Input Parameters
About This Calculator
- Uses versioned NEC 2023/2026 Table 310.16 reference columns
- Applies continuous-load, temperature and bundling factors
- Checks voltage drop using one-way length, resistivity, and optional AC reactance/power factor
Reference estimate only. Verify the locally adopted code, terminal rating, conductor listing, installation method and manufacturer instructions before installation.
Code adoption boundary: NEC 2023 or 2026 is a reference edition, not automatically the code in force at your location. Check the jurisdiction, effective date, state or local amendments, Errata/TIA, permit transition rules and the authority having jurisdiction before treating a result as compliant.
How this wire size calculator works
The result uses the larger of corrected ampacity and voltage-drop size. Ampacity is based on a 60°C reference column with optional 125% continuous-load, ambient-temperature and current-carrying-conductor factors. Voltage drop uses 20°C resistivity and can include user-supplied AC reactance and power factor.
FAQ
How is wire size estimated?
Compare corrected ampacity with voltage-drop area and use the larger size.
What wire size is needed for 30 amps?
#10 AWG copper is only a 60°C reference starting point; continuous load and derating may require a larger size.
How do copper and aluminum differ?
They use separate reference ampacity values. Aluminum may require a larger size and compatible terminals.
Common Wire Sizing Mistakes — What Electricians & Engineers Get Wrong
Using 90°C ampacity but forgetting 75°C terminal limit
NEC Table 310.16 shows three columns: 60°C, 75°C, and 90°C. THHN wire is rated 90°C, so engineers sometimes use the 90°C column — but NEC 110.14(C) requires using the lower of conductor and terminal rating. Most breakers and equipment terminals are rated 75°C (or 60°C on older panels). Using the 90°C column on a 75°C terminal can result in a conductor that is technically legal by ampacity but illegal at the connection point.
Forgetting conduit fill derating
NEC Table 310.15(C)(1) requires ampacity correction when more than 3 current-carrying conductors share a conduit. Four conductors: derate to 80%. Seven conductors: derate to 70%. A common mistake is to size each wire independently and then bundle them — the bundled assembly may need the next larger AWG to compensate. The derating applies to current-carrying conductors only; a ground wire does not count.
Voltage drop on one-way distance vs. round-trip
Voltage drop occurs across both the hot conductor and the neutral/return conductor. For single-phase circuits, use 2 × one-way length in the formula. A 100 ft circuit has 200 ft of total conductor path. This is the single most common calculation error — using one-way distance produces a wire that drops twice the intended voltage and is undersized for the run.
AWG to mm² substitution without checking standard sizes
12 AWG has an actual cross-section of 3.31 mm², but the nearest IEC standard size is 4 mm². Substituting the exact mm² equivalent (3.31 mm²) is non-standard — IEC 60228 defines discrete cable sizes (1.5, 2.5, 4, 6, 10, 16 mm²…). Always round up to the next standard IEC size. Using 3.31 mm² on a datasheet will confuse suppliers outside North America who can only source standard sizes.