How the wire size calculator works
Normalize actual amperes, then calculate the continuous-load design current. For every candidate, adjust the insulation-rated base ampacity for ambient temperature and current-carrying count; cap it at the terminal column. Check terminal ampacity against design current and adjusted effective ampacity against actual current independently, then apply small-conductor protection caps.
I_design = I_load × 1.25 for a wholly continuous load. A_adjusted = A_insulation × kT × kN. A_effective = min(A_adjusted, A_terminal). Require A_terminal ≥ I_design and A_effective ≥ I_load; ordinary small-conductor protection cap must also cover I_design.
Worked example
A 40 A continuous copper load needs a 50 A design current. At 30°C with three current-carrying conductors, 90°C insulation and confirmed 75°C terminals, #8 has base 55 A, factors 1.00 × 1.00, adjusted 55 A and terminal/effective 50 A. Terminal 50 ≥ design 50 and effective 50 ≥ actual 40: #8 is the first supported candidate. This does not specify a complete circuit or breaker.
More original examples
For 20 A noncontinuous copper at 30°C, #14 fails its ordinary 15 A protection cap; #12 is the first candidate. For 45 A continuous copper at 60°C, with 90°C insulation and 75°C terminals, #6 has 75 × 0.71 = 53.25 A adjusted and 65 A terminal capacity: 65 ≥ 56.25 design and 53.25 ≥ 45 actual, so #6 passes the two independent checks. A 40 A noncontinuous load with confirmed 60°C terminals selects #8 at the base conditions.
NEC references
NEC 2023: 310.16; 310.15(A); 310.15(B)(1); 310.15(C)(1); 310.15(E); 310.15(F); 110.14(C); 210.19(A); 210.20(A); 240.4(D). These are reference identifiers, not reproduced code text. Read our methodology and verification limits.
For product-specific evidence, consult Southwire manufacturer resources and Hubbell RACO box documentation. We do not publish a full reference table.
Limitations
No full ampacity chart is published. No parallel sets, NM-B/SE cable, rooftops, free air, special underground tables, 310.12 service discount, motor/HVAC-specific circuit design, next-size breaker exceptions or 100%-rated assembly exceptions. Terminal rating must be selected from actual equipment evidence. Optional drop uses actual current and does not automatically enlarge the ampacity candidate.
For estimation only. Results compare loads under the selected NEC method; they do not guarantee permit approval or code compliance. Verify with your local adopted code and a licensed electrician.
FAQ
Why must I choose a terminal temperature?
Insulation rating does not establish equipment terminal limits. Use the lowest applicable marked/listed terminal rating under 110.14(C).
Does 90°C insulation mean I can use 90°C final ampacity?
No. That column starts the adjustment calculation; final effective ampacity remains limited by terminals and applicable circuit rules.
Which conductors enter the count?
Count qualified current-carrying conductors, including spares. Eligible neutral and grounding exclusions require your confirmation.
Can I use this for NM-B cable?
No. Cable-specific limits are outside this model; selecting NM-B is rejected.
Does the optional voltage-drop check choose a bigger wire?
No. It reports the candidate’s resistance-model drop as a reminder. Use the voltage-drop tool for a target-based candidate and reconcile both constraints.
Related calculators
Planning an EV charger or panel upgrade? Run a load calculation.