A longer generator cable usually needs a larger conductor to hold voltage drop to the same target at the same current. Compare the complete cable route using one-way distance, then check the proposed cable’s current-carrying capacity separately.
Open the Generator Cable Size Calculator.
Why does length change cable size?
Every conductor has resistance. At a fixed material, cross-sectional area, and temperature, resistance increases with length; at a fixed length, a larger cross-sectional area reduces resistance. Current flowing through that resistance produces a voltage difference between the generator and the load.
For a simple single-phase resistance-only estimate, the path includes an outgoing conductor and a returning conductor. If the load is 25 metres from the generator along the actual cable route, the circuit contains about 50 metres of current-carrying conductor. This is why a formula using one-way distance includes a factor of two.
Length does not automatically increase the load’s rated current. Instead, it changes the amount of voltage lost delivering that current. Motors, electronic supplies, and other equipment may respond differently to the lower terminal voltage, making a marginal long run troublesome even though it once worked over a short lead.
Schneider Electric’s steady-load voltage-drop guide includes resistance, reactance, power factor, and circuit configuration. The simplified calculations below isolate resistance so the effect of distance is visible; they do not replace that fuller design method.
Measure the route, not the straight-line distance
Measure the intended installed path from the generator connection to the supplied equipment or distribution point. Include changes in direction, vertical sections, and the permitted routing around obstacles. A building that is 15 metres away on a plan may require a 30-metre cable route.
Enter that one-way route length into this calculator. Do not double it before entering it: the single-phase calculation already includes its return-path factor. Double-counting length creates an unnecessarily inflated screening result.
Account for downstream wiring as well. If a generator feeder ends at a distribution point and a long branch circuit continues to a pump, the pump experiences both drops. Giving the feeder the entire voltage-drop budget leaves no allowance for the rest of the circuit.
Avoid choosing a route solely to minimize electrical distance. Physical protection, connector suitability, environmental exposure, and the generator’s permitted location still matter. A cable plan should preserve the required operating clearances and use equipment intended for the actual installation.
Compare short and long runs at the same load
Consider an illustrative 4.8 kW resistive load supplied at 240 V single phase. Its current is 4,800 ÷ 240 = 20 A. Assume copper conductivity of 56 m/(Ω·mm²), the simplified value built into the tool, and a 3% drop target, equal to 7.2 V.
For this example, the resistance-only area relationship is:
Area in mm² = 2 × one-way metres × amps ÷ (56 × allowed drop in volts)
At 10 metres, the calculation is 2 × 10 × 20 ÷ (56 × 7.2) = 0.99 mm². At 50 metres, it becomes 4.96 mm², and at 100 metres it becomes 9.92 mm².
| One-way route | Current | Calculated area for drop only | Relative area |
|---|---|---|---|
| 10 m | 20 A | 0.99 mm² | 1× |
| 25 m | 20 A | 2.48 mm² | 2.5× |
| 50 m | 20 A | 4.96 mm² | 5× |
| 100 m | 20 A | 9.92 mm² | 10× |
The small number in the first row is deliberately a mathematical result, not a cable recommendation for 20 A. Current capacity, permitted minimum sizes, terminals, cable construction, and installation conditions may demand a larger conductor even on the shortest route.
The linear pattern is the useful lesson. With the same assumptions and target, doubling distance doubles the calculated area required for voltage drop. Available cable sizes come in steps, so a real selection will not necessarily change at every small increase in distance.
What happens if you keep the same cable?
Reverse the example and hold conductor area at 6 mm². The estimated resistance of each metre of conductor is 1 ÷ (56 × 6), or approximately 0.002976 Ω/m. At 20 A, a 25-metre one-way circuit loses about 2.98 V; at 100 metres, it loses about 11.90 V.
Those drops correspond to approximately 1.24% and 4.96% of 240 V. A lead that appears satisfactory on the short route can exceed the example’s 3% design target when moved farther away, even though the load and cable markings have not changed.
This calculation assumes the same current, temperature, and conductivity for both runs. Warm conductors have different resistance, and real cable data may use a specified operating temperature. For the final design, use the manufacturer’s resistance or voltage-drop figures under the appropriate conditions.
Long extension arrangements also add connections. Contact resistance at a damaged or unsuitable connector is not represented by simply entering another metre in the calculator. A visibly overheated plug, damaged insulation, or recurring protective trip warrants inspection rather than an optimistic adjustment to the cable-length input.
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.
Voltage drop and ampacity answer different questions
Voltage-drop sizing asks whether the load receives an adequate voltage. Ampacity asks whether the conductor can carry the design current without exceeding its permitted thermal conditions. A conductor must satisfy both requirements, along with protection and fault-performance requirements.
Schneider’s conductor-sizing overview treats conductor area, overcurrent protection, voltage drop, and short-circuit assessment as connected design tasks. There is no universal rule that a cable meeting one percentage-drop target automatically passes all the others.
Installation method matters. Cable in free air, conduit, insulation, or a group of loaded cables can have different thermal conditions. The insulation rating, conductor material, number of loaded cores, ambient temperature, and terminal temperature limits all belong in the final check.
A long run may be selected primarily for voltage drop while a short run is selected primarily for ampacity. This explains why extending a cable sometimes requires a substantial size increase, while shortening it does not justify reducing the existing conductor to the calculator’s smallest mathematical answer.
Check motor starting and downstream performance
A running-load calculation describes steady operation. A motor starting event draws a different current and may coincide with a generator voltage dip. The cable then adds its own contribution to the reduced voltage at the motor.
Record the actual symptom: a motor that hesitates only on a long lead, a controller that resets during starts, or a light that dims briefly. These observations help a qualified person distinguish feeder drop, generator transient response, equipment faults, and ordinary starting behavior.
Do not raise a breaker rating to compensate for a long cable or a difficult start. The protective device and conductor need coordinated selection, and the generator has its own output limits. A different cable, load sequence, starting method, or generator may be the relevant change after proper assessment.
Does twice the voltage mean half the cable area?
At fixed real power and power factor, doubling voltage halves current. With the same percentage-drop target, it also doubles the allowable drop in volts, so the simple mathematical area requirement becomes one quarter. This is a design comparison only; equipment must be designed and connected for its specified voltage.
Can extra cable be left tightly coiled?
Use the cable or reel manufacturer’s loading instructions, including any separate wound and unwound ratings. The voltage-drop calculation does not assess heat retained in a coil or reel, so unused length is still part of both the electrical and thermal evaluation.
References
- Schneider Electric: Calculation of voltage drop in steady load conditions
- Schneider Electric: Sizing and protection of conductors
- Generator Cable Size Calculator