Voltage Drop
Calculate voltage drop and percent drop for wire runs using NEC K-factor circular-mil method. Flags runs exceeding the NEC 210.19 recommended guideline and suggests the minimum passing conductor size.
Formula: VD = (√3 or 2) × K × I × L / CM — NEC 210.19(A) Info Note 4, Chapter 9 Table 8
How this calculator works
Calculate voltage drop and percent drop for wire runs using NEC K-factor circular-mil method. Flags runs exceeding the NEC 210.19 recommended guideline and suggests the minimum passing conductor size.
Formula
VD = (√3 or 2) × K × I × L / CM — NEC 210.19(A) Info Note 4, Chapter 9 Table 8How to use it
- Collect current field measurements and known project values.
- Enter each value and confirm that every selected unit matches your measurements.
- Review the calculated result, then verify it against plans, product data, site conditions, and applicable codes.
Worked example
For a representative electrical job, enter the measured values from one area first and calculate the result. Repeat the calculation for each area whose dimensions or conditions differ, add the results, and apply only the waste or contingency allowance the project actually requires. This keeps unlike conditions from being hidden inside one average.
Frequently asked questions
What formula does the Voltage Drop calculator use?
The calculator uses: VD = (√3 or 2) × K × I × L / CM — NEC 210.19(A) Info Note 4, Chapter 9 Table 8. Results are planning estimates and should be checked against project requirements, manufacturer data, and applicable codes.
How accurate is the Voltage Drop calculator?
Accuracy depends on the measurements and assumptions you enter. Use current field measurements, include an appropriate waste or safety allowance when the form provides one, and verify the result before purchasing or installing materials.
Can I use this electrical result for a final quote?
Use it as a starting point for planning. A final quote should also account for site conditions, labor, taxes, permits, local code requirements, and supplier packaging or minimum-order rules.