Voltage Drop Calculator
Check circuit sizing against NEC-recommended voltage drop limits before it's a field problem. Built by the same team that fabricates the control panels and pulls the conductors.
This estimate uses standard AC resistance constants and does not account for reactance, temperature derating, or parallel runs. For final conductor sizing on a permitted install, confirm with a licensed electrical engineer.
How the calculation works
K is the resistivity constant for the conductor material (12.9 for copper, 21.2 for aluminum, in ohm-cmil/ft). I is load current in amps, D is one-way conductor length in feet, and CM is the conductor's cross-sectional area in circular mils. The result is divided by system voltage to get percent drop.
| Circuit Segment | NEC-Recommended Max |
|---|---|
| Branch circuit alone | 3% |
| Feeder alone | 2% |
| Feeder + branch combined | 5% |
These are efficiency recommendations in NEC 210.19(A) and 215.2(A), Informational Notes — not hard code violations, but exceeding them risks equipment underperformance and nuisance trips.
Common questions
What's an acceptable voltage drop for an industrial circuit?
The NEC recommends keeping total voltage drop under 5% from service entrance to the farthest outlet, split roughly 3% for branch circuits and 2% for feeders. On long feeder runs common in industrial and midstream facilities — compressor buildings, remote SWD sites, control panel runs — these limits are easy to exceed if conductor size isn't checked against actual run length.
Does conduit fill affect voltage drop?
Not directly — conduit fill governs heat dissipation and ampacity derating, while voltage drop is a function of conductor length, size, and load current. But they're usually solved together on the same run, which is why we built a companion conduit fill calculator (below).
Why does aluminum show a higher voltage drop than copper for the same size?
Aluminum has roughly 1.6x the resistivity of copper, so an aluminum conductor needs to be upsized relative to copper to achieve the same voltage drop over the same distance — a common miscalculation on cost-driven substitutions.
Is this calculator sufficient for a stamped electrical design?
No. This tool gives a fast field or planning-stage estimate. It doesn't account for ambient temperature derating, harmonics, parallel conductor runs, or site-specific code amendments. Final circuit design on a permitted industrial project should be reviewed by a licensed electrical engineer — which is exactly what our E&I team does on every project.
Electrical & Instrumentation Tools
When the spreadsheet turns into a facility, talk to the people who build it.
PLC's Electrical & Instrumentation team designs, fabricates, and installs control panels, PLC integration, and power distribution for oil & gas, midstream, and industrial facilities across Texas, the Permian Basin, and Colorado.
- In-house control panel fabrication — not subcontracted out
- Licensed PE review on every electrical design package
- SCADA / PLC integration for new and retrofit facilities