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Electrical

Wire Size Calculator (NEC)

Size the conductor, breaker, and equipment grounding conductor for a circuit or feeder — with the full NEC 2023 derating chain and voltage drop shown step by step.

How the sizing chain works

A conductor is sized by the worst constraint in a chain, not by a single lookup. This tool runs the whole chain the way an inspector reads it, citing the NEC 2023 section at each step.

  1. Required ampacity — the load, multiplied by 1.25 for a continuous (3+ hour) load, per 210.19(A)(1) / 215.2(A)(1).
  2. Ambient correction — computed from the 310.15(B)(1)(2) formula, so any temperature works, not just the table rows.
  3. Conductor-count adjustment — the 310.15(C)(1) factor once more than three current-carrying conductors share a raceway.
  4. Termination limit — derate from the insulation column, then cap at the 60/75 °C termination column (110.14(C)) and the small-conductor limits (240.4(D)).
  5. Voltage drop — checked against your 3%/5% target and upsized if needed, shown separately from the code minimum.
  6. Breaker and EGC — the next standard rating (240.6(A)) and the matching grounding conductor (250.122).

Ampacities are the NEC 2023 Table 310.16 minimums; corrections are computed from the code formulas, not copied from the code text. Verify the final design against the edition your jurisdiction has adopted.

Frequently asked

What size wire do I need for a 50-amp, 240V circuit?
On 75 °C terminations at 86 °F with three conductors, a 50 A non-continuous load needs #8 copper (50 A at 75 °C) or #6 aluminum, protected by a 50 A breaker with a #10 copper equipment grounding conductor. If the load is continuous (runs 3+ hours), size at 125% — 62.5 A — which moves you to #6 copper. Enter your exact conditions above; heat, conductor count, and distance can all push it up a size.
What wire size for a 60-amp sub-panel 150 feet away?
By ampacity alone a 60 A feeder is #6 copper (65 A at 75 °C). But over 150 feet voltage drop usually governs: #6 copper at 60 A on 240V drops about 3.7%, past the 3% design target, so the calculator recommends #4 copper (about 2.3%) to hold the drop. You get both numbers — the NEC code minimum and the voltage-drop recommendation — because 3% is an informational note, not a hard requirement.
Why start from the 90 °C column if my breaker is only rated 75 °C?
This is the rule most calculators skip. Per NEC 110.14(C) you apply ambient and conductor-count derating starting from the conductor's own insulation column (90 °C for THWN-2), then compare the result to the 75 °C termination column and take the smaller. Starting high and derating down often keeps you at a smaller size than derating from 75 °C would — but you can never exceed the termination rating for the final ampacity.
How does ambient temperature change the wire size?
Hot conductors carry less current. The calculator applies the NEC 310.15(B)(1)(2) correction by formula — k = √((T_ins − T_ambient) / (T_ins − 30 °C)) — rather than a lookup table. At 110 °F (43 °C) with 90 °C insulation, k ≈ 0.88, so a #8's 55 A base drops to about 48 A. A 130–140 °F attic can knock a conductor down a full size.
Do more than three wires in one conduit change the size?
Yes. NEC 310.15(C)(1) requires an adjustment factor once you have more than three current-carrying conductors in a raceway or cable: 0.80 for 4–6, 0.70 for 7–9, 0.50 for 10–20, and lower beyond that. The neutral usually doesn't count unless it carries harmonic or unbalanced current. Set the conductor count above and the factor is applied automatically.
Is the 3% voltage-drop limit actually required by code?
No. NEC 210.19 and 215.2 mention 3% (branch) and 5% (feeder + branch combined) only in informational notes — they are recommended design targets, not enforceable minimums. The calculator always gives you the true code-minimum size plus the size that meets your chosen drop target, so you can decide based on the run length and load.

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Related guides

Sources & references

Undersized conductors and overcurrent devices are a fire hazard. Have all electrical work sized, installed, and inspected under the NEC edition adopted in your jurisdiction.

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed electrician. See our methodology, sources, and code editions.