About

Accurate wire sizing is the backbone of electrical safety and efficiency. This tool calculates the voltage drop across a circuit, ensuring compliance with the National Electrical Code (NEC) recommendations. Excessive voltage drop results in inefficient equipment operation, overheating motors, and potential fire hazards. For sensitive electronics, a drop > 1.5% can cause resets; for general loads, the NEC recommends keeping the drop under 3% for branch circuits.

This calculator utilizes standard resistance values at 75°C to provide a realistic worst-case scenario for both Copper and Aluminum conductors. Unlike basic estimators, it accounts for circuit phase type and provides a visual cross-section reference to assist in physical wire identification.

Formulas

The voltage drop V_d is calculated using the resistance of the conductor over a specific distance.

{

V_d = 2 × L × R × I1000 for 1-Phase / DCV_d = √3 × L × R × I1000 for 3-Phase

To find the percentage drop %V_d:

%V_d = V_dV_source × 100

Legend:
L = Length of circuit (one way)
R = Resistance per 1000ft (or km)
I = Load Current (Amperes)
V_source = System Voltage

Reference Data

AWG / kcmil	Area (mm²)	Res. Copper (Ω/kft)	Res. Aluminum (Ω/kft)	Max Load (Est. @ 75°C)
14 AWG	2.08	3.14	N/A	20 A
12 AWG	3.31	1.98	3.25	25 A
10 AWG	5.26	1.24	2.03	35 A
8 AWG	8.37	0.778	1.28	50 A
6 AWG	13.3	0.491	0.808	65 A
4 AWG	21.2	0.308	0.508	85 A
2 AWG	33.6	0.194	0.319	115 A
1/0 AWG	53.5	0.122	0.201	150 A
2/0 AWG	67.4	0.0967	0.159	175 A
4/0 AWG	107	0.0608	0.100	230 A
250 kcmil	127	0.0515	0.0847	255 A
500 kcmil	253	0.0258	0.0424	380 A

Frequently Asked Questions

The National Electrical Code (NEC) recommends a maximum 3% voltage drop for branch circuits to ensure efficiency. Excessive drop causes heating (energy loss) and can prevent motors from starting or damage sensitive electronics.

Copper is superior for conductivity and resistance to corrosion, making it the standard for household wiring. Aluminum is lighter and cheaper, often used for heavy feeder lines, but requires larger gauges to carry the same current and special anti-oxidant connections.

Resistance increases with temperature. This tool uses the NEC standard reference of 75°C (167°F), which assumes the wire is under load and warm. Using room temperature (20°C) values would underestimate the voltage drop in real-world scenarios.

Yes, the "Conduit Estimator" feature checks if 3 conductors of your selected size will fit into standard trade-size conduits (EMT/PVC) based on the 40% fill rule.