NEC Wire Size Calculator

How the calculator sizes a conductor

A conductor has to clear two independent thresholds, and the larger of the two governs. The first is ampacity: the conductor's derated, termination-capped capacity must carry the required current. For a continuous load, that required current is 125% of the load per NEC 210.19(A)(1). The second is voltage drop: at the actual load current over the one-way run, the drop must stay within your target. The calculator sweeps every size in NEC Table 310.16 from smallest up, finds the minimum size that clears each threshold independently, and returns the larger of the two.

Both criteria are monotonic in conductor size (bigger conductor, more ampacity and less drop), so the smallest qualifying conductor is unambiguous. The result tells you which constraint governed: ampacity, voltage drop, or both at the same size.

Ampacity basis

Base ampacity comes from NEC Table 310.16 at the conductor's insulation column. Derating starts from that column: ambient correction per Table 310.15(B)(1) and the adjustment for more than three current-carrying conductors per Table 310.15(C)(1) multiply against the base value. The derated result is then capped by NEC 110.14(C): the usable ampacity cannot exceed the Table 310.16 value at the lower of the conductor insulation rating and the termination rating. For most modern equipment that termination rating is 75°C, which is why a 90°C THHN conductor is derated from the 90°C column but ultimately limited at the 75°C value.

Small conductors carry an additional overcurrent limit. NEC 240.4(D) caps overcurrent protection for 14, 12, and 10 AWG regardless of their table ampacity (15, 20, and 30 A for copper). The calculator folds this into sizing: a size that has the raw ampacity but whose 240.4(D) limit falls below the required current is rejected, and the result notes why.

Voltage drop basis

Voltage drop is computed on the conservative published-value basis: K-factor for conductors smaller than #2 AWG and NEC Chapter 9, Table 9 AC resistance at 75°C for #2 AWG and larger. Single-phase circuits use the round-trip multiplier (2×), three-phase uses √3. Drop is always evaluated at the actual load current, not the 125% sizing figure because sizing for capacity and sizing for drop answer different questions. For the full treatment of the formula and the K-factor versus Table 9 transition, see the voltage drop calculator.

Worked example

A 50 A continuous load runs 150 ft on a 240 V single-phase circuit, copper, 75°C terminations, 30°C ambient, three current-carrying conductors, target 3% voltage drop.

Required ampacity is 50 × 1.25 = 62.5 A. On ampacity alone, #6 copper (65 A at 75°C) clears it. But voltage drop at the actual 50 A over 150 ft pushes #6 past 3%, so the calculator steps up to #4. The result reports voltage drop as the governing constraint and returns a 70 A overcurrent device per NEC 240.6(A), with the 240.4(B) next-size-up context noted where it applies.

Common mistakes

• Sizing voltage drop at 125%. The continuous-load multiplier applies to ampacity and OCPD sizing, not to the voltage-drop calculation. Drop is a function of the real current on the conductor.
• Derating from the 75°C column. Derating starts from the insulation column (90°C for THHN/THWN-2), and the 110.14(C) termination cap is applied to the result. Starting derating at 75°C double-counts the limit and oversizes.
• Ignoring 240.4(D) on small conductors. #12 copper has 25-30 A of ampacity at 90°C but cannot be protected above 20 A. A load that needs more forces a larger conductor regardless of the table value.
• Using one-way distance as round-trip. Enter the one-way run; the calculator applies the 2× (single-phase) or √3 (three-phase) factor.

FAQs

What size wire do I need for a 50 amp circuit?

On ampacity alone, 6 AWG copper (65 A at 75°C) or 4 AWG aluminum covers a 50 A continuous load sized to 62.5 A per NEC 210.19(A)(1). But the answer depends on distance: past roughly 100-150 ft, voltage drop can force a larger conductor than ampacity requires. Enter your actual run length and target drop to get the governing size.

Does the NEC require voltage drop in wire sizing?

No. NEC 210.19 and 215.2 contain informational notes recommending a maximum of 3% on the branch circuit and 5% combined feeder plus branch, but these are recommendations, not requirements. Some jurisdictions adopt them as mandatory through local amendments, and drop is a functional requirement on long runs regardless of code. Check your adopted amendments.

Why does the calculator recommend a larger wire than an ampacity chart?

A bare ampacity chart answers only whether the conductor can carry the current. This tool also enforces your voltage-drop target and the NEC 240.4(D) small-conductor limits. When voltage drop or 240.4(D) governs, the recommended size is larger than ampacity alone would indicate. The result identifies which constraint drove the size.

Should I size for 60°C, 75°C, or 90°C?

Derate from the conductor's insulation rating (90°C for THHN/THWN-2), but the usable ampacity is capped at the termination rating per NEC 110.14(C). Most modern equipment is rated 75°C, so set the termination rating to 75°C unless the equipment is specifically listed otherwise. The 90°C column is a starting point for derating, not a usable column on its own.

Related tools

• Voltage drop calculator: compute drop for a known conductor and distance.
• Wire ampacity calculator: derated ampacity for a single conductor with the full 310.15 chain.
• DC voltage drop calculator: drop for DC circuits (solar, battery, automotive).