1. What is the NEC Voltage Drop Formula?

The NEC (National Electrical Code) voltage drop formula calculates the voltage loss that occurs in an electrical circuit due to the resistance of the conductors. It helps ensure proper voltage levels at the load end of the circuit.

2. How Does the Calculator Work?

The calculator uses the NEC voltage drop equation:

Where:

• \( VD \) — Voltage drop (volts)
• \( K \) — Material constant (12.9 for copper, 21.2 for aluminum)
• \( I \) — Current (amperes)
• \( L \) — One-way length of wire (feet)
• \( CM \) — Cross-sectional area of conductor (circular mils)

Explanation: The formula accounts for the resistance of the conductor material, current flow, and conductor length to determine voltage loss.

3. Importance of Voltage Drop Calculation

Details: Proper voltage drop calculation ensures electrical equipment receives adequate voltage to operate correctly and prevents excessive power loss in wiring.

4. Using the Calculator

Tips: Enter the material constant (K), current in amps, one-way length in feet, and conductor size in circular mils. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the maximum allowable voltage drop?
A: NEC recommends no more than 3% voltage drop for branch circuits and 5% for feeder and branch circuits combined.

Q2: Why multiply by 2 in the formula?
A: The factor of 2 accounts for the round-trip path of the current (both the "hot" and "neutral" conductors).

Q3: How do I find the CM value for my wire?
A: Refer to NEC Chapter 9, Table 8 which lists standard wire sizes and their circular mil areas.

Q4: Does this formula work for DC circuits?
A: Yes, the formula works for both AC and DC single-phase circuits. For three-phase circuits, use √3 instead of 2.

Q5: When should I be concerned about voltage drop?
A: For long circuit runs, high current loads, or sensitive equipment where voltage fluctuations could cause problems.