Dc Wire Size Calculator

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Reviewed by: David Chen, P.E. (Professional Electrical Engineer)

The DC Wire Size Calculator quickly determines the minimum required wire gauge (AWG or kcmil) for a specific DC electrical load, ensuring the voltage drop remains within acceptable limits. This tool is essential for solar PV systems, RVs, marine applications, and any long-distance DC wiring runs.

DC Wire Size Calculator

Amps (A)
Volts DC (V) – e.g., 12V, 24V, 48V
Distance in Feet (ft) from source to load
Recommended maximum is 3%

Recommended Minimum Wire Size

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AWG (American Wire Gauge)

Detailed Calculation Steps

Press Calculate to see the steps.

DC Wire Size Calculator Formula

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

$$ V_{drop} = \text{System Voltage} \cdot (\frac{\text{Max Drop } \%}{100}) $$

Formula Source: National Electrical Code (NEC) General Principle | Resistivity Constant Reference: Copper Development Association

Variables Explained

  • CM (Circular Mils): The required conductor cross-sectional area. A higher CM value means a larger (thicker) wire.
  • K (Resistivity): The specific resistance of the conductor material. K is typically 12.9 for Copper and 21.2 for Aluminum (at 75°C). This calculator uses 12.9 (Copper).
  • I (Current): The load current in Amps.
  • L (Length): The one-way distance in feet from the power source to the load. Since current must travel both ways (out and return), the formula includes the factor of 2.
  • Vdrop (Voltage Drop): The maximum allowed drop in voltage, in Volts.

Related Calculators

What is DC Wire Sizing?

DC (Direct Current) wire sizing is the process of selecting a conductor with an adequate cross-sectional area (gauge) to handle the required current over a given distance while maintaining the voltage drop within acceptable limits. Unlike AC (Alternating Current) circuits, which are prone to power factor and inductance effects, DC circuits primarily suffer from resistive losses. These losses cause the voltage delivered to the load to be lower than the source voltage, known as “voltage drop.”

Excessive voltage drop is problematic, especially in low-voltage DC systems (like 12V), because even a small loss in voltage represents a significant percentage of the total supply. For instance, a 1-volt drop on a 12-volt system is over 8%. This can cause loads like motors to run inefficiently or electronics to fail to power on correctly. Industry standards (like the NEC) often recommend limiting the total voltage drop to 3% for power and lighting circuits.

How to Calculate DC Wire Size (Example)

Let’s find the required AWG wire size for a 35A load on a 24V system, located 60 feet away, with a maximum 3% voltage drop allowed (using Copper wire, K=12.9).

  1. Determine the Maximum Allowable Voltage Drop ($V_{drop}$): $$ V_{drop} = 24V \cdot (3\% / 100) = 0.72 \text{ Volts} $$
  2. Apply the Circular Mils (CM) Formula: $$ \text{CM} = \frac{2 \cdot K \cdot I \cdot L}{V_{drop}} $$ $$ \text{CM} = \frac{2 \cdot 12.9 \cdot 35 \text{ A} \cdot 60 \text{ ft}}{0.72 \text{ V}} $$
  3. Calculate the Required CM: $$ \text{CM} = \frac{54180}{0.72} = 75,250 \text{ Circular Mils} $$
  4. Select the Wire Gauge: Since 75,250 CM is greater than the standard size for AWG 2 (66,360 CM), the next larger standard size must be chosen, which is AWG 1/0 (105,600 CM).

Frequently Asked Questions (FAQ)

What is the standard maximum voltage drop percentage?
The National Electrical Code (NEC) generally recommends a maximum voltage drop of 3% for feeder or branch circuits in power and lighting applications. However, some critical loads may require less than 1% drop.
Why is wire sizing more critical for 12V DC than 120V AC?
In a 12V system, a 0.5V drop is about 4.1% of the total voltage. In a 120V system, a 0.5V drop is only 0.4%. Since the current (I) is inversely proportional to voltage (V) for a fixed power (P=VI), 12V systems require much higher currents, which exacerbates resistive losses and voltage drop, making proper sizing crucial.
What is the difference between AWG and kcmil?
AWG (American Wire Gauge) is used for wire sizes up to 4/0. Wire sizes larger than 4/0 are measured in kcmil (thousands of circular mils), which is often referred to simply as MCM.
Does temperature affect wire size requirements?
Yes, the resistivity constant (K) increases as temperature rises. The formula used here utilizes $K=12.9$, which is an industry standard value for 75°C (167°F) rated conductors, providing a safe, conservative size recommendation for typical applications.
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