Voltage Drop Southwire Calculator
Voltage Drop Calculator (trade K-factor method) 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 (trade K-factor method) 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 (trade K-factor method)
Enter System voltage, Load current, One-way circuit length and Conductor material. Voltage drop updates as you type. 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".
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
What information do I need for the Voltage Drop Calculator (trade K-factor method)?
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 (trade K-factor method) 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.
Can I check how the Voltage Drop Calculator (trade K-factor method) result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.