Service Entrance Wire Size Calculator
Determine the minimum AWG or kcmil copper or aluminum service entrance conductor size based on total load (amperes), service voltage, and run length using NEC voltage drop and ampacity guidelines.
Formulas Used
Voltage Drop (Single-phase):
VD (volts) = (2 × K × I × L) / CM
Voltage Drop (Three-phase):
VD (volts) = (1.732 × K × I × L) / CM
Required Circular Mils:
CM = (multiplier × K × I × L) / VDallowed
Where:
K = Resistivity constant (Copper = 12.9, Aluminum = 21.2 CM·Ω/ft at 75°C)
I = Load current (Amperes)
L = One-way conductor length (feet)
CM = Circular mils of conductor
VDallowed = Voltage × (Max VD% / 100)
The final wire size is the larger of the ampacity-based size (NEC Table 310.16, 75°C column) and the voltage-drop-based size.
Assumptions & References
- Ampacity values from NEC Table 310.16 (2020 edition), 75°C insulation column (THWN, XHHW, USE-2).
- Resistivity constants: Copper K = 12.9, Aluminum K = 21.2 (circular-mil·ohms/ft at 75°C).
- NEC 210.19(A) and 215.2(A) recommend maximum 3% voltage drop for branch circuits and feeders; 5% total from service to final outlet.
- NEC 230.42 requires service entrance conductors rated for the calculated load at 100% (no 80% continuous-load derate at the service level).
- NEC 230.79 sets minimum service sizes: 100A for single-family dwellings, 60A for limited loads.
- No derating applied for conduit fill (>3 conductors), high ambient temperature, or wet locations — apply NEC 310.15 correction factors as needed.
- Neutral conductor sizing (NEC 220.61) and equipment grounding conductor sizing (NEC 250.122) are not included.
- Three-phase calculations assume balanced load and use the √3 (1.732) multiplier.
- Always consult a licensed electrician and the local Authority Having Jurisdiction (AHJ) before installation.