Why This Matters

Undersized conductors overheat. Oversized conductors waste money and complicate installation. Conductor sizing sits at the intersection of safety, economics, and code compliance — getting it right means the circuit works safely under worst-case conditions, not just on a mild Tuesday.

Service conductors carry additional weight because they generally lack upstream overcurrent protection. A feeder has a breaker watching its back; a service conductor has to fend for itself.

The Two Sizing Criteria

A compliant conductor must satisfy both of these — the smallest size that passes both criteria is selected (and it must additionally be protectable by the overcurrent device the load requires, per 240.4):

• Criterion A — terminal column (110.14(C)). The ampacity from the permitted terminal column (usually 60°C or 75°C) must be at least the noncontinuous load plus 125% of the continuous load.
• Criterion B — correction/adjustment (310.15). The insulation-column ampacity, multiplied by the ambient correction (Table 310.15(B)(1)(1)) and the conductor-count adjustment (Table 310.15(C)(1)), must be at least the total connected load.

The 90°C column may be used for the correction/adjustment math even when the terminals are only 75°C — but the final usable ampacity is still capped at the terminal column.

Common Mistakes

• Ignoring terminal temperature limits. A 90°C wire on 75°C terminals uses the 75°C column for the terminal check. The insulation sets the ceiling; the terminal sets the operating limit.
• Forgetting the 125% continuous factor. If a load runs 3+ hours, the terminal check is sized at 125% (unless the assembly is listed for 100%).
• Wrong material column. Reading copper values for aluminum (or vice-versa) is silent and expensive.
• Overlooking ambient correction. A 46°C rooftop is not a 30°C basement.
• Bundling without adjusting. More than 3 current-carrying conductors triggers Table 310.15(C)(1).
• Assuming the neutral never counts. Per 310.15(E), a wye neutral on a majority-nonlinear load is a current-carrying conductor.

The Third Gate — The Conductor Must Be Protectable

Passing both ampacity criteria is not enough. The overcurrent device the load actually requires must also be a device that is permitted to protect the chosen conductor under 240.4. The classic trap: a 25 A noncontinuous load needs a 25 A device, and #12 Cu is 25 A at 75°C — but 240.4(D) caps #12 Cu protection at 20 A, so #12 is not a lawful answer. This calculator applies that check as a third selection gate, so the headline size is always one you can actually protect.

Reading the Status Badge

The badge grades the margin of the governing criterion, using thresholds chosen by this tool — they are editorial, not code:

• PASS — at least 10% margin over the governing requirement.
• TIGHT — the conductor complies but has less than 10% margin. A code-minimum conductor is fully compliant; TIGHT is a design comment, not a violation.
• FAIL — no size satisfies the criteria (see the warnings).

What This Tool Deliberately Does Not Do

• Voltage drop (210.19(A) / 215.2(A) Informational Notes) is not evaluated. It is a performance recommendation, not a requirement, but it frequently governs long runs.
• Article 220 load calculation is upstream of this tool — you supply the calculated load.
• Rooftop sunlit-raceway ambient (310.15(B)(2)) is not added automatically; include it in the ambient you enter.
• 310.15(C)(1) exceptions (raceway nipples ≤ 600 mm / 24 in., certain spaced cables) are not applied — the tool stays conservative.
• Grounded (neutral) conductor sizing per 220.61, equipment grounding conductors (250.122), parallel sets (310.10(G)), and derating for conduit fill are outside this scope.