Voltage Drop Calculator for NC Wiring Projects
Calculate voltage drop, percentage drop, and receiving-end voltage for copper or aluminum conductors per NEC guidelines used in North Carolina wiring projects.
Formulas Used
Single-Phase Voltage Drop:
VD = 2 × I × Rone-way
Three-Phase Voltage Drop:
VD = √3 × I × Rone-way
One-Way Conductor Resistance:
Rone-way = (r / 1000) × L
Where: r = resistivity in Ω/1000 ft (from NEC Chapter 9, Table 9 at 75°C), L = one-way length in feet, I = load current in amperes.
Percentage Voltage Drop:
VD% = (VD / Vsource) × 100
Receiving-End Voltage:
VR = Vsource − VD
Assumptions & References
- Conductor resistivity values are taken from NEC Chapter 9, Table 9 (Alternating-Current Resistance and Reactance for 600-Volt Cables) at 75°C conductor temperature.
- Reactance (XL) is not included; this is a resistive-only calculation suitable for most practical low-voltage wiring estimates.
- The NEC (NFPA 70) recommends a maximum voltage drop of 3% for branch circuits and 5% combined for feeders and branch circuits (NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2).
- North Carolina adopts the NEC with state amendments via the NC State Building Code: Electrical, administered by the NC Department of Insurance.
- One-way length is used; the formula accounts for the round-trip path via the phase multiplier (2 for single-phase, √3 for three-phase).
- Assumes balanced load and negligible power factor correction (unity power factor).
- For aluminum conductors, ensure proper terminations rated for aluminum per NEC 110.14.
- Always verify calculations with a licensed NC electrician and local AHJ (Authority Having Jurisdiction).