Electrical
Wire Size Calculator for Voltage Drop
The smallest conductor that keeps voltage drop within a chosen limit for a given load, distance, and system voltage, with the required circular mils shown.
Calculated reference results
- Smallest size for voltage drop
- 6
- Required conductor area
- 24,826cmil
- Allowable drop
- 14.40V
- Actual drop at that size
- 13.62V
- Actual drop
- 2.84%
- Table 310.16 ampacity at 75 C
- 65 A
Shown so you can confirm the size also satisfies ampacity. Verify against your code book.
The arithmetic
- Allowable drop = 480 V x 3.0% = 14.40 V
- CM required = (12.9 x 1.732 x 40.0 A x 400 ft) / 14.40 V = 24,826 cmil
- Smallest standard size at or above 24,826 cmil is 6 at 26,240 cmil
What this does not account for
- This sizes for voltage drop only. Ampacity, overcurrent protection, terminal temperature ratings, conduit fill, and short-circuit withstand are separate requirements and any of them can force a larger conductor.
How this is calculated
To size a conductor for voltage drop, rearrange the drop formula for area: CM = (K x phase constant x current x one-way distance) / allowable volts dropped. Pick the smallest standard size whose circular mils meet or exceed that figure. The conductor must still separately satisfy ampacity, overcurrent protection, and terminal temperature limits.
Formulas
CM = (K x n x I x L) / V_allowable
- CM — required conductor area in circular mils
- V_allowable — system voltage multiplied by the allowable drop percentage
Assumptions and limitations built into this calculator
- Sizes for voltage drop alone. Every other conductor requirement is checked separately.
- Uses the circular-mil K method and ignores conductor reactance.
- Assumes one conductor per phase and a balanced load.
Frequently asked questions
- Why did the calculator pick a size larger than the ampacity table needs?
- On a long run, voltage drop governs rather than ampacity. That is normal, and it is why remote lift stations and well sites often have conductors far larger than the load current alone would suggest.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.