Input Parameters
About This Calculator
- Checks both ampacity and voltage drop
- Recommends the larger of the two
- NEC recommends ≤3% branch circuit
How this wire size calculator works
Two criteria determine wire size: ampacity (must carry the current) and voltage drop (must not exceed limit over length).
The calculator picks the larger wire. A = I × 2 × ρ × L / (V × drop%) for single-phase.
FAQ
How to calculate wire size?
Check ampacity and voltage drop, use larger wire.
Wire for 30 amps?
#10 AWG copper minimum. Check V-drop for long runs.
Copper vs aluminum?
Aluminum needs 2 sizes larger for same ampacity.
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.