Watts alone do not define a safe circuit. Voltage, phase, power factor, starting behaviour, duty, route and the applicable wiring rules must be considered together.
1. Record what the appliance actually requires
Photograph the nameplate and installation instructions. Record input voltage, phase, frequency, real input power or current, power factor when stated, starting information and whether the manufacturer requires a dedicated circuit.
Do not substitute heating output, cooling capacity or motor shaft power for electrical input. A heat pump, pump or compressor may have a much larger momentary demand than its normal running value.
2. Estimate the steady current transparently
For a single-phase load, apparent power is real input power divided by power factor. Dividing apparent power by RMS voltage gives a steady-current estimate. A near-resistive heater may have power factor close to one; electronic and motor loads need their documented value.
Keep any planning allowance visible as a separate input. It helps compare scenarios but does not replace the continuous-load, motor or protective-device rules that apply where the circuit is installed.
3. Describe the complete cable route
Measure route length rather than straight-line distance. Record conductor material, installation method, ambient temperature, grouping, insulation exposure and every termination or disconnect in the path.
The voltage-drop screen includes outgoing and return copper resistance. It does not calculate ampacity or the reactive part of AC voltage drop. Long runs, motors and larger conductors may need a fuller impedance calculation.
4. Select cable and protection as one coordinated design
A low calculated voltage drop does not prove that a conductor can carry the current safely. The designer must check ampacity after installation corrections, protective-device trip behaviour, fault current, disconnection requirements, earthing, isolation and equipment terminal limits.
Follow the appliance instructions and local electrical rules. Fixed wiring, distribution-board work and final testing should be completed or verified by a qualified person as required locally.
5. Request professional verification of the finished circuit
Before service, ask a qualified electrician to verify polarity, protective conductors, connections, insulation and required test results. Ask for a record of any relevant load-current, voltage-drop and starting checks; these are not instructions to probe a live circuit yourself.
Unexpected voltage sag, heat, odour, discoloration, nuisance trips or unstable equipment require professional investigation. Keep the electrician’s measured results with the circuit brief so later changes can be recognised.
Keep the homeowner task to collecting documents and externally visible information. Do not open a distribution board or expose conductors to obtain calculator inputs. A switched-off breaker is not proof that conductors are dead: safe isolation, prevention of re-energisation and appropriate voltage checks require a competent person. The linked UK workplace guidance supports this safety boundary and does not replace local wiring rules.
HSE — competent persons and safe-isolation principles (UK workplace guidance, English) ↗What the circuit brief should contain
| Input | Why it matters | Evidence |
|---|---|---|
| Electrical input | Sets steady demand | Nameplate or measurement |
| Starting behaviour | Tests short-duration demand | Manufacturer data |
| Cable route | Affects drop and ampacity | Measured installed route |
| Installation conditions | Change conductor capacity | Site inspection |
| Protection and earthing | Controls fault risk | Applicable design and tests |
Worked example
A 2.2 kW appliance on a 30 m run
Assume 230 V, 95% power factor, a 125% planning scenario and 2.5 mm² copper for the voltage-drop screen.
Apparent power = 2,200 / 0.95 = 2,316 VA Steady current = 2,316 / 230 = 10.07 A Planning current = 10.07 × 1.25 = 12.59 A Resistive drop = 2 × 30 × 12.59 × 0.0175 / 2.5 = 5.29 V (2.30%)
The figures describe a scenario for review. They do not select the conductor, breaker, residual-current protection, earthing or installation method.
What to have ready
- Appliance nameplate and installation manual
- Supply voltage, phase and frequency
- Running, starting and simultaneous loads
- Measured cable route and installation conditions
- Applicable electrical rules and qualified designer or installer
Common mistakes
- Using useful output instead of electrical input.
- Assuming every load has power factor equal to one.
- Treating voltage drop as proof of cable ampacity.
- Ignoring starting current and manufacturer instructions.
Tools for these measurements
Prepare the measurements with these instrument guides.
Use and limitations. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.