Solar and Electrical Installation: Voltage Drop Practice
This worked exercise explains how to check voltage drop without mistaking one calculation for a complete cable design. It is independently prepared revision practice, not an authenticated 2023 KNEC paper or an official marking scheme. Use the assumptions stated in the question and the cable data provided by your lecturer or examination paper.
Practice question and assumptions
A single-phase 240 V supply feeds a 7.2 kW resistive load through a cable run of 35 m. The exercise allows a voltage drop of 2.5% of the stated supply voltage. Calculate the load current, the allowed drop, and the largest acceptable voltage-drop coefficient in mV/A/m. Compare the two illustrative cable choices below.
- Candidate A: 6 mm², with a supplied coefficient of 7.3 mV/A/m.
- Candidate B: 10 mm², with a supplied coefficient of 4.4 mV/A/m.
Assume unity power factor, steady load current and a coefficient defined for this single-phase circuit using the stated cable-run length. The 240 V and 2.5% values belong to this exercise; they are not a declaration of the voltage or permitted drop for every Kenyan installation.
Step 1: calculate the load current
Convert kilowatts to watts before substitution: 7.2 kW = 7,200 W.
I = P / V = 7,200 / 240 = 30 A.
For the assumed resistive load, this is the current used in the voltage-drop calculation. A different load model, power factor or operating condition would require a different current calculation.
Step 2: find the allowed voltage drop
Allowed drop = (2.5 / 100) × 240 = 6.0 V.
Write the percentage as 0.025, not 2.5. The result should be a small fraction of the supply voltage. This quick check helps catch a percentage-conversion error.
Step 3: calculate the coefficient limit
With a suitable tabulated coefficient, voltage drop in volts is calculated as coefficient × current × cable-run length / 1,000. Rearranging for the coefficient gives:
Maximum coefficient = (6.0 × 1,000) / (30 × 35) = 5.714 mV/A/m.
The selected coefficient must be at or below this limit for the voltage-drop condition in the exercise. Keep extra digits until the final comparison. Use the length definition belonging to the table; do not double the run length if the single-phase coefficient already accounts for the circuit conductors.
Step 4: check the candidates directly
| Exercise candidate | Calculation | Result against 6.0 V |
|---|---|---|
| A: 6 mm² at 7.3 mV/A/m | 7.3 × 30 × 35 / 1,000 = 7.665 V | Exceeds the exercise limit |
| B: 10 mm² at 4.4 mV/A/m | 4.4 × 30 × 35 / 1,000 = 4.620 V | Meets the exercise limit |
Conclusion: Candidate B is the smaller of the two listed choices that meets this voltage-drop test. Its drop is 4.620 / 240 × 100 = 1.925%. This does not establish that it is suitable for installation: the exercise has not completed the other design checks.
What a complete selection still needs
Actual cable selection also depends on current-carrying capacity, installation method, temperature, grouping, protective-device coordination, fault conditions, earthing and equipment terminals. Use the applicable current standards, manufacturer data and competent supervision for real work. An AC voltage-drop example should not be transferred unchanged to a solar DC string or battery circuit; the circuit model, equipment ratings and cable data must match.
Try a variation
Keep the same 30 A load and coefficient of 4.4 mV/A/m, but extend the run to 50 m. The calculated drop is 4.4 × 30 × 50 / 1,000 = 6.6 V, so it exceeds the 6.0 V limit. Explain why a cable that passes one run length can fail another without any change in load current.
Further reading and corrections
The IET voltage-drop explanation illustrates the coefficient method with a specified cable table. Schneider Electric’s installation design methodology places cable sizing within the wider design process. These references support further study; they do not replace local requirements.
Use the course catalogue for related revision resources. If a unit, assumption or calculation is unclear, send Jemshah the page and the exact step so it can be reviewed.
