Voltage Drop Calculator for Missouri Electricians

Missouri has adopted the NEC 2023 edition. The NEC recommends keeping voltage drop to 3% or less on branch circuits and 5% or less combined (branch + feeder) for conductors carrying lighting and receptacle loads (NEC 210.19 Informational Note).

Use the Calculator

Enter your circuit details below. The calculator uses the NEC 2023 conductor resistance values from Table 9.

→ Open the Voltage Drop Calculator

Common Missouri Wiring Scenarios

  • Residential 120V branch circuit: 15A or 20A circuits feeding receptacles and lighting. Size for 3% drop at the farthest outlet.
  • 240V appliance circuit: Dryers, ranges, HVAC units. Use 6 AWG or larger for runs over 50 feet at 30A.
  • Commercial 480V feeder: Use Table 9 resistance values at 75°C column for THWN-2 conductors.

NEC 2023 Key Wire-Sizing Rules for Missouri

  • Table 310.12 (residential): 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A at 60°C.
  • Table 310.16 (commercial/industrial): Use the 75°C column for conductors terminated in equipment rated 75°C or higher.
  • Voltage drop is an informational note, not a hard code requirement, but AHJs and utilities increasingly treat 3% as mandatory.

Frequently Asked Questions

What is the maximum allowed voltage drop in Missouri?

The NEC 2023 (adopted by Missouri) recommends, but does not mandate, a maximum of 3% voltage drop on branch circuits and 5% total (branch + feeder combined). Some local Missouri amendments and utility interconnection rules make the 3% limit mandatory. Check with your local AHJ.

How do I calculate voltage drop for a 240V circuit in Missouri?

Voltage Drop = (2 × Length × Current × Resistance per 1000 ft) / 1000. Use our free calculator which pulls the correct resistance values from NEC 2023 Table 9 automatically.

Which wire gauge should I use for a 100-foot 20A run in Missouri?

For a 20A, 120V circuit at 100 feet, 12 AWG gives approximately 3.3% voltage drop, just over the recommended 3%. Upgrade to 10 AWG to bring it under 3%. Use the calculator to confirm for your exact load.