NEC Informational Note 4 guidelines
The National Electrical Code (NEC NFPA 70 Section 210.19(A) Informational Note 4) recommends that the maximum voltage drop on branch circuit conductors should not exceed 3%, and the total drop across feeders and branch circuits combined should not exceed 5%.
In the US, voltage drop is calculated using circular mils (CM): Single Phase Vd = (2 × K × I × L) / CM, and Three Phase Vd = (√3 × K × I × L) / CM, where K is conductor resistivity (12.9 for copper at 75°C, 21.2 for aluminum) and L is one-way distance in feet.
Selecting proper wire gauges ensures high efficiency and prevents damage to sensitive motors and electronic appliances.
Worked example: 120V 20A branch circuit
A 16A continuous load running 100 feet on 12 AWG copper wire (6,530 CM) on a 120V single-phase circuit:
Vd = (2 × 12.9 × 16 A × 100 ft) ÷ 6,530 CM = 41,280 ÷ 6,530 = 6.32 Volts.
Percentage drop = (6.32 V ÷ 120 V) × 100% = 5.27%, exceeding the 3% branch circuit guideline. Upsizing to 10 AWG (10,380 CM) reduces the drop to 3.98V (3.31%).
Inputs for the US NEC voltage drop calculator
Wire size (AWG)
Select standard American Wire Gauge (14, 12, 10, 8, 6, 4, 2, 1/0, 2/0, 4/0 AWG).
One-way length in feet
Distance in feet from breaker panel to outlet/equipment.
CALCULATOR QUESTIONS
US NEC wire voltage drop calculator questions
What is the NEC voltage drop rule?
The NEC recommends a maximum 3% drop on branch circuits and 5% total system drop for reasonable efficiency of operation.
What is K in the US voltage drop formula?
K is the direct current resistance of a circular mil-foot of conductor at operating temperature (12.9 ohms-cmil/ft for copper at 75°C).