To estimate generator cable size from kW, first convert the required electrical power to current using voltage, phase, and power factor. Then check cable area for voltage drop and current-carrying capacity, because kW alone cannot establish a safe conductor size.
Open the Generator Cable Size Calculator.
Collect the information that kW leaves out
Start by defining what the cable must carry. A temporary connection supplying a known 6 kW load is a different design question from a feeder intended to deliver a generator’s full 30 kW rating. Record the required operating load and the generator’s nameplate limits separately.
You need circuit voltage, phase configuration, power factor, one-way route length, conductor material, and the voltage-drop budget for the cable being assessed. Final selection also needs installation method, ambient conditions, insulation and terminal ratings, protective-device information, and applicable installation requirements.
Use electrical input power for connected equipment. A motor’s shaft-output kW cannot be treated as electrical input without accounting for motor efficiency. Likewise, a generator engine’s mechanical rating is not the same as the alternator’s available electrical output.
| Input | Question to resolve | Illustrative value |
|---|---|---|
| Design real power | What must this feeder deliver? | 6 kW |
| Circuit voltage | Which conductors supply the load? | 230 V |
| Phase | Single phase or balanced three phase? | Single phase |
| Power factor | What current accompanies the real power? | 0.90 |
| One-way route | How far does the cable travel? | 35 m |
| Drop budget | How much voltage may this section lose? | 3% |
A power-factor assumption should be visible in the calculation. Replacing an unknown value with 1.0 makes the current estimate smaller than it would be for a load delivering the same real power at a lower power factor.
Convert generator kW to current
For single-phase real power, the relationship is P = V × I × PF. Rearranging with power in kW gives:
Current in amps = 1,000 × kW ÷ (volts × power factor)
For a balanced three-phase load, use line-to-line voltage and line current:
Current in amps = 1,000 × kW ÷ (√3 × line-to-line volts × power factor)
Do not add generator efficiency to these output-current formulas. Generator efficiency relates fuel or mechanical input to electrical output; the electrical kW and kVA relationship already describes power at the output terminals.
In the illustrative single-phase example, 6,000 ÷ (230 × 0.90) = 28.99 A. If the same 6 kW were a unity-power-factor load at 230 V, current would be 26.09 A. The difference matters when checking both voltage drop and conductor capacity.
For a three-phase example, 30 kW at 400 V and 0.80 PF requires about 54.13 A per line when balanced. It does not require 162.4 A in each conductor. Adding the three line currents and using that sum as the conductor current would triple-count the balanced load.
Work through a copper area estimate
Return to the 6 kW, 230 V, 0.90 PF example and use its 28.99 A current. A 3% voltage-drop target allows 230 × 0.03 = 6.9 V across the selected cable section.
For a simple resistance-magnitude screening estimate using an illustrative copper conductivity of 56 m/(Ω·mm²), rather than treating it as a universal operating-temperature value:
Area = 2 × current × one-way length ÷ (56 × allowable drop in volts)
Substitution gives 2 × 28.99 × 35 ÷ (56 × 6.9) = 5.25 mm². That is a calculated area for one simplified voltage-drop check. It is not a declaration that an arbitrary 6 mm² cable is suitable for the installation.
For comparison, a 6 mm² conductor under those same assumptions gives a drop magnitude of about 2 × 28.99 × 35 ÷ (56 × 6) = 6.04 V, or 2.63% of 230 V. This provides a useful mathematical cross-check: a chosen area above the 5.25 mm² threshold produces a smaller drop than the example’s limit.
The estimate deliberately leaves out cable reactance and operating-temperature changes. Schneider Electric’s voltage-drop method uses the relevant resistance-angle and reactance terms. Use cable-specific data and that fuller method for a final AC assessment, especially when loads or conductor arrangements make the simplification unsuitable.
Use the corrected calculator
The corrected area tool now uses the proper single-phase or balanced three-phase line-current and return-path relationships. It includes explicit operating-temperature resistance and reactance inputs and the standard lagging-power-factor voltage-drop expression. At PF 1 the reactive term is zero. A result is a mathematical area estimate for that drop budget, not an ampacity or standard cable/AWG selection. If reactive drop alone exceeds the budget, no finite area satisfies this fixed-reactance model.
Check ampacity before choosing a standard size
Voltage drop and ampacity impose separate requirements. Even if the calculated drop area were only 1 mm², the conductor still needs sufficient current capacity for the required load and protection arrangement. A short route does not eliminate the thermal check.
The Schneider Electric conductor-sizing overview identifies conductor selection, overcurrent protection, voltage drop, and short-circuit assessment as linked parts of design. Use the applicable cable tables and correction factors rather than a generic area-to-amps chart detached from installation conditions.
Record the cable type as well as its size. Copper versus aluminium, insulation temperature rating, number of loaded conductors, routing in insulation or conduit, grouping with other circuits, and ambient temperature can change the allowed current. Terminal limits can govern even when the cable insulation permits a higher temperature.
Choose the next suitable standard size only after all required checks. Rounding 5.25 mm² upward does not establish that the next available product meets every requirement; it simply avoids rounding the arithmetic downward. Connector acceptance and conductor flexibility may also matter for portable cable assemblies.
Include the starting load and the rest of the route
A motor may start with a current well above its running value. The steady-load calculation does not establish motor-terminal voltage during that event. Generator voltage response, cable impedance, and the motor’s driven load interact, so a cable meeting a running-drop target can still be part of a difficult-starting problem.
Also identify the full path from generator to equipment. If the entered 35 metres covers only a feeder to a board, any branch wiring after the board adds further drop. Allocate a voltage budget deliberately instead of allowing every section the entire permitted end-to-end drop.
For a split-phase system, evaluate how loads are distributed. A 240 V load and a 120 V load use different voltage relationships, and an imbalance changes neutral current. A balanced three-phase shortcut cannot assess these arrangements.
Load diversity should be supported by a real operating plan. Excluding a heater from the design because someone expects to remember to turn it off may be inappropriate if the installation must support that heater automatically. Make the intended operating limitation explicit and have it assessed with the controls and protection.
Review the calculation before handing it over
Prepare a short record showing the design kW, voltage, phase, PF, calculated current, route length, chosen drop budget, and the cable-data source. Add the result of the ampacity check and identify any outstanding fault or protection assessment. This creates a useful design brief instead of an isolated calculator screenshot.
Check units in each line. Convert watts to kW only once; use metres where the formula expects metres; convert a percentage such as 3% to 0.03; and use line-to-line voltage for the balanced three-phase relationship. A factor-of-1,000 error can look plausible if it is followed by unexplained rounding.
Is a generator’s built-in breaker enough information?
Its rating is useful, but the conductor still needs assessment for the actual installation and equipment terminals. The breaker model and trip characteristics can matter as well as the number printed on its handle. Use the generator documentation as part of the design record.
Can a cable be selected from kVA instead?
Yes, current can be calculated directly from apparent power: single-phase amps equal 1,000 × kVA ÷ V, and balanced three-phase amps equal 1,000 × kVA ÷ (√3 × line voltage). Do not divide by power factor again when starting with kVA.
References
- Schneider Electric: Calculation of voltage drop in steady load conditions
- Schneider Electric: Sizing and protection of conductors
- Generator Cable Size Calculator