Voltage Drop Calculator
Voltage Drop Calculator works out Voltage drop and Voltage at the load from System voltage, Load current and One-way circuit length. The default figures on this page give voltage drop of 3.292 %; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
6 steps
- Multiplier for a single-phase circuit: 2 (out and back).
- K for copper at 75 degC = 12.9 ohm-cmil/ft; 12 AWG = 6,530 cmil.
- VD = 2 x 12.9 x 20 A x 100 ft / 6,530 cmil = 7.902 V.
- Percent drop = 7.902 / 240 = 3.292 %, against an allowance of 3 %.
- Voltage at the load = 240 - 7.902 = 232.098 V.
- Area needed for 3 % (7.200 V): 7,167 cmil, met by 10 AWG.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the Voltage Drop Calculator calculates
Calculates conductor voltage drop, percentage drop and voltage at the load for a copper or aluminium circuit, and the smallest conductor that keeps the drop inside a chosen limit.
How to use this Voltage Drop Calculator
Enter System voltage, Load current, One-way circuit length and Conductor material. Voltage drop updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Voltage drop, Voltage at the load and Conductor area.
The method follows NFPA 70, National Electrical Code — Chapter 9, Table 8 (Conductor Properties), NFPA 70, National Electrical Code — 210.19(A) Informational Note and 215.2(A)(2) Informational Note and IAEI Magazine, "Voltage Drop Calculations".
20 A on 12 AWG copper, 200 ft, 240 V single-phase example
With System voltage: 240, Load current: 20, One-way circuit length: 200, Conductor material: copper, the calculator returns Voltage drop: 15.8039816, Voltage drop: 6.58499234, Smallest size meeting the limit: 8 AWG. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Voltage drop, Voltage at the load and Conductor area. The inputs you can vary are System voltage, Load current, One-way circuit length, Conductor material, Conductor size and Circuit type.
Understanding Voltage drop, Voltage at the load and Conductor area
Advisory only. Voltage drop is one of several constraints on conductor size; ampacity, termination temperature, fault duty and the edition of NFPA 70 adopted by your authority having jurisdiction all govern the final design. Have a licensed electrical professional confirm any installed circuit.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
Can you calculate voltage drop?
Calculates conductor voltage drop, percentage drop and voltage at the load for a copper or aluminium circuit, and the smallest conductor that keeps the drop inside a chosen limit. Open “How this is worked out” beside the result to see each step for your own figures.
Is the Southwire voltage drop calculator free?
The calculation is free and does not require an account. An account is only needed for optional features such as saving, comparing, exporting, or sharing supported scenarios.
Can you provide A calculator for calculating voltage drop through wire?
Calculates conductor voltage drop, percentage drop and voltage at the load for a copper or aluminium circuit, and the smallest conductor that keeps the drop inside a chosen limit. On the figures this page opens with — system voltage 240 V, load current 20 A and one-way circuit length 200 ft — the voltage drop is 3.292%. Put your own figures in their place and the working updates beside the result.
How much voltage drop is there in 300 feet?
On the figures this page opens with — system voltage 240 V, load current 20 A, one-way circuit length 200 ft and allowable voltage drop 3% — the voltage drop is 3.292%. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
What information do I need for the Voltage Drop Calculator?
You need System voltage, Load current, One-way circuit length, Conductor material, Conductor size, Circuit type, Conductors in parallel per phase and Allowable voltage drop. The page opens with default figures that give voltage drop of 3.292 %, so you can see the scale before entering your own.
What does this Voltage Drop Calculator show?
It shows Voltage drop, Voltage at the load, Conductor area, Circuit resistance, Power lost in the conductors, Inside the allowable drop, Minimum area for the limit and Smallest size meeting the limit, with the working and any warnings for your entries.