Wire Size Calculator
Ampacity the code way — the 310.16 table, the 240.4(D) small-conductor caps, the 125% continuous rule, copper vs aluminum, in one pick.
Wire Size Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- 75°C copper/clad aluminum column only — no 60°C or 90°C columns.
- Breaker ceiling shown as the 240.4(D) cap or the standard rating, not a specific product.
- THWN-2/XHHW-2 in raceway or cable; no direct-burial arrays.
In short: 30 A → 10 AWG copper (rated 35 A, breaker capped at 30). An EV's 48 A continuous → 60 A after the 125% rule → 6 AWG copper or 4 AWG aluminum. 70 A → 4 copper. And the caps nobody reads: 14/12/10 AWG can never protect above 15/20/30 A no matter the table.
Formula
continuous: required = load × 1.25
effective = 310.16 ampacity × ambient factor × bundling factor
pick = smallest size with effective ≥ required and 240.4(D) cap ≥ load
[('75°C', 'the termination column dwellings use'), ('15/20/30', 'the 240.4(D) caps on 14/12/10 copper'), ('1.25', 'the continuous-load multiplier, 3 h+'), ('×0.80 / ×0.70', 'bundling derates, 4-6 / 7-9 conductors')]
Worked Example
- State the load; if it runs 3 h+, multiply by 1.25.
- Choose copper or aluminum — aluminum runs two sizes up.
- Derate: ambient factor, then bundling factor.
- Pick the smallest conductor that still covers the load.
- Cap the breaker at 240.4(D); check the run with the Voltage Drop page.
EV: 48 A × 1.25 = 60 A → 6 AWG copper (65 A) or 4 AWG aluminum (65 A), 50-A-class breaker. Plain 30 A → 10 copper; 70 A → 4 copper.
Strengths & Limits Of This Model
Where this engine is strong
- Caps enforced INSIDE the pick, not as a footnote
- Aluminum table alongside copper, same law
- The EV 125% case as a first-class path
Where it stops
- One temperature column
- No parallel-conductor sizing
Practical Use Cases
EV charger circuits
The 48 → 60 A continuous case.
Water heaters & ranges
Big fixed appliances.
Subpanels
Feeders from the main panel.
Aluminum checks
The two-sizes-up rule, quantified.
Attic runs
The 40 °C derate, honestly applied.
Methodology & Editorial Standards
Smallest 75°C conductor whose ampacity × ambient × bundling factors covers the required load (load × 1.25 when the continuous term is 125), with 240.4(D) caps enforced inside the search: a size whose cap sits below the load is skipped even if its ampacity fits. The drop advisory recomputes the K-method law (2KIL/CM ÷ volts) at the entered run. Zero/negative loads, loads past 400 A, unknown conductor or character terms refused; picks beyond 1/0 refused with the derated ceiling stated.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Wire Size Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What size wire for a 30-amp circuit?
10 AWG copper — rated 35 A at 75°C and breaker-capped at exactly 30 A by 240.4(D). Aluminum needs 8 AWG for the same circuit.
What size wire for an EV charger?
A 48-A charger is continuous: 48 × 1.25 = 60 A, so 6 AWG copper or 4 AWG aluminum, on a 60-A breaker — the most common hardwired Level 2 install.
Why can't 12 AWG carry 25 amps like the table says?
240.4(D) caps small conductors below their table ampacity: 14/12/10 copper max out at 15/20/30-A protection. The table is what the metal survives; the cap is what the code trusts.
How much bigger is aluminum wire?
About two sizes at the same job — 1/0 aluminum carries what 3 AWG copper does (150 vs 100 A here is the 1/0-vs-1 gap). Its terminations must be AL-rated with antioxidant compound.
Does a long run change the wire size?
The ampacity table does not care about length, but the 3% drop advisory does — a 60-A EV feed at 150 ft drops 3.69% on 6 AWG and wants 4. That check lives on the Voltage Drop page.
What derating applies in an attic?
The 310.15(B) ambient correction: 40 °C is × 0.91 at the 75°C column. Bundle 4-6 current-carrying conductors and another ×0.80 lands on top.
What size wire for a 100-amp subpanel?
A 100-A feeder wants 3 AWG copper or 1 AWG aluminum in the 75°C column — aluminum steps up one size here. Long feeders then answer to the Voltage Drop page before the breaker ever closes.
Wire page or Voltage Drop page?
This page answers what survives the current; the Voltage Drop page answers what arrives at the far end. A wire can pass ampacity and still fail a long run.