Electrical
Voltage drop field guide for electrical crews
A practical guide to calculate voltage drop, understand limits, and know what to record on the job.
Direct answer
Voltage drop is the voltage lost in a conductor while current flows through it. Many designs use 3 percent for branch circuits and 5 percent total feeder plus branch circuit as a target, but project documents and equipment requirements control the decision.
Voltage drop basics
Voltage drop increases with current, distance, conductor resistance, and temperature. It affects equipment performance, motor starting, PoE power, lighting levels, and troubleshooting decisions.
The field risk is usually not the calculation itself. The risk is using the wrong length, current, conductor material, or target, then failing to record the assumption that drove the decision.
Formula
For single-phase circuits, use 2 x current x one-way length x conductor resistance per 1000 ft, then divide by 1000. For three-phase circuits, use 1.732 x current x one-way length x conductor resistance per 1000 ft, then divide by 1000.
VD = (2 x I x L x R) / 1000VD = (1.732 x I x L x R) / 1000- I
- Load current in amps
- L
- One-way conductor length in feet
- R
- Conductor resistance in ohms per 1000 ft
Field example
A three-phase 208 V feeder carries 60 A over a 150 ft one-way route using copper #4 AWG at 0.248 ohms per 1000 ft. The calculated drop is 3.87 V, or 1.86 percent of 208 V, which is inside a common 3 percent branch target.
| Input | Value |
|---|---|
| System | Three-phase, 208 V |
| Load current | 60 A |
| One-way length | 150 ft |
| Conductor | Cu #4 AWG |
| Resistance | 0.248 ohms/1000 ft |
Acceptable limits
Treat 3 percent branch and 5 percent total feeder plus branch as common design targets, not a substitute for project specifications, equipment instructions, or applicable code language.
If the result is close to a target, check measured route length, design current, conductor material, temperature assumptions, and whether the equipment has a tighter voltage tolerance.
Field checklist
- Confirm load current, voltage, and phase.
- Measure the actual one-way route length.
- Use the conductor material and size that will actually be installed.
- Compare against project specifications and equipment instructions.
- Record assumptions when the result changes conductor size, routing, or acceptance.
What to document
Record conductor material, size, phase, voltage, load current, one-way length, resistance source, calculated volts dropped, percent drop, target used, who checked it, and why the calculation was needed.
Common mistakes
- Using round-trip length in a three-phase formula that expects one-way length.
- Using estimated plan distance instead of routed length.
- Mixing copper and aluminum resistance values.
- Treating an informational design target as a universal code rule.
FAQ
What voltage drop is acceptable?
Many designs use 3 percent for branch circuits and 5 percent total feeder plus branch circuit as a target. Verify project specifications, equipment requirements, and applicable code before treating a target as mandatory.
What should be documented for voltage drop?
Document conductor material, size, voltage, phase, load current, one-way length, calculated drop, percent drop, assumptions, target, and who checked it.