Conductor size

The method, and where it stops

The order, and why it is this order

Sizing a conductor is a search, not a formula. There is no closed form, because the ampacity of a candidate depends on the candidate and the voltage drop depends on it again, differently. So the page walks the table from the smallest size upward and asks current, protection, and voltage-drop questions of each one.

1. Is it big enough for the current? The required current is the load, at 125 % if it is continuous (210.19(A)(1), 215.2(A)(1)). The candidate’s allowable ampacity is computed by exactly the ladder the conductor ampacity page shows — base at the conductor’s own insulation column, ambient correction from that same column, conductor-count adjustment, then the uncorrected 110.14(C) termination cap, and the answer is the lesser. This page calls that page’s engine rather than repeating it. The uncorrected insulation/termination ampacity must also cover the sizing current; a cool-ambient credit cannot replace that check.
2. Can the selected device protect it? The conductor must support the standard device rating, including the small-conductor limits of 240.4(D). This tool does not assume eligibility for the next-size allowance.
3. Does it drop too much? V_d = K × I × L × (R·cos θ + X·sin θ) ÷ 1000, with R and X from Chapter 9 Table 9 at the raceway you chose, and divided by the number of parallel sets. I is the actual load current, not the 125 % figure: the 125 % is a sizing rule, and the drop a load causes is caused by the current it draws.

The first size that clears all three checks is the answer. If no single conductor does, the search stops at your maximum conductor size, if selected, and restarts at two parallel sets, then three, up to twenty — and 310.10(G)(1) removes every size below 1/0 AWG from the search the moment sets exceed one.

The voltage-drop target and its scope

The general-purpose guidance is cited at . This tool does not assess special applications with mandatory voltage-drop limits, local energy-code requirements, or project specifications. It models steady-state, balanced AC at 60 Hz using 75 °C impedance. DC, unbalanced/shared-neutral circuits, motor starting, and actual conductor-temperature effects require separate calculations.

Table 9 assumes three single conductors in conduit. Its use for two-wire circuits or parallel sets is an approximation; a shared raceway with multiple sets can require a geometry-specific impedance calculation.

Which limit governs, and how that is decided

Not by comparing the chosen conductor’s margins. Sizes come in steps, so the smallest conductor that clears a 3 % target often clears it by a wide margin, and reading that margin as “voltage drop was not close” would be wrong. The question is counterfactual: of the sizes below the answer, what did each one fail? If they all failed on current or device protection, those checks govern. If they all failed on drop, voltage drop governs. If different sizes failed on different things, both are load-bearing and relaxing either alone buys you nothing — and the page says so.

Overcurrent protection and conservative sizing

The device is the smallest supported standard 240.6(A) rating at or above the sizing current. The search rejects a conductor if the device exceeds its allowable ampacity or its 240.4(D) small-conductor ceiling. A load needing 25 A protection therefore cannot receive a 12 AWG copper recommendation limited to 20 A.

This tool does not apply the conditional next-standard-rating allowance in 240.4(B). Above 800 A, 240.4(C) requires conductor ampacity to cover the device rating. At or below 800 A, the same conservative selection is used here.

For continuous loads, this method conservatively requires the full 125 % sizing current after adjustment and correction, as well as at the terminations. NEC 210.19(A) and 215.2(A) have separate checks before and after adjustment/correction; applying those checks and permitted exceptions in a full design can allow a smaller conductor. This is the smallest size within the method used here, not a claim of the smallest size permitted by every NEC provision.

The tool is for general-purpose insulated conductors in raceways. It does not assess wiring-method restrictions such as the 60 °C ampacity limit for NM cable, 100 %-rated assemblies, motors, taps, service conductors, or special occupancies. Confirm the equipment listing, terminal ratings, and conductor suitability at both ends.

The equipment grounding conductor

Table 250.122, indexed by the device rating — not by the phase conductor, and not by the load. Two adjustments follow from the rest of the answer.

250.122(B): where the ungrounded conductors are increased in size from the minimum supporting the separate before/after load checks and device rules, the grounding conductor is increased proportionately by circular-mil area. That is exactly what happens when voltage drop or the conservative selection margin increases the conductor, so the EGC can be larger than the bare Table 250.122 lookup — and the caveat states the ratio and both sizes. The calculation retains the full circular-mil ratio. Direct Table 250.122 sizes of 800 and 1,250 kcmil are available; proportional upsizing requires supported area data. Candidates with unresolved EGC sizing are rejected and the search continues.

250.122(F)(1)(b): with parallel sets in separate raceways, each raceway needs a full-size grounding conductor sized from the same device rating, never one EGC divided among them. Under 250.122(F)(1)(a), with all sets in one raceway, a single grounding conductor is permitted. The page follows whichever layout you chose.

Use the insulation rating applicable to the actual location: for example, XHHW is rated 75 °C in wet locations, while XHHW-2 is rated 90 °C in dry and wet locations. Apply the most restrictive conditions along the run; this tool does not evaluate the short-section exception in 310.14(A)(2). Count applicable neutrals and spare conductors under 310.15(C)(1) and (E); exclude grounding and bonding conductors under 310.15(F). The dwelling allowance in 310.12 is not applied.

Branch-circuit application: the device rating shown is a load-based selection, not approval for a particular outlet or appliance. A 10 A branch circuit cannot supply receptacle outlets (210.18 and 210.23(A)). Multiple-outlet circuit ratings and permitted loads are restricted by 210.18 and 210.23; required dwelling circuits may need 20 A even when the entered load is smaller. Verify equipment and outlet ratings under 210.20(C) and (D).

For 210.19(A) and 215.2(A), check the uncorrected ampacity against the noncontinuous load plus 125 percent of the continuous load, and the corrected ampacity against the actual maximum load. This calculator conservatively requires the full 125-percent sizing current after correction as well. Its checkbox treats the entire entered load as continuous or noncontinuous; it does not separately model mixed loads or apply the listed 100-percent-rated assembly and separate-connector exceptions.

Feeder neutral sizing, common-neutral arrangements, and the service-conductor minimum in 215.2(C) are not evaluated. The tool does not determine requirements for feeder ground-fault protection, surge protection or separate-building bonding.

Where the numbers come from

Verification status: the supported ampacity, correction, conductor-area, AC impedance, standard-device and grounding-size values have been compared against owner-supplied NEC 2023 excerpts. The stated sizing method has been reviewed against Articles 110, 210, 215, 240, 250 and 310. Issued NEC 2023 errata and TIAs were screened for this model on September 13, 2026; see the source register for dispositions. Later corrections and actual installation conditions still require verification. Local amendments are outside this review. This is a conservative sizing aid; a successful calculation does not establish complete installation compliance.

Every table on this page is generated from the same source the firm’s own engineers use in the Bellazipeda project-management suite, not retyped for the web. Table 310.16, Table 310.15(B)(1), Table 310.15(C)(1), 240.6(A), Table 250.122, 240.4(D) and Chapter 9 Table 9 are copied by a generator, and a drift test compares the published bytes against a fresh generation, so a stale copy and a hand-edit fail identically. The edition is the stated NEC edition. Jurisdictions adopt on their own schedule and amend freely; check yours.

One gap is worth naming, because it looks like a bug. Table 310.16 prints a 700 kcmil row and Chapter 9 Table 9 does not — its printed rows run … 600, 750, 1000 kcmil. So a 700 kcmil conductor has an ampacity and no published impedance, and this page refuses to grade its voltage drop rather than interpolating one and calling it Table 9. The search skips it and says why.