What is the K-factor for Copper vs Aluminum?

At 75°C, the K-factor (Direct Current Resistance Constant) is typically 12.9 for Copper and 21.2 for Aluminum. This means Aluminum has higher resistance and often requires a larger gauge for the same load.

Voltage Drop Calculator + Payback Planner (Single-Phase & Three-Phase)

Q: Do I enter one-way or round-trip length?

Enter the one-way run length. The calculator handles the round-trip resistance for single-phase and the phase-to-phase geometry for three-phase circuits automatically.

Q: Does power factor change the voltage drop?

Yes, if you include reactance. With resistance only, PF does not change the magnitude of drop significantly in simple DC-like models, but in AC circuits with reactance, the formula becomes I * (R cosφ + X sinφ).

System

Configuration Nominal voltage (V) Load input

Current (A) Power factor (0–1)

Duty & energy

Hours per year Energy price ($/kWh) Extra cost to upsize wire ($)

Conductor & length

Material Ambient temp (°C) One-way run length

Size by AWG or mm² (no dataset; physics-based)

AWG size

Area (mm²)

Advanced: reactance & direct resistance override

Reactance per length (Ω / 1000 ft) Resistance override (Ω / 1000 ft) Use override?

If you have a datasheet value (Ω per 1000 ft or per km), paste it here and enable the override. Otherwise the tool computes resistance from conductor area, material resistivity, and temperature.

Enter your details and select Calculate.

Method & assumptions

AWG formula (no lookup table): area in mm² = 0.012668 × 92^((36 − n)/19.5). We convert to m² for resistance.
Resistivity @ 20°C: Copper 1.724e−8 Ω·m, Aluminum 2.826e−8 Ω·m. Temperature coefficient: Cu 0.00393/°C, Al 0.00403/°C.
Voltage drop (resistive only): 1‑phase uses 2×I×R_cond, 3‑phase uses √3×I×R_phase. If reactance X is provided, uses I×(R cosφ + X sinφ) with the same geometric factor.
Losses: Resistive line loss P = I²×R_loop. Loop R is round-trip for 1-phase; for 3-phase, total copper loss is 3×I²×R_phase.
Payback uses your extra material/labor cost to upsize and compares annual energy savings.

About this free voltage drop calculator

This online voltage drop calculator estimates single‑phase and three‑phase voltage drop with power factor, optional reactance, AWG or mm² sizing, and copper or aluminum conductors. It also computes line losses in watts, converts them to kWh and annual cost, and shows the payback if you upsize wire. Everything runs locally in your browser.

The Physics of Voltage Drop: Ohm’s Law Application

Voltage drop occurs because every conductor has internal resistance. As current flows through the wire, this resistance dissipates energy as heat, reducing the voltage available at the load. This is a direct application of Ohm's Law ($V = I \times R$).

The master formula used for single-phase calculations is:

$$V_{drop} = \frac{2 \times K \times L \times I}{CM}$$

K: Direct Current Resistance Constant (Ohm-circular mils per foot)
L: Length of the conductor (one-way)
I: Load Current (Amps)
CM: Circular Mils (Cross-sectional area of the wire)

For three-phase circuits, the multiplier changes from 2 to $\sqrt{3}$ (approx. 1.732).

NEC Compliance: The Authority Signal

While this tool provides accurate physics-based calculations, electrical installations in the US must comply with the National Electrical Code (NEC).

Branch Circuits: NEC 210.19(A)(1) Informational Note No. 4 recommends limiting voltage drop to 3% for branch circuits to ensure reasonable efficiency.
Feeders: NEC 215.2(A)(1) Informational Note No. 2 recommends a 3% drop for feeders and a total combined drop of 5% for both feeder and branch circuits.

Note: These are generally recommendations (Informational Notes) for efficiency, not mandatory safety codes, except for specific critical systems like fire pumps. However, following them is standard professional practice.

The Science of K-Factors: Copper vs. Aluminum

The "K-factor" represents the DC resistance of a circular mil-foot of wire at a specific temperature. This is why material selection matters:

Material	K-Factor (@ 75°C)	Characteristics
Copper	12.9	Lower resistance, standard for most wiring.
Aluminum	21.2	Higher resistance, requires upsizing, lighter weight.

Because Aluminum has a higher K-factor (21.2 vs 12.9), it mathematically necessitates a larger Circular Mil (CM) area to achieve the same voltage drop performance as Copper.

Temperature Correction Features

Resistance increases with temperature. A wire running across a hot rooftop will have more voltage drop than one in a cool basement. This tool allows you to adjust the ambient temperature, applying the formula $R_{new} = R_{ref} \times [1 + \alpha(T_{new} - T_{ref})]$ to give you a real-world estimate.

Voltage drop & payback FAQ

Do I enter one-way or round-trip length?

Enter the one-way run. The tool handles round-trip internally for single-phase and total per-phase handling for three-phase.

Does power factor change the voltage drop?

Yes if you include reactance. With resistance only, PF does not change the magnitude of drop; including reactance uses R cosφ + X sinφ.

What if I only know resistance from a datasheet?

Use the override. Paste Ω per 1000 ft (or per km after converting) and set Use override = Yes.