Wire Sizing Methods: Ampacity vs Voltage Drop

Two Methods, One Goal

Electrical conductors must be sized to safely carry the required current while maintaining acceptable voltage at the load. Two primary methods govern conductor sizing: ampacity (current-carrying capacity) and voltage drop. Understanding when each method controls the design is essential for creating efficient, code-compliant, and cost-effective electrical systems. This guide explains both methods and provides practical guidance for professional electrical design.

In many installations, particularly those with short conductor runs, ampacity requirements determine the minimum wire size. However, for longer circuits or those with tight voltage tolerance requirements, voltage drop often mandates larger conductors than ampacity alone would require. Professional designers must check both criteria and use the larger size.

Ampacity-Based Sizing

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC Article 310 provides ampacity tables based on conductor size, insulation type, installation method, and ambient temperature. This method ensures conductors don't overheat and is a mandatory code requirement.

Voltage Drop-Based Sizing

Voltage drop sizing ensures the load receives adequate voltage for proper operation. While not a mandatory NEC requirement (it's an informational note), voltage drop limits are essential for equipment performance and energy efficiency. For long runs, voltage drop often requires larger conductors than ampacity alone would dictate.

The Design Process

1. 1
   Determine load current — Calculate the expected current based on load characteristics and apply any required factors (continuous load multiplier, etc.)
2. 2
   Size for ampacity — Select the minimum wire size that provides adequate ampacity per NEC Table 310.16, including derating factors
3. 3
   Calculate voltage drop — Using the ampacity-sized conductor, calculate the expected voltage drop percentage
4. 4
   Upsize if needed — If voltage drop exceeds limits, increase conductor size until requirements are met
5. 5
   Document both — Record the controlling factor in your design documentation

Size Your Conductors

Our wire sizing calculator considers both ampacity and voltage drop requirements to recommend the optimal conductor size for your application. Get instant results with NEC compliance verification.