How to Calculate Voltage Drop Across a Cable (With Free Online Calculator) - OneSDR

⚡Make sure your power gets where it needs to go — safely and efficiently!

Ever plug something in at the far end of a long extension cord and it doesn’t work as well? That’s often due to voltage drop — when some of the voltage is lost along the cable because of resistance in the wire.

Too much voltage drop can cause dim lights, slow motors, and even overheating. Let’s go over how to calculate it so you can avoid surprises!

Table of Contents

🙋 What Is Voltage Drop?

Voltage drop is the reduction in voltage as electrical current travels through a cable. All wires have some resistance, so the farther electricity has to go, the more voltage it can lose — especially if the cable is thin or the current is high.

Voltage drop becomes important when:

You’re wiring long distances (like outdoor lighting or sheds)
You’re using high current loads (like heaters or motors)
You want your equipment to perform at full power

🧮 Voltage Drop Formula

Here’s the most common formula:

Voltage Drop (V) = (2 × L × I × R) ÷ 1000

Where:

L = one-way length of the wire (in meters or feet)
I = current in amps
R = resistance per unit length of the wire (in ohms per meter or foot)
2 = because the current goes to the load and back
1000 = for converting millivolts to volts (if needed)

The result gives you how much voltage is lost along the cable.

🌐 Real Example

Let’s say you’re running a copper wire that’s 50 feet long, carrying 10 amps. You’re using 12 AWG wire, which has a resistance of about 1.588 ohms per 1000 feet.

Step 1: Convert resistance to ohms per foot

R = 1.588 ÷ 1000 = 0.001588 Ω/ft

Step 2: Plug into the formula

Voltage Drop = 2 × 50 × 10 × 0.001588 = 1.588 volts

So you’ll lose about 1.6V across that cable run.

If your power source is 120V, your device only sees about 118.4V — which might be fine, but if the drop gets too big, it can cause problems.

⚙️ Cable Voltage Drop Calculator

Use this calculator to find the voltage drop (also as a percentage). Enter the cable dimensions either as AWG or in metric.

Voltage Drop Calculator

Imperial (ft & AWG) Metric (m & mm²)

Select Wire Gauge (AWG): Cable Length (one-way, in feet):

Current (amps): Source Voltage (volts):

💡 Voltage Drop Recommendations

Most electricians try to keep voltage drop under 3% of the total voltage.

For 120V circuits → keep drop below 3.6V
For 240V → keep it below 7.2V

If you’re over that limit:

Use thicker wire (lower resistance)
Shorten the cable
Reduce the current (use smaller loads or split circuits)

📌 Common Cable Resistance Values

Wire Gauge (AWG)	Ohms per 1000 ft
14 AWG	2.525 Ω
12 AWG	1.588 Ω
10 AWG	0.999 Ω
8 AWG	0.628 Ω
6 AWG	0.395 Ω