How We Build and Check Our Electrical Calculators

Our calculators are used for real decisions: what wire to buy, what box to install, whether a service can take an EV charger. This page explains where the numbers come from, how we test them, who reviews them, and what they don't do.

Sources and code edition

Code basis: the 2023 edition of NFPA 70, the National Electrical Code, unless a page says otherwise. Where the 2026 edition changes a result (for example, load calculations moving to Article 120), the page says so and the calculator lets you choose the edition.
Tables used: Table 310.16 (ampacity), Table 310.12 (dwelling services), 240.4(D) (small conductors), Table 250.122 (equipment grounding conductors), Table 250.66 (grounding electrode conductors), Chapter 9 Tables 1, 4, 5 and 8 (conduit fill and conductor properties), and 314.16(B) (box fill).
Where we got the values: we transcribed table values from published reproductions and cross-checked them against manufacturer and distributor references. Values we could not independently re-check are listed for our reviewer to confirm against an authorized copy of the NEC.

How we test

Reference cases. Every calculator has a set of test cases with known answers. For example, our conduit fill engine is checked against a distributor's published PVC Schedule 40 fill chart; all 20 values we compared match, including one 6 AWG conductor in ½" PVC and sixteen 8 AWG in 1¼".
Worked examples are recomputed. Every number in a worked example on our pages is recalculated by our build system before publishing. If a number on a page doesn't match the formula, the page doesn't publish. A 20 A load on 100 feet of 12 AWG copper, for instance, must come out at 7.90 V of drop.
Edge rules are coded, not remembered. Rules like the 240.4(D) small-conductor limits, the 60°C limit for NM-B cable and the Note 7 rounding rule for conduit fill are built into the logic.

Who reviews

We are bringing in licensed electricians to review each technical page, and a page will name its reviewer once that review is done. Until a page names one, treat it as checked by our automated tests only, not by a licensed professional.

What the calculators don't account for

Local amendments. States and cities amend the NEC. Your inspector's interpretation governs.
Conditions of use. Ampacity corrections for high ambient temperature, more than three current-carrying conductors, rooftop runs or cable bundling need to be applied where they apply.
Equipment instructions. Listed equipment may require a specific conductor size, temperature rating or overcurrent device.
Design judgment. Voltage-drop limits of 3% and 5% are recommendations in the NEC informational notes, not requirements, except where specific articles make them mandatory.

Corrections

If you find an error, tell us at. We correct the page, note the change in the page's update line, and re-run the tests.

How AI is used

Some page drafts are prepared with AI assistance. Every calculated number is checked by our build tests, and every calculator result comes from code we wrote and tested against the tables above, not from a language model.

Last updated: October 6, 2026.

How We Build and Check Our Calculators