NEC Voltage Drop Calculator & Guide — Free [2026]

The NEC addresses voltage drop in Informational Notes rather than mandatory requirements. Section 210.19(A) Informational Note No. 4 recommends:

• 3% maximum voltage drop for branch circuits
• 5% maximum total voltage drop for feeder plus branch circuit combined

While these are recommendations and not mandatory code requirements, they are widely adopted as best practice and may be enforced by local AHJs (Authorities Having Jurisdiction). Exceeding these limits can cause equipment malfunction, motor overheating, and flickering lighting.

The voltage drop for a conductor is calculated using impedance values from NEC Chapter 9, Table 9:

Where:

• I = circuit current in amperes
• L = one-way cable length in feet
• R = AC resistance per 1000 feet from Table 9
• X = reactance per 1000 feet from Table 9
• θ = power factor angle

For most circuits with unity or near-unity power factor, the simplified formula is: Vd = 2 × I × L × R / 1000 (single-phase) or multiply by √3/2 for three-phase.

Table 9 provides AC resistance and reactance values in ohms per 1000 feet for 600V cables at 75°C. Key values for copper conductors in PVC conduit:

AWG/kcmil	R (Ω/1000ft)	X (Ω/1000ft)
14 AWG	3.14	0.058
12 AWG	1.98	0.054
10 AWG	1.24	0.050
8 AWG	0.786	0.052
6 AWG	0.510	0.051
4 AWG	0.321	0.048
2 AWG	0.202	0.045
1/0 AWG	0.127	0.044

For aluminium conductors, AC resistance values are approximately 1.6 times higher than copper. Use the ECalPro Voltage Drop Calculator for automatic lookup of all Table 9 values.

Calculate voltage drop to NEC standards instantly with the ECalPro Voltage Drop Calculator. Supports:

• Single-phase and three-phase calculations
• Automatic Table 9 resistance/reactance lookup
• AWG and metric conductor sizes
• Power factor correction
• Pass/fail against 3% and 5% limits
• Professional PDF report generation

Also supports BS 7671, IEC 60364, and AS/NZS 3008 voltage drop calculations in the same tool. Free — no signup required.