Voltage Drop Calculator — South Africa
Check voltage drop for any cable run and verify SANS 10142-1 compliance. Covers single-phase and three-phase circuits with instant pass/fail verdict.
⚠️ For planning purposes only. All electrical work in SA must be carried out by a registered electrician under SANS 10142. A Certificate of Compliance (CoC) is required.
What Is Voltage Drop and Why Does It Matter in South Africa?
Every electrical conductor has resistance. As current flows through that resistance, energy is lost as heat and the voltage available at the load drops below the supply voltage. This reduction is called voltage drop. In South Africa, the national grid supplies 230V single-phase and 400V three-phase (nominal). Most equipment is designed to operate within ±10% of its rated voltage — fall below that and you get problems.
South Africa's national wiring standard, SANS 10142-1, limits voltage drop to 5% of the nominal supply voltage for final circuits. That is 11.5V on a 230V circuit and 20V on a 400V circuit. Exceed that, and your installation is non-compliant and will fail a Certificate of Compliance (CoC) inspection — which is mandatory when selling a property.
Voltage drop is one of the most commonly misunderstood aspects of electrical installation in South Africa. Many installations pass the current-carrying capacity check but quietly fail on voltage drop — particularly on longer cable runs to outbuildings, borehole pumps, irrigation systems, and inverter battery banks.
The SANS 10142 Voltage Drop Formula — Manual Calculation
SANS 10142-1 uses a cable-specific constant called mV/A/m (millivolts per amp per metre) to calculate voltage drop. This value is derived from the conductor resistance at 70°C operating temperature and is published in the SANS standard.
VD (V) = mV/A/m × Current (A) × Length (m) ÷ 1000
VD % = (VD ÷ Supply Voltage) × 100
Receiving-End Voltage = Supply Voltage − VD
SANS 10142-1 Limit = 5% of supply (11.5V / 20V)
Worked example — outbuilding socket circuit
A 16A plug circuit runs 30m to an outbuilding on 2.5mm² cable, 230V single-phase:
VD = 17.0 × 16A × 30m ÷ 1000 = 8.16V
VD % = (8.16 ÷ 230) × 100 = 3.55% ✅ Compliant
Receiving-end voltage = 230 − 8.16 = 221.84V
This passes. But if the run were extended to 50m, the voltage drop would be 13.6V (5.91%) — a SANS failure. The solution would be to upsize to 4mm² cable, which brings the drop to 8.8V (3.83%) at 50m — compliant.
mV/A/m Reference Table — Copper PVC 70°C (SANS 10142-1)
| Cable Size | mV/A/m | CCC Clipped (A) | CCC Conduit (A) | Typical Use |
|---|---|---|---|---|
| 1.5mm² | 28.0 | 17.5A | 14A | Lighting circuits |
| 2.5mm² | 17.0 | 25A | 20A | Plug circuits, geysers (short runs) |
| 4mm² | 11.0 | 32A | 26A | Geysers, stoves (short runs) |
| 6mm² | 7.1 | 41A | 34A | Stoves, pool pumps, sub-feeds |
| 10mm² | 4.2 | 57A | 46A | Large appliances, outbuilding mains |
| 16mm² | 2.7 | 75A | 61A | Sub-distribution boards |
| 25mm² | 1.7 | 100A | 80A | Main DB feeds, large buildings |
| 35mm² | 1.3 | 125A | 100A | High-current three-phase runs |
Source: SANS 10142-1 aligned values for copper PVC-insulated cables at 70°C conductor operating temperature.
Common Voltage Drop Problems in South African Installations
Outbuilding and granny flat sub-boards
This is one of the most common non-compliance issues found during CoC inspections. A granny flat 40m from the main house, fed with 2.5mm² cable on a 20A circuit, will produce a voltage drop of 13.6V (5.91%) at full load — failing SANS. The correct cable for this run is 4mm², which drops the VD to 8.8V (3.83%). Always size outbuilding feeds based on run length, not just load.
Borehole and irrigation pump runs
Agricultural and large residential properties frequently have borehole pumps 100–300m from the distribution board. A 5.5kW pump drawing 25A over 200m of 6mm² cable produces a voltage drop of 35.5V — 15.4% of supply, three times the SANS limit. In this case, 35mm² cable would be required. Alternatively, a local DB board closer to the pump reduces the run length dramatically.
Inverter and solar battery cable runs
DC voltage drop in solar and inverter installations is especially critical because DC systems operate at lower voltages — 12V, 24V, or 48V. A 2V drop on a 48V system is 4.2% — already close to the limit — and a 2V drop on a 12V system is 16.7%, catastrophically reducing system efficiency. Battery and inverter cable runs must be kept as short as physically possible and sized generously. Use this calculator with the system DC voltage for an approximate check, but always consult the inverter manufacturer's cable sizing guide for DC runs.
Motor starting and running current
Motors draw 5–8 times their rated current at startup (locked rotor current). While this is a momentary event, it causes a momentary voltage dip that can trip sensitive equipment and stresses the cable. When sizing cable for motor loads — pool pumps, borehole pumps, air conditioners — calculate voltage drop at running current for compliance, but verify the cable is appropriate for the starting current surge with a registered electrician.
What Happens When Voltage Drop Is Excessive?
The consequences of excessive voltage drop are practical and immediate, not just regulatory:
- Motors overheat: A motor running below its rated voltage draws more current to compensate, generating excess heat and shortening its service life
- Tripping and nuisance faults: VFDs, inverters, and sensitive electronics often have under-voltage protection that trips before the appliance fails
- Lighting flickers: Fluorescent and LED drivers are particularly sensitive to low voltage
- Reduced appliance performance: Stoves, geysers, and appliances take longer to reach operating temperature
- Cable heating: Because low voltage causes higher current draw (to maintain power), the cable itself runs hotter — compounding the problem and degrading insulation faster
- CoC failure: A non-compliant installation cannot receive a Certificate of Compliance, which is legally required when selling property in South Africa