Free Tool · NEC Chapter 9
Voltage drop & wire size calculator
Check the voltage drop on a copper run, or set a target and get the smallest wire that meets it. The NEC math is shown so you can check it.
6.60%
7.92 V dropped over 100 ft. 112.08 V reaches the load. Over the 5% total target: size up or shorten the run.
The math
2 × 20 A × 1.98 Ω/kft ÷ 1000 × 100 ft = 7.92 V
Planning aid, not a substitute for a licensed electrician or your authority having jurisdiction. Uses NEC Chapter 9, Table 8 DC resistance for uncoated stranded copper at 75°C (the conservative case). The 3% branch and 5% total figures are the design targets in the NEC 210.19 and 215.2 Informational Notes, which are recommendations, not hard limits. Ampacity (conductor current rating) is a separate requirement this tool does not check.
Need someone to design the run?
Norvet plans and manages low-voltage and cabling projects, then brings in licensed crews to install and hands you the documentation.
Request a cabling quoteCopper conductor resistance (NEC Chapter 9, Table 8)
Uncoated stranded copper, DC resistance at 75°C. These are the values the calculator uses.
| AWG | Circular mils | Ω / 1000 ft |
|---|---|---|
| 18 | 1,620 | 7.95 |
| 16 | 2,580 | 4.99 |
| 14 | 4,110 | 3.14 |
| 12 | 6,530 | 1.98 |
| 10 | 10,380 | 1.24 |
| 8 | 16,510 | 0.778 |
| 6 | 26,240 | 0.491 |
| 4 | 41,740 | 0.308 |
| 3 | 52,620 | 0.245 |
| 2 | 66,360 | 0.194 |
| 1 | 83,690 | 0.154 |
Common questions
What is the voltage drop formula?
For a 2-wire DC or single-phase circuit, voltage drop = 2 × current × resistance × length ÷ 1000, where resistance is ohms per 1000 ft from NEC Chapter 9 Table 8 and length is the one-way run in feet. The factor of 2 accounts for the current traveling out and back. Three-phase uses √3 instead of 2.
How much voltage drop does the NEC allow?
The NEC recommends no more than 3% on a branch circuit (210.19 Informational Note) and 5% total for feeder plus branch (215.2 Informational Note). These are recommendations in the informational notes, not hard code limits, but most designers and inspectors treat them as the target.
Does this cover PoE and low-voltage runs?
It covers copper 18 AWG through 1 AWG using NEC Table 8 resistance, which fits many low-voltage power runs and 18/2 or 18/4 device wiring. Note that Cat6 conductors are 23 AWG, below Table 8, and PoE has its own IEEE 802.3 power budget, so for PoE-over-Ethernet use the switch and device power specs rather than this tool.
Why use the 75°C resistance value?
NEC Table 8 lists DC resistance at 75°C. Using it is conservative: a hotter conductor has higher resistance and therefore more voltage drop, so a run that operates cooler will drop slightly less than shown. Sizing to the 75°C value keeps a safety margin.
Does this check ampacity?
No. This tool sizes for voltage drop only. A conductor must also carry the load current safely (ampacity, NEC 310.16 and related). Always confirm ampacity separately; the correct wire is the larger of what ampacity and voltage drop each require.
Cite this tool
Free to reference or link to, with credit. No permission needed.
APA citation
For coursework, training material, and reports.
Norvet MSP. (2026). Voltage Drop & Wire Size Calculator (NEC) [Computer software]. https://norvetmsp.com/voltage-drop-calculatorLink to this tool
Paste into a blog post or resource page to credit the source.
<a href="https://norvetmsp.com/voltage-drop-calculator" title="Voltage Drop & Wire Size Calculator (NEC) by Norvet MSP">Voltage Drop & Wire Size Calculator (NEC)</a> by Norvet MSP