Voltage Drop Length Calculator | Maximum One-Way Circuit Length

Result

Formulas

\(V_{\mathrm{allowable}} = V_{\mathrm{system}}\times \frac{p_{\mathrm{drop}}}{100}\)
\(L_{\mathrm{max,one-way}} = \frac{V_{\mathrm{allowable}}\times 1000}{k_{\mathrm{phase}} I R_{1000}}\)
\(k_{\mathrm{phase}} = 2\text{ for single-phase or DC};\ \sqrt{3}\text{ for three-phase}\)

A Voltage Drop Length Calculator determines the maximum one-way conductor length that can be installed while staying within an entered voltage-drop limit. The result supports preliminary branch-circuit or feeder layout decisions after the load current, nominal system voltage, conductor resistance, and phase model are known.

For a proposed conductor, the calculated maximum length establishes a voltage-drop boundary. If the actual one-way route length from source to load exceeds that value, the circuit will exceed the entered drop percentage unless a lower-resistance conductor, different circuit arrangement, or different electrical design is used.

This mode performs inverse voltage-drop arithmetic. It starts with the permitted voltage drop and solves for length; it does not select a conductor or establish ampacity.

Maximum One-Way Length

The calculator produces Maximum one-way length in feet. One-way length is the physical route from the source to the load, such as from a panelboard to a receptacle, disconnect, motor controller, distribution panel, or utilization equipment.

The voltage-drop formula accounts for the full current path through its Phase multiplier:

Single phase or DC uses a multiplier of 2× because current travels out on one conductor and returns on another conductor.
The selected phase model determines the multiplier used by the inverse formula.

Do not enter the round-trip conductor distance as the installation length when the result is labeled Maximum one-way length. The phase multiplier already accounts for the circuit path represented by the calculation model.

Calculation Inputs

Field	Electrical use
Load current (A)	Current used by the voltage-drop model. This should represent the current being evaluated for the circuit condition.
System voltage (V)	Nominal circuit voltage used to convert the entered percentage limit into allowable volts of drop.
Conductor resistance (ohm/1000 ft)	Resistance value for the conductor and temperature basis being reviewed, expressed in ohms per 1,000 feet.
Entered drop limit (%)	Maximum voltage-drop percentage used for this calculation mode.
Phase model	Circuit multiplier selection used by the inverse voltage-drop formula.

The Conductor resistance (ohm/1000 ft) input is the property that ties the result to a particular conductor choice and temperature basis. A lower resistance value permits a longer circuit length at the same current and voltage-drop limit. A higher resistance value reduces the permitted length.

Load current has the opposite effect: increasing current reduces the maximum one-way length available within the same voltage-drop allowance.

Inverse Voltage-Drop Formula

The calculator first converts the entered percentage limit to volts:

\(\displaystyle V_{\text{drop allowed}} = V_{\text{system}} \times \frac{\text{Entered drop limit}}{100}\)

It then solves for maximum one-way length:

\(\displaystyle L_{\text{max}} = \frac{V_{\text{drop allowed}} \times 1000} {I \times R \times M}\)

Where:

\(L_{\text{max}}\) = maximum one-way length in feet
\(V_{\text{drop allowed}}\) = entered voltage-drop limit in volts
(I) = Load current in amperes
(R) = Conductor resistance in ohm/1000 ft
(M) = Phase multiplier

This is resistance-based voltage-drop arithmetic using the entered values. It does not add reactance, power factor, connection resistance, transformer impedance, starting current, or other system effects unless those conditions have already been incorporated into the values chosen for review.

Calculation Example

For the entered values:

Input or result	Value
Load current	20 A
System voltage	120 V
Conductor resistance	1.588 ohm/1000 ft
Entered drop limit	3%
Phase model	Single phase or DC
Phase multiplier	2×

The permitted voltage drop is:

\(\displaystyle 120 \times 0.03 = 3.6\text{ V}\)

The maximum one-way length is:

\(\displaystyle L_{\text{max}} = \frac{3.6 \times 1000} {20 \times 1.588 \times 2} = 56.6751\text{ ft}\)

Maximum one-way length: 56.6751 ft

For this input set, a 20 A, 120 V single-phase or DC circuit using the entered conductor resistance reaches the 3% voltage-drop limit at approximately 56.7 feet one way. A longer route requires a lower-resistance conductor or a revised circuit design to remain at or below the entered limit.

Conductor and Installation Review

The calculated length is used alongside, not instead of, conductor sizing and installation review. The conductor represented by Conductor resistance (ohm/1000 ft) must be evaluated separately for required ampacity, insulation temperature rating, terminal rating, overcurrent protection, and actual installation conditions.

For installed conductors, verify items such as:

Conductor size in AWG or kcmil and the resistance basis used.
Required ampacity after applicable adjustment factor and correction factor calculations.
Number of current-carrying conductors and any resulting adjustment requirements.
Raceway fill, routing length, pull points, bends, and accessible splice locations.
Branch-circuit or feeder load characteristics, including continuous-load treatment where applicable.
Actual conductor operating temperature and whether the resistance value matches that temperature basis.
Motor starting, inrush, or other operating conditions that may produce greater momentary voltage drop.
Local requirements, project specifications, utility conditions, and AHJ interpretation.

The calculation boundary is specific: it determines the maximum one-way length from entered voltage, drop limit, current, resistance, and phase multiplier. Conductor selection, raceway design, ampacity, protection, and installation compliance remain separate electrical and code decisions.