wins in long, cost-driven feeders. The calculator compares both.
How does motor starting affect voltage drop?
Motor starting current is 5-7 times rated, so the starting voltage drop is 5-7 times the steady-state drop. A feeder sized for 2% running drop may dip 10-15% during start, which can stall the motor. Soft-starters or larger cables limit this. The calculator flags starting drop and recommends mitigation.
What cross-section do I need for a 100 A three-phase feeder?
For a 100 A, 0.4 kV three-phase feeder, a 35 mm2 copper cable is the standard choice, carrying ~110-125 A with a 50 m run dropping about 1%. For longer runs or tighter drop limits, use 50 mm2. The calculator picks the minimum section meeting both ampacity and drop per IEC 60364-5-52.
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