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Voltage Drop Calculator

Estimate voltage drop, percentage loss, and terminal voltage over a distance for common wire sizes.

How Voltage Drop works

Electrical current moving through wire experiences friction (resistance), which causes heat generation and drops voltage. Sizing cables properly ensures safety and prevents machinery damage.

Calculations are based on one-way distance in single-phase systems (incorporating 2 runs of wire). Conductor size is measured in AWG (American Wire Gauge), and resistivity constant K is specified at 75°C.

Formula

1Voltage Drop (Vd) = (2 × K × I × L) ÷ Area

2Percentage Loss = (Vd ÷ Source Voltage) × 100

K is the resistivity constant (Cu = 12.9, Al = 21.2); Area is circular mils.

Voltage Drop conversion table

Reference Table5 rows

Estimated voltage drop for 120V 15A loads across copper wire (100 ft run).
Wire Size (AWG)↕	Voltage Drop (V)↕	Percentage Loss (%)↕	Terminal Voltage (V)↕
14 AWG	9.42 V	7.85 %	110.58 V
12 AWG	5.93 V	4.94 %	114.07 V
10 AWG	3.73 V	3.11 %	116.27 V
8 AWG	2.34 V	1.95 %	117.66 V
6 AWG	1.47 V	1.23 %	118.53 V

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FAQ: Voltage Drop

What is acceptable voltage drop?

In general, the National Electrical Code (NEC) recommends a maximum voltage drop of 3% across branch circuits, and 5% total across both feeder and branch circuits for optimal performance.

Why is copper better than aluminum for voltage drop?

Copper has a lower resistivity constant (12.9) compared to aluminum (21.2). This means copper conducts electricity with less friction and results in lower voltage drop for the same wire thickness.

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