Electrical calculators

Seven tools covering the numbers you need to size a circuit and defend the choice: how much room the conductors need in the box and in the pipe, how much capacity they lose to heat and to bunching, how much voltage they lose over the run, and what a three-phase load actually draws.

Two codes, kept separate on purpose. Everything here runs on the US National Electrical Code except the circuit check, which runs on BS 7671, the UK wiring regulations. They are not interchangeable and mixing them produces answers that look plausible and are wrong, so each tool names the code it applies and will not silently switch.

Which edition your inspector enforces can change the answer, and two calculators here have to ask. For box fill it is the rule that moved: from the 2020 code, grounding conductors beyond the fourth each add a quarter allowance, so a junction box that passed under the 2017 code can fail under the 2023 one on identical wiring. For solar interconnection it is the opposite problem, where the arithmetic has not moved and the subsection numbering has, in every recent cycle, so two people can quote the same rule and cite different numbers. Adoption is by state and sometimes by city, so both carry an edition selector rather than picking one and hoping.

Where a derivation and a published table disagree, these take the conservative one. At 21 °C the ambient correction formula gives 1.14 for a 60 °C conductor where the NEC table allows 1.08, so below 30 °C the correction is clamped to 1.0 rather than handing out capacity the code does not grant. That rule is applied throughout, and each page says where it bit.

The two corrections stack, and that is where undersized runs come from. Ampacity is derated twice over: once for ambient temperature and again for the number of current-carrying conductors bundled in the same raceway. Applying one and forgetting the other is the common error, and it always errs upward, which is the dangerous direction. Four current-carrying conductors in one conduit at 40 °C do not carry the table value, and nothing on the label tells you that.

Voltage drop is a performance limit, not a safety one, and the NEC treats it that way. The 3 percent branch and 5 percent total figures are in informational notes rather than in the enforceable text, so they are recommendations you can exceed without failing an inspection and then spend years explaining. The calculator shows the drop against both figures and against the conductor you actually chose, so the tradeoff is visible rather than pass or fail.

BS 7671 tabulated values are not freely licensed, so the circuit check uses only the physical formulas and the facts inside the standard, never its tables. It tells you what it computed and what you still need the book for.