Enrichment · STEM Explorations · Electricity

Long wire, thin wire,
fewer volts at
the far end.

Choose the wire and type the voltage, current and distance. The calculator gives the volts lost on the way, the voltage the load actually gets, and the gauge that keeps the loss small.

Check the dropWhat you get

The voltage drop calculator

The worked example is a made-up 100 ft feed carrying 16 A to a garden shed on 12 AWG copper. Nothing you type leaves this browser.

Loading the calculator. If nothing appears, turn on JavaScript.

What you get

Enrichment · STEM ExplorationsFree

What you finish with
The voltage drop in volts and percent, the voltage at the load, and the drop for every gauge with the smallest one within 3%. It prints as a summary and downloads as a spreadsheet.
Who it is for
Electrical and trades students, physics students learning resistance, and robotics or maker teams running power over distance.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The STEM Explorations space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose copper or aluminium, the gauge and the system.
  2. Type the source voltage, the current and the one-way length.
  3. Read the drop and the voltage at the load.
  4. Use the table to find a gauge within 3%.
  5. Print the summary for class or a planning talk with an electrician.

Check the drop

How the drop is worked out

Resistance of the wire times the current.

The worked example: a shed on 12 AWG2 × K × I × L41,2802 × 12.9 × 16 × 100÷ 6,530 cmil6.32 V12 AWG copper÷ 120 V5.27%more than 3%10 AWG instead3.31%÷ 10,380 cmilThe worked example: a shed on 12 AWG2 × K × I × L41,2802 × 12.9 × 16 × 100÷ 6,530 cmil6.32 V12 AWG copper÷ 120 V5.27%more than 3%10 AWG instead3.31%÷ 10,380 cmil
Current flows out and back, so a single-phase run has twice its length of wire. A thicker wire (more circular mils) loses fewer volts.

The formula

A wire's resistance grows with its length and shrinks with its cross-section, measured in circular mils. The standard estimate is voltage drop = 2 × K × current × one-way length ÷ circular mils for single-phase and DC (current goes out and back), or √3 × K × current × length ÷ circular mils for three-phase. K is about 12.9 ohm-circular mils per foot for copper and 21.2 for aluminium at 75°C.

What it does not decide

The National Electrical Code suggests, in informational notes, no more than 3% drop on a branch circuit and 5% overall; these are recommendations for performance, not the rules that make a circuit safe. The wire must be rated for the current, protected by the right breaker and installed to code. Electrical work needs a qualified electrician and local permits.

Sources

  1. National Fire Protection Association. NFPA 70, National Electrical Code: Chapter 9, Table 8, conductor properties, and informational notes to sections 210.19 and 215.2 on voltage drop (nfpa.org).
  2. National Institute of Standards and Technology. Copper Wire Tables, Handbook 100 (nist.gov).
  3. Occupational Safety and Health Administration. Electrical safety (osha.gov/electrical).
  4. OpenStax. College Physics 2e, section 20.3: resistance and resistivity (openstax.org).

The worked example is made up. This is a learning estimate, not an electrical design; have a qualified electrician plan and install wiring. Your figures stay in this browser. Support: team@caveatacademic.com.