Voltage Drop Calculator: method, checked example and practical limits
Estimate conductor voltage drop from current, one-way length, resistance per thousand feet and circuit type. The result represents an electrical screening calculation under the stated circuit and material assumptions.
Inputs used by this calculator
Voltage Drop Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Source voltage (V) | 120 | 0.001 to 1000000000 | Supplies the source voltage (v) quantity used by the published equation or result breakdown. |
| Load current (A) | 15 | 0 to 1000000000 | Supplies the load current (a) quantity used by the published equation or result breakdown. |
| One-way conductor length (ft) | 100 | 0 to 1000000000 | Selects the unit or conversion path used by Voltage Drop Calculator. |
| Conductor resistance (ohm/1,000 ft) | 1.588 | 0 to 1000000000 | Selects the unit or conversion path used by Voltage Drop Calculator. |
| Circuit | Single-phase AC | DC two-wire · Single-phase AC · Three-phase AC | Supplies the circuit quantity used by the published equation or result breakdown. |
Inputs and measurement conventions
The quantities used on this page are Source voltage (V), Load current (A), One-way conductor length (ft), Conductor resistance (ohm/1,000 ft), Circuit. Start with values for the same situation and read the units attached to each field. The field reference below shows supported choices and numerical limits.
One-way length versus full loop length changes the answer. Temperature, AC reactance, installation method and ampacity need separate checks.
Method and equation
Multiply entered resistance per 1,000 feet by one-way feet and current, using the circuit factor of two or square root of three.
Voltage Drop Calculator uses the following stated relationship: drop=K×current×resistance×length/1000; K=2 for two-wire or √3 for three-phase. Its assumptions determine which question the answer can resolve.
drop=K×current×resistance×length/1000; K=2 for two-wire or √3 for three-phaseWorked calculation with the starting values
Source voltage (V): 120; Load current (A): 15; One-way conductor length (ft): 100; Conductor resistance (ohm/1,000 ft): 1.588; Circuit: Single-phase AC. These starting values give 4.764 V estimated voltage drop. The calculation breakdown is Load voltage: 115.236; Voltage drop: 3.9699999999999998%; Circuit factor: 2.
A 120 V, 15 A load across 100 ft with 1.588 ohm/1,000 ft drops about 4.76 V in a two-wire circuit.
Reading the answer
The result represents an electrical screening calculation under the stated circuit and material assumptions.
Load voltage: 115.236; Voltage drop: 3.9699999999999998%; Circuit factor: 2. These intermediate quantities make the stated Voltage Drop Calculator model inspectable; they should be read together with the main answer rather than treated as independent guarantees.
Common errors and model boundaries
One-way length versus full loop length changes the answer. Temperature, AC reactance, installation method and ampacity need separate checks.
This screening estimate omits reactance, temperature, power factor, terminals and code requirements; final conductor sizing needs applicable electrical rules.
Independent verification and next decision
No. Current capacity, protection, insulation, installation conditions and applicable electrical code also control cable selection.
For the Voltage Drop Calculator method, compare drop=K×current×resistance×length/1000; K=2 for two-wire or √3 for three-phase with the starting inputs and the calculation table. The linked NFPA: National Electrical Code information and NIST Guide to the SI references identify the relevant definitions, conventions or domain limitations. Retain intermediate precision when comparing a manually worked answer.
Voltage Drop Calculator worked-example ledger
| Quantity | Example value | Role |
|---|---|---|
| Source voltage (V) | 120 | Entered assumption |
| Load current (A) | 15 | Entered assumption |
| One-way conductor length (ft) | 100 | Entered assumption |
| Conductor resistance (ohm/1,000 ft) | 1.588 | Entered assumption |
| Circuit | Single-phase AC | Entered assumption |
| Main answer | 4.764 V estimated voltage drop | Calculated from the starting inputs |
| Load voltage | 115.236 | Intermediate or contextual output |
| Voltage drop | 3.9699999999999998% | Intermediate or contextual output |
| Circuit factor | 2 | Intermediate or contextual output |
Frequently asked questions
Does an acceptable voltage drop prove a cable is safe?
No. Current capacity, protection, insulation, installation conditions and applicable electrical code also control cable selection.
Which relationship does the Voltage Drop Calculator use?
Multiply entered resistance per 1,000 feet by one-way feet and current, using the circuit factor of two or square root of three. The displayed relationship is drop=K×current×resistance×length/1000; K=2 for two-wire or √3 for three-phase.
What do the starting values produce in the Voltage Drop Calculator?
Source voltage (V): 120; Load current (A): 15; One-way conductor length (ft): 100; Conductor resistance (ohm/1,000 ft): 1.588; Circuit: Single-phase AC. These starting values give 4.764 V estimated voltage drop. The calculation breakdown is Load voltage: 115.236; Voltage drop: 3.9699999999999998%; Circuit factor: 2.
How should I interpret this Voltage Drop Calculator answer?
The result represents an electrical screening calculation under the stated circuit and material assumptions.
Which input mistake most affects the Voltage Drop Calculator?
One-way length versus full loop length changes the answer. Temperature, AC reactance, installation method and ampacity need separate checks.
What is outside this Voltage Drop Calculator model?
This screening estimate omits reactance, temperature, power factor, terminals and code requirements; final conductor sizing needs applicable electrical rules.
What to keep in mind
This screening estimate omits reactance, temperature, power factor, terminals and code requirements; final conductor sizing needs applicable electrical rules.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.