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Wire Size Calculator — NEC 310.16
Size a branch-circuit or feeder conductor the way the code book does it: minimum AWG by NEC Table 310.16 ampacity, the 240.4(D) small-conductor caps, then a voltage-drop check that upsizes long runs until they hold 3% or less.
Load
One-way run length
System voltage
Phase
Conductor
How this calculator sizes wire
The calculator works in two passes. First it finds the minimum conductor size whose ampacity meets your load, reading the 75°C column of NEC Table 310.16 — the column used for common THWN and XHHW building-wire circuits — for copper or aluminum. On the small sizes it also applies NEC 240.4(D), the small-conductor rule that caps overcurrent protection at 15 A on 14 AWG, 20 A on 12 AWG, and 30 A on 10 AWG copper no matter what the ampacity table says. That rule is why a 20 A circuit needs 12 AWG copper, not 14.
Second, it computes voltage drop for your one-way run length using the DC resistance values from NEC Chapter 9, Table 8 (uncoated stranded copper and aluminum — what actually gets pulled in conduit). Single-phase circuits use 2 × I × R × L; three-phase uses √3 instead of 2. If the drop at the ampacity-minimum size exceeds 3%, the calculator steps up sizes until it passes. The 3% figure comes from the NEC 210.19 Informational Note — it is recommended design guidance, not a mandatory limit, but it is the number most specs hold you to.
Worked example
A 20 A load at 120 V, single-phase, 100 ft one-way in copper: ampacity alone allows 12 AWG, but that run drops 6.6% — far past the guidance. 10 AWG still drops 4.13%. The calculator recommends 8 AWG, which lands at 2.59% and holds the circuit inside the 3% recommendation.
Frequently asked questions
What size wire do I need for a 20 amp circuit?
12 AWG copper is the minimum for a 20 A circuit — NEC 240.4(D) caps 14 AWG copper at 15 A regardless of its Table 310.16 ampacity. On long runs, voltage drop can push you larger: at 120 V and 100 ft one-way, holding 3% takes 8 AWG.
Is the 3% voltage-drop limit mandatory?
No. The 3% figure comes from the NEC 210.19 Informational Note, which is recommended practice rather than an enforceable requirement. Many project specifications adopt it as a hard limit, though, and the Authority Having Jurisdiction has the final say.
Why does the calculator use the 75°C column of Table 310.16?
The 75°C column is the standard sizing basis for common THWN/XHHW building wire on 75°C-rated terminations. Always confirm the termination and insulation ratings on your actual equipment before finalizing a size.
Does aluminum wire need to be bigger than copper?
Yes. Aluminum has lower ampacity at the same size and higher resistance, so it sizes larger both ways: at 65 A, copper needs 6 AWG while aluminum needs 4 AWG, and aluminum runs also drop more voltage per foot.
Learn the code behind this calculator
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