NEC Wire Size Calculator – Voltage Drop Compliant Sizing for 120V & 240V Circuits

Choosing the right wire size is critical for safe and efficient electrical wiring. An undersized wire can overheat, cause voltage drops, and pose serious hazards. That’s why professionals use the NEC wire size calculator to follow industry standards and stay compliant with the National Electrical Code (NEC). Whether you’re working on 120V or 240V circuits, proper electrical cable sizing as per NEC ensures performance, safety, and long-term reliability.

This guide explains how wire sizing works, how voltage drop impacts your system, and how to accurately size wires using NEC-compliant voltage drop calculators. We’ll also cover the NEC guidelines, give technical examples, and help you understand everything in simple terms.

What Is NEC Wire Size and Why Is It Important?

The NEC wire size refers to the minimum cross-sectional area of a conductor that safely carries current without overheating. The NEC (National Electrical Code) provides tables and formulas that electricians use to determine the correct wire gauge for various loads, distances, and voltages.

Every wire has a resistance. The smaller the wire, the higher the resistance. When current flows, voltage is lost across that resistance — this is called voltage drop. Excessive voltage drop can cause poor performance or even damage appliances.

Using an NEC wire size calculator helps you account for both ampacity and voltage drop, ensuring the installation is safe, efficient, and code-compliant.

Understanding Voltage Drop and Its Effects

Voltage drop is the reduction in voltage in a circuit as current flows through wire resistance. It increases with:

• Longer wire lengths
• Higher current loads
• Smaller wire sizes

For example, a long run of wire from a panel to a subpanel or motor can lose several volts. On a 120V line, even a 5% drop means a 6V loss, which can severely affect equipment.

Use our free online tool Wire Size Calculator by Load and Length

NEC Recommended Voltage Drop Limits

The NEC suggests the maximum total voltage drop for both branch circuits and feeders should not exceed 5%. This includes:

• 3% voltage drop for branch circuits
• 2% voltage drop for feeders

While the NEC does not strictly enforce these values, they are considered best practices. Following them ensures optimal performance and reduces energy waste.

NEC Wire Size Calculator for 120V and 240V Circuits

A voltage drop calculator based on NEC standards uses the following formula:

Voltage Drop (V) = (2 × Length × Current × Resistance per 1000 ft) / 1000

Where:

• Length is the one-way distance of the wire in feet
• Current is the load in amperes
• Resistance depends on wire size and material (e.g., copper or aluminum)
• The multiplier is 2 for single-phase systems (out and back)

Using this result, you can determine the required wire gauge to keep the voltage drop under control.

Let’s break it down using common wire sizes.

Wire Resistance per 1000 ft (Copper Wire)

Let’s apply this to a practical scenario.

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Example 1: 120V Circuit for a 15A Load Over 100 Feet

Input:

• Voltage: 120V
• Load: 15 Amps
• Length: 100 ft (one-way)

Let’s try 12 AWG copper wire first.

Voltage Drop = (2 × 100 × 15 × 1.588) / 1000 = 4.76V
Percentage Drop = (4.76 / 120) × 100 = 3.97%

That’s close to the NEC limit of 3%. A safer choice is to go with 10 AWG, which gives:

Voltage Drop = (2 × 100 × 15 × 0.999) / 1000 = 2.998V
Percentage Drop = (2.998 / 120) × 100 = 2.5%

Conclusion: 10 AWG is safer and more efficient for this load and distance.

NEC Guidelines for Electrical Cable Size

When sizing electrical wires using NEC standards, consider these main factors:

1. Ampacity Rating – The maximum current a wire can carry without overheating. Check NEC Table 310.16 for ampacity values.
2. Insulation Type – Different insulations (e.g., THHN, XHHW) have different temperature ratings.
3. Ambient Temperature – High temperatures reduce a wire’s ampacity.
4. Conduit Fill – Multiple wires in a conduit may require derating.
5. Voltage Drop – Always adjust wire size if the calculated drop exceeds 3% for branches or 5% overall.

These guidelines are applied whether you use an NEC voltage drop calculator manually or through an online tool.

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Example 2: 240V Water Pump at 30A Load Over 150 ft

Let’s size wire for a 240V pump drawing 30A, 150 feet from the panel.

Try 8 AWG copper wire:

Voltage Drop = (2 × 150 × 30 × 0.628) / 1000 = 5.652V
Percentage Drop = (5.652 / 240) × 100 = 2.35%

That’s acceptable. But if the wire run was 200 ft, let’s see:

Voltage Drop = (2 × 200 × 30 × 0.628) / 1000 = 7.536V
Percentage Drop = (7.536 / 240) × 100 = 3.14%

In that case, go up to 6 AWG:

Voltage Drop = (2 × 200 × 30 × 0.395) / 1000 = 4.74V
Percentage Drop = (4.74 / 240) × 100 = 1.975%

Result: Use 6 AWG for long runs over 200 feet to remain within safe voltage drop limits.

NEC Wire Size Table for 120V and 240V Circuits

Here’s a quick-reference table for common loads, distances, and recommended wire sizes based on NEC voltage drop compliance. This assumes copper wire and typical residential applications.

Always use a voltage drop calculator to confirm for your specific case.

Copper vs. Aluminum – Impact on Wire Size

Copper wire has lower resistance, so it carries current better over long distances. Aluminum is cheaper and lighter, but you need a larger size for the same current capacity.

For example:

• 4 AWG copper = 0.248 ohms/1000 ft
• 2 AWG aluminum = 0.317 ohms/1000 ft

So, when using aluminum, you must increase the wire size to compensate for higher resistance and increased voltage drop.

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Tips for Using NEC Voltage Drop Calculators Effectively

• Always include total wire length (out and back)
• Choose wire type and material correctly
• Factor in the load type (resistive, motor, lighting)
• Consider future load expansions
• Test different wire gauges for cost-effective sizing

Most online electrical cable size NEC calculators will ask for:

• Load in amps
• Distance in feet
• System voltage (120V/240V)
• Wire material (copper/aluminum)
• Acceptable voltage drop (%)

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Conclusion: Always Size Wires for Safety and Performance

Choosing the correct wire size is not just a matter of code — it ensures safety, system performance, and longevity. A properly used NEC wire size calculator will help you stay compliant and avoid costly mistakes.

For both 120V and 240V systems, always calculate:

• Ampacity based on NEC
• Voltage drop across the circuit
• Correct wire material and insulation

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