Voltage Drop Calculator
Adjust the inputs below. Results update as you type.
How it works
Voltage drop = 2 × resistance per foot × length × current (for single-phase). NEC recommends maximum 3% drop for branch circuits, 5% combined for feeder+branch. Resistance = 2 × 12.9 / (1000 × conductor area in circular mils) for copper. Wire gauge (AWG) inversely relates to size: #12 AWG = 6,530 CM; #10 = 10,380 CM. Excessive voltage drop causes equipment malfunction and heat buildup.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Voltage drop = (2 × length × current × resistance per unit length) for a single-phase circuit. It rises with longer wires and higher current, and falls with thicker wire.
Worked example
A long run carrying high current on thin wire can drop several volts; codes typically recommend keeping the drop under about 3% of supply voltage.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
What is voltage drop?+
The loss of voltage as current travels along a conductor, caused by the wire's resistance. Excessive drop dims lights and strains equipment.
How do I reduce voltage drop?+
Use thicker wire (lower gauge number), shorten the run, or raise the supply voltage. Thicker wire has less resistance per foot.
What drop is acceptable?+
Many electrical codes suggest keeping total voltage drop at or below about 3–5% from source to load for efficiency and safety.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.