Voltage Drop Calculator

Voltage drop is one of the most critical considerations when designing any electrical system, especially low-voltage DC systems used in off-grid solar, RVs, boats, and emergency backup power. When current flows through a wire, some energy is lost due to the wire's resistance. This loss appears as a reduction in voltage at the load end and heat generated in the wire.

In 12V systems, voltage drop is especially critical. A 3% drop at 12V is only 0.36 volts - barely noticeable in calculations but significant in practice. Many 12V devices and batteries have tight operating ranges. LiFePO4 batteries, for example, have a flat discharge curve between 12.8V and 13.2V. Excessive voltage drop can push your system below safe operating thresholds, reducing efficiency and potentially damaging equipment.

The solution is proper wire sizing. Using thicker wire (lower AWG number) reduces resistance and voltage drop. However, thicker wire costs more and is harder to work with. Our voltage drop calculator helps you find the optimal wire gauge for your specific application, balancing cost, practicality, and electrical efficiency.

The voltage drop formula for DC and single-phase AC circuits is:

VD = (2 × L × I × R) / 1000

Where VD is voltage drop in volts, L is one-way wire length in feet, I is current in amperes, and R is wire resistance in ohms per 1000 feet. The factor of 2 accounts for the round-trip distance (current flows to the load and back).

To calculate voltage drop percentage: divide the voltage drop by the source voltage and multiply by 100. For example, if you have a 0.5V drop in a 12V system: (0.5 / 12) × 100 = 4.17% voltage drop.

Power loss in watts is calculated as: P = VD × I (voltage drop multiplied by current). This represents energy wasted as heat in your wiring. In high-current applications, this can be substantial and is why efficient wire sizing matters for both safety and energy conservation.

Low-voltage DC systems like 12V face unique challenges with voltage drop. Because the system voltage is already low, even small absolute voltage drops represent a significant percentage of the total voltage. Consider these examples:

Example 1: LED lighting circuit. A 12V LED strip drawing 5 amps, located 20 feet from the battery. Using 14 AWG copper wire: VD = (2 × 20 × 5 × 3.07) / 1000 = 0.61V. That's a 5.1% drop, resulting in only 11.39V at the LED strip. The lights will be noticeably dimmer. Upgrading to 10 AWG wire reduces the drop to 2.0% (0.24V), delivering 11.76V.

Example 2: Inverter connection. A 2000W inverter at 12V draws about 167 amps at full load. Even a 3-foot cable run requires heavy wire. With 4 AWG copper: VD = (2 × 3 × 167 × 0.308) / 1000 = 0.31V (2.6% drop). This is acceptable, but anything longer requires 2 AWG or larger.

When calculating voltage drop, wire material significantly affects results. Copper and aluminum are the two most common conductor materials, each with distinct characteristics.

Copper wire has approximately 61% lower resistance than aluminum of the same gauge. This means less voltage drop and smaller wire sizes for the same application. Copper is also more durable, easier to terminate, and has better corrosion resistance. For most DC applications, especially in vehicles, boats, and off-grid systems, copper is the standard choice.

Aluminum wire is lighter and less expensive than copper. It's commonly used in utility power lines and large commercial installations where weight and cost savings outweigh the need for smaller wire sizes. When using aluminum, you must size up approximately 2 AWG numbers to achieve the same voltage drop performance as copper (e.g., use 2 AWG aluminum instead of 4 AWG copper).

The National Electrical Code (NEC) provides guidelines for acceptable voltage drop in electrical installations. While the NEC allows up to 5% total voltage drop (3% for branch circuits + 2% for feeders), many professionals recommend staying well below these maximums for optimal system performance.

Safety considerations extend beyond voltage drop percentages. Wires carrying current generate heat proportional to I²R (current squared times resistance). Undersized wires can overheat, damaging insulation and creating fire hazards. Always ensure your wire gauge meets both voltage drop requirements AND ampacity ratings for the expected current.

For off-grid systems, consider using our wire gauge calculator in conjunction with this voltage drop calculator. The wire gauge calculator helps you select wire based on current capacity, while this calculator verifies that your choice also meets voltage drop requirements for your specific wire run length.

If your calculated voltage drop is too high, you have several options to improve your system design:

1. Use larger wire. This is the most direct solution. Each step down in AWG number (larger wire) roughly halves the resistance and voltage drop. Going from 12 AWG to 10 AWG, for example, reduces resistance from 1.93 to 1.21 ohms per 1000 feet.

2. Shorten wire runs. Place batteries and power sources as close to loads as practical. In an RV or boat, centralizing the battery bank near high-current loads like inverters minimizes heavy wire runs.

3. Increase system voltage. Higher voltage systems carry the same power with less current. A 24V system carries 1000W at 42 amps versus 83 amps at 12V - half the current means roughly half the voltage drop for the same wire. This is why larger off-grid systems often use 24V or 48V.

4. Split loads across circuits. Instead of running one heavy wire to multiple loads, run separate smaller wires. This distributes current and can sometimes be more practical than running a single large cable.