Voltage Drop Calculator
Run 32 A at 230 V single-phase down 40 m of 6 mm² copper and you lose 9.4 V — 4.1%, over the Philippine Electrical Code's 3% line for a branch circuit. One size up, 10 mm², brings it to 2.4% and stops the cable wasting power as heat.
Enter your circuit and conductor. The calculator returns the drop in volts and percent, checks it against the PEC recommended limits (3% branch, 5% total), finds the minimum size that passes, and prices the cable's I²R losses in pesos per year.
Your Circuit
Voltage Drop Results
| Property | Value | Basis |
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The Formula
Voltage drop (k = 2 single-phase, √3 three-phase; R and X in Ω/km, L in metres):
Conductor resistance at operating temperature (IEC 60228 values at 20°C, α = 0.00393/K copper, 0.00403/K aluminium):
Cable power loss (n = 2 conductors single-phase, 3 three-phase):
Method & Sources
Every constant has a source — nothing is approximated by eye.
- 3% / 5% limits — Philippine Electrical Code recommended practice (fine-print note adopted from NEC 210.19(A) Informational Note 4): 3% on a branch circuit, 5% combined feeder plus branch. Guidance rather than a hard rule — but equipment warranties, motor life and inspector expectations are built on it.
- Conductor resistances — IEC 60228 (Table 2, class 2 stranded): copper 1.5 mm² = 12.1 Ω/km down to 300 mm² = 0.0601 Ω/km; aluminium from 16 mm² = 1.91 Ω/km. Corrected to 70°C (PVC) or 90°C (XLPE) with α = 0.00393/K (Cu) and 0.00403/K (Al) — conservative, because a loaded cable runs hot.
- Reactance X = 0.08 Ω/km — typical manufacturer figure for LV multicore cable; the term only matters above ~50 mm² or at low power factor.
- Minimum-size finder — iterates up the IEC 60228 size list until the drop passes your selected limit. Ampacity is a separate check: a size that passes voltage drop can still fail current rating — run it through the Cable Sizing Calculator.
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Frequently Asked Questions
What voltage drop does the Philippine Electrical Code allow?
The PEC's recommended practice — a fine-print note adopted from NEC 210.19(A) Informational Note 4 — is 3% on a branch circuit and 5% combined feeder plus branch. It is guidance rather than a hard rule, but inspectors expect it, manufacturers assume it, and motors run hotter and die younger beyond it.
What formula does this calculator use?
VD = k × I × L × (R cosφ + X sinφ) ÷ 1000 — k is 2 for single-phase, √3 for three-phase; R comes from IEC 60228 corrected to operating temperature; X is 0.08 Ω/km, typical for LV multicore. Below 50 mm² the resistive term dominates.
Why is the one-way length multiplied by 2 for single phase?
Current flows out on the line and back on the neutral — a 40 m route is 80 m of conductor. Balanced three-phase uses √3 × one-way length instead, because the return currents cancel.
Copper or aluminium?
Aluminium has ~1.64× copper's resistance per mm² (IEC 60228), so it needs to be roughly two sizes larger for the same drop. Aluminium wins on cost for big feeders; copper wins on size, terminations and reliability for branch circuits.
What does voltage drop actually cost me?
The lost volts become heat: P = I²R. A 32 A circuit on 40 m of 6 mm² copper dissipates about 300 W whenever fully loaded — at 2,000 loaded hours and ₱12/kWh, roughly ₱7,200 a year for an undersized cable. One size up usually pays for itself.
Is voltage drop the same as cable sizing?
No. Sizing starts with ampacity (current rating with derating), then voltage drop, then earth-fault loop impedance. A cable can pass ampacity and still fail drop on a long run. Our Cable Sizing Calculator does the full check — this tool goes deep on the drop step.
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Heat pump and solar installs live or die on the electrical detail. Our engineers size the supply, the cable and the protection as part of every Karnot quote — at no extra charge.
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