Calculators
The two NEC calculators below run client-side in plain JavaScript — no accounts, no uploads, nothing sent anywhere, and no record kept of what you typed. The tables behind these pages are generated directly from the Project Manager suite this firm uses for its own design work, so a correction made for a live project reaches this page by the same route it reaches an engineer's desk.
For underground cable geometry and mutual heating, use Terra Thermal Studio, a separate engineering preview. Terra and its user guide require assigned sign-in credentials. Terra sends calculation inputs to the server and is not a certified ampacity or code-compliance assessment. Read its user guide and methodology.
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Conductors · NEC
Allowable ampacity
The allowable ampacity of one insulated conductor in a raceway or cable, from Table 310.16 downward: corrected for ambient temperature at the conductor's own insulation column, adjusted for the number of current-carrying conductors bundled with it, then capped at the termination temperature rating the equipment is listed for. The step that governs is named, and so is the reason.
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Conductors · NEC
Conductor size
The inverse problem — the smallest conductor that satisfies all of ampacity after derating, the 110.14(C) termination column and a voltage-drop limit, with the equipment grounding conductor and the overcurrent device that go with it. Where no single conductor is enough it says so and escalates to parallel sets per 310.10(G) rather than printing a clean-looking wrong answer.
About these calculators
Preliminary sizing only. Not for construction. These pages are a check on an intuition and an aid to a conversation — they are not a substitute for a licensed engineer's sealed design, and nothing produced here is a calculation of record. A real installation is governed by the adopted edition of the Code in your jurisdiction, by local amendments, by the listing of the equipment actually installed, and by conditions of use that no web form asks about. Have the design sealed by an engineer licensed in the jurisdiction before anything is built to it.
Every result states its Code edition, printed beside the article it came from and read out of the same generated data file as the numbers themselves. This page is deliberately not one of those places. It runs no script and reads no table, so an edition named here would have to be typed by hand — the second-copy problem described below, in its quietest form: a hand-typed edition goes on naming the old one long after the tables underneath it have moved, and nothing would catch it. Open a calculator and it will tell you, from the data it computed with, which edition it carries. That is not decoration: ampacity tables, correction factors and termination rules all move between editions, and a number quoted without its edition is not a code number at all. If the calculator names an edition your jurisdiction has not adopted, these pages are the wrong tool — read them as a worked example of the method, not as an answer.
They refuse rather than extrapolate. Off the end of a table is a refusal, not a clamp to the nearest row; an input that cannot be read is a refusal that names what it could not read. This is the design decision that costs the most and matters the most. A calculator that silently continues past the edge of its data returns a number that looks exactly like a good one, and the failures worth preventing in conductor sizing are all of that shape — undersized, plausible, and never questioned again.
The tables are generated, not retyped. Every cell comes from the canonical tables in this firm's own project-management suite — the copy its engineering work is produced from — read out by a generator that refuses anything it cannot parse exactly rather than guessing at it, and held in step by a drift check that compares the published copy byte for byte against a freshly generated one. A public calculator is a second copy of a code table, and a second copy that is typed by hand disagrees with the first one quietly, in one cell, for as long as nobody reads both. This one cannot drift from the tables the firm's engineers work to.
Be clear about what that does and does not establish. It establishes provenance: these cells are the firm's cells, and they are still the firm's cells today. The checks that ride along with the generator are structural — counts, contiguity, monotonicity, membership, agreement between one table and another — and a structural check catches a cell that has moved, not a cell that was wrong when it was first written; a plausible wrong value passes every one of them. So nothing here substitutes for reading a figure you intend to rely on out of the printed Code, and if you read one and it disagrees with this page, please say so.
Each page states its own exclusions where they fall, but three are worth saying up front. These calculators answer for a conductor in a raceway, cable or earth — the scope of Table 310.16 — and not for free-air, cable-tray or other installation methods that have tables of their own. They size a conductor, not a system: continuous-load allowances, motor and tap rules, raceway fill and short-circuit withstand are separate calculations. And they answer a steady-state question; a conductor's behaviour during a fault is a different problem with a different limit.
Corrections are genuinely welcome, and a disagreement about a table cell is the most useful mail this site receives: [email protected].