C-10 Voltage Drop Calculations: The Easy Way

Voltage drop calculations are a staple of the C-10 exam. The good news: there's really only one formula you need to memorize, and once you understand it, these questions become easy points.

The NEC Voltage Drop Recommendations

The NEC recommends (but does not mandate) maximum voltage drop limits in Section 210.19(A)(1) Informational Note No. 4 and 215.2(A)(4) Informational Note No. 2:

• Branch circuits: Maximum 3% voltage drop
• Feeder + branch circuit combined: Maximum 5% total voltage drop
• Feeders alone: Maximum 3% (to leave room for 2% branch circuit drop)

While these are recommendations, the C-10 exam treats them as requirements for calculation purposes. If a question asks for "maximum voltage drop," use 3% for branch circuits and 5% for the total system.

The Core Formula

The voltage drop formula for single-phase circuits:

Understanding each variable:

• VD (Voltage Drop): The voltage lost in the conductor, measured in volts.
• K (Resistivity): 12.9 for copper conductors, 21.2 for aluminum conductors. Memorize these two values.
• I (Current): The load current in amperes flowing through the conductor.
• D (Distance): The one-way distance from the source to the load in feet (not the total wire length). The "2" in the formula accounts for the round-trip.
• CM (Circular Mils): The cross-sectional area of the conductor in circular mils. Common values: #14 = 4,110 CM, #12 = 6,530 CM, #10 = 10,380 CM, #8 = 16,510 CM, #6 = 26,240 CM.

Three-Phase Voltage Drop

For three-phase circuits, the formula changes slightly because the geometry of the circuit is different:

The only difference is replacing the "2" with "1.732" (√3). All other variables remain the same. The exam will typically specify whether the circuit is single-phase or three-phase.

Worked Example

Question: What is the voltage drop on a 120V, single-phase circuit using #12 copper conductors carrying 16 amps over a distance of 100 feet?

• VD = (2 × 12.9 × 16 × 100) ÷ 6,530
• VD = 41,280 ÷ 6,530
• VD = 6.32 volts
• Percentage: (6.32 ÷ 120) × 100 = 5.27%
• This exceeds the 3% recommendation (3.6V max). You would need to upsize to #10 AWG.

Solving for Other Variables

The exam may ask you to solve for different variables. Rearrange the formula:

• Find minimum conductor size: CM = (2 × K × I × D) ÷ VD(max)
• Find maximum distance: D = (VD × CM) ÷ (2 × K × I)
• Find maximum current: I = (VD × CM) ÷ (2 × K × D)

For "find minimum conductor size" questions, calculate the required CM, then select the next larger standard conductor size from your memorized CM values.

Percentage Voltage Drop

To convert voltage drop in volts to a percentage, use:

Be careful with the source voltage: use 120V for 120V circuits, 240V for 240V circuits, 208V for 208V three-phase, and 480V for 480V three-phase. The percentage references the source voltage, not the load voltage.