Voltage Drop Calculator CalculatorScale.com

Electrical Voltage Drop Calculator (NEC Standards)

Compute voltage loss, conductor resistance, and maximum safe circuit runs.

Caution: Voltage drop is between 3% and 5% (acceptable for feeders, consider upsizing wire for branch circuits).
Voltage Drop
5.79 V
4.83% of Supply
Voltage at Load End
114.21 V
Delivered Voltage
Conductor Resistance
0.386 Ω
Total Circuit Ohms
Power Loss
86.8 W
Thermal Heat Loss

Maximum Circuit Distance for Selected Wire (12 AWG COPPER @ 15A)

Max Distance for 3% Drop (NEC Branch):62 ft
Max Distance for 5% Drop (NEC Total):104 ft

Understanding National Electrical Code (NEC) Voltage Drop Rules

Under National Electrical Code (NEC) Informational Note 210.19(A) and 215.2(A)(1), electrical systems are recommended to limit voltage drop to ensure reasonable efficiency of operation:

Branch Circuits: Max 3% Drop

The voltage drop on branch circuit conductors from the final overcurrent device to the load should not exceed 3%.

Combined Feeder + Branch: Max 5% Drop

The maximum total voltage drop on both feeders and branch circuits to the farthest outlet should not exceed 5%.

Mathematical Voltage Drop Formulas

Single Phase AC & DC Circuits (Round Trip):

V_drop = (2 × K × L × I) ÷ CM

Three Phase AC Circuits:

V_drop = (1.732 × K × L × I) ÷ CM

Frequently Asked Questions

What causes voltage drop in electrical wiring?

All electrical conductors possess internal electrical resistance. As current (amperage) flows through the wire over distance, a portion of the electrical energy is converted into heat ($P = I^2 R$), reducing the remaining voltage available at the load end.

How do I fix excessive voltage drop?

The most effective solution is to upsize the conductor wire gauge (e.g., upgrading from 14 AWG to 12 AWG or 10 AWG), shortening the cable run distance, using copper instead of aluminum, or increasing the operating system voltage (e.g., 240V instead of 120V).

Why does aluminum wire have higher voltage drop than copper?

Copper has an electrical conductivity of ~100% IACS, while aluminum is ~61% IACS. Because aluminum has higher electrical resistivity, an aluminum conductor must be approximately two AWG sizes larger than a copper conductor to carry the same current with equivalent voltage drop.