Generator Cable Length Calculator
A generator can provide power far from a building or work area, but the cable connecting it to a load has electrical resistance. As current flows, some voltage is lost along the cable. The Generator Cable Length Calculator estimates how far a single-phase generator circuit can run before that voltage loss reaches a percentage you choose.
The result is a maximum one-way distance in feet, from the generator to the load. It is based on nominal copper conductor resistance at 20°C and steady load current. The calculator’s defaults are illustrative, so replace them with values that match your circuit before using the result for planning.
What the Generator Cable Length Calculator does
The calculator estimates the maximum cable length for a specified voltage-drop limit. It uses the circuit voltage, load current, allowed voltage drop, and selected copper conductor size to calculate the distance.
Voltage drop matters because equipment at the end of a long cable receives less voltage than is available at the generator. A cable with more resistance, a longer run, or a higher current produces a larger drop. For the same voltage, current, and allowable percentage, a larger copper conductor generally permits a longer run because its resistance is lower.
The result is a voltage-drop estimate, not a determination that a cable is safe for the load or compliant with electrical rules. A cable must also have an appropriate current rating, protection, grounding arrangement, and construction for its installation and conditions of use.
How to use the Generator Cable Length Calculator
Enter the values for the circuit you want to assess, then read the Maximum One-Way Length result in feet. The calculator has four inputs:
- Circuit voltage (V): Enter the voltage across the load, from 100 to 480 volts. The illustrative default is 120 V.
- Load current (A): Enter the circuit’s load current in amperes, from 1 to 100 A. The illustrative default is 10 A. Use the expected steady operating current for this estimate.
- Maximum voltage drop (%): Choose the percentage of circuit voltage you are willing to lose along the conductors, from 1% to 10%. The illustrative default is 5%.
- Copper conductor size (AWG): Select 14, 12, 10, 8, or 6 AWG. The first option, 14 AWG, is selected by default. AWG means American Wire Gauge; within these options, a lower AWG number represents a larger conductor.
Measure the one-way route the cable will actually follow, including turns and changes in elevation. Compare that route with the result rather than doubling the route yourself: the formula already accounts for both the outgoing and return conductors.
For a 120/240 V generator, assess each relevant load circuit using the voltage across that load and its current. For example, a 120 V load and a 240 V load should be entered using their respective circuit voltages. Neutral loading may require a separate assessment.
How the Generator Cable Length Calculator formula works
The estimate uses Ohm’s law. It first converts the permitted percentage into an allowable voltage drop, then finds the length of conductor that would produce that drop at the entered current. Because current travels through an outgoing conductor and a return conductor, the total conductor length is twice the one-way distance.
Maximum one-way length (ft) = Circuit voltage × Maximum voltage drop (%) × 10 ÷ (2 × Load current × Conductor resistance)
In this formula, the voltage-drop percentage is entered as a number such as 5, rather than 0.05. The resistance is in ohms per 1,000 feet. The factor of 10 accounts for both the percentage conversion and the resistance’s 1,000-foot basis.
The calculator assumes the following nominal DC resistance values for copper conductors at 20°C:
- 14 AWG: 2.525 ohms per 1,000 ft
- 12 AWG: 1.588 ohms per 1,000 ft
- 10 AWG: 0.999 ohms per 1,000 ft
- 8 AWG: 0.6282 ohms per 1,000 ft
- 6 AWG: 0.3951 ohms per 1,000 ft
These coefficients are calculation assumptions, not measurements of a particular cable. Actual operating conditions can make the voltage drop different from the estimate.
Worked example using the default inputs
Suppose a circuit supplies a steady 10 A load at 120 V. You allow a 5% voltage drop and select 14 AWG copper. The assumed resistance for that conductor is 2.525 ohms per 1,000 ft.
Maximum one-way length = 120 × 5 × 10 ÷ (2 × 10 × 2.525) = 118.81 ft
The allowable drop in this example is 6 V, or 5% of 120 V. Under the calculator’s assumptions, approximately 118.81 ft is the one-way distance at which the conductors reach that drop at 10 A. This example does not establish that 14 AWG cable is suitable for every 10 A generator circuit or installation.
Use cases for the Generator Cable Length Calculator
The calculator is useful when comparing possible generator cable runs before choosing equipment or discussing an installation with an electrician. For example, you can use it to:
- Plan a temporary power run: Compare the distance from a generator to a work area with the estimated limit for the expected steady load.
- Compare conductor sizes: Keep the voltage, current, and drop percentage the same while selecting different AWG sizes to see how assumed resistance affects the result.
- Evaluate a smaller voltage-drop target: Compare the result at 5% with a lower percentage to see how a stricter limit reduces the estimated distance.
- Assess different loads: Enter the expected current for each relevant circuit to see how a heavier load changes the permitted length.
- Consider generator placement: Estimate whether moving the generator closer to a load could reduce voltage drop, while still following the generator manufacturer’s placement and ventilation instructions.
These comparisons are most useful when the inputs reflect the actual load and cable route. They are an early planning aid; the final cable choice requires additional electrical checks.
Other factors to consider when calculating Maximum One-Way Length
Ampacity and protection: A conductor can satisfy a voltage-drop target and still be unsuitable for the current it carries. Check the cable’s ampacity under its installation conditions, along with the circuit’s overcurrent protection and applicable electrical requirements. The calculator does not perform those checks.
Temperature: The resistance values assume copper at 20°C. Conductor resistance increases as the wire heats, so the actual voltage drop during operation can be higher than this estimate.
Starting current: Motors, compressors, and other equipment may draw more current when starting than they do during steady operation. That surge can cause a larger temporary voltage drop. The calculator uses the current you enter and does not model starting behavior.
Connections and AC effects: Plugs, receptacles, terminals, and other connections add resistance that the formula excludes. It also excludes AC reactance. These omissions can matter when assessing an actual circuit.
Grounding and installation: The result does not assess grounding, bonding, cable construction, environmental exposure, or local code requirements. Generator connection arrangements vary, so check the manufacturer’s instructions and consult a qualified electrician where appropriate. Never connect a generator to building wiring through a receptacle; use an appropriate transfer arrangement installed for that purpose.
Generator operation: Voltage at the generator can vary with load and operating conditions. The calculation assumes the entered circuit voltage and does not predict generator performance. Place a portable generator outdoors according to its instructions, away from openings where exhaust could enter a building.
Generator cable length FAQ
Is the result the total length of both conductors?
No. The result is the one-way cable route from the generator to the load. The formula includes a factor of two to account for the outgoing and return conductors.
Does a larger wire allow a longer generator cable?
Generally, yes. Within the calculator’s copper options, a larger conductor has lower assumed resistance. If the other inputs stay the same, that lower resistance produces a longer estimated maximum distance for the same voltage-drop percentage. Ampacity and protection still need separate checks.
What voltage-drop percentage should I enter?
Enter the limit you are using for your circuit design. The default 5% is illustrative, not a universal requirement or recommendation. The appropriate target depends on the equipment, installation, and applicable requirements.
Can I use this calculator for a 240 V generator load?
Yes, for an applicable single-phase 240 V load, enter 240 V and that load’s current. For a generator supplying both 120 V and 240 V loads, assess each relevant circuit with its own load voltage and current; neutral loading may need separate assessment.
Does the result guarantee that my equipment will start and run properly?
No. The result estimates steady-current voltage drop using resistance at 20°C. It does not include starting surges, conductor heating, connection resistance, reactance, or changes in generator voltage. Check the equipment and generator instructions and assess the complete circuit before use.