Teaching an IEC wire-size calculator to speak NEC, and what App Review asked for
A calculator can have correct tables and still give the wrong answer for the job, because the defaults belong to a different trade. Switching Electrician Calc to NEC meant residential defaults, US words everywhere, results that carry over between calculators and the working printed in the PDF. App Review then asked for a screen recording from a real iPhone and notes on purpose, features, services, regions and regulation. The app is now on the App Store.
Electrician Calc sizes the wire and the breaker for a circuit, checks voltage drop and shows every step, so the result can be shown to an inspector. It works by two standards: IEC 60364 (mm², metres, 230/400 V) and the NEC (AWG, feet, 120/208/240/480 V). The app picks one by the phone's region and switches with one tap.
Both modes read from real tables: NEC 310.16 with 75 °C terminations, IEC installation methods A1–E with temperature and grouping factors. Most of the work in the last weeks went into something else.
Correct tables, wrong answer: a 40 A range on a 50 A breaker
To see the NEC mode through an American electrician's eyes I ran a user test: an agent played a licensed US residential electrician who now uses a free manufacturer app and memory. It read only the store page, installed the app and tried a real job: a 40 A, 240 V range, 60 ft away, on NM-B.
The numbers it checked were right. The first answer for the range was still a 50 A breaker. "Continuous load" was on by default and the wire type was THHN, so the app applied the 125 % factor a house electrician would not use for a range. The tables were correct; the defaults belonged to a commercial job.
| Where | Found in the test | Change |
|---|---|---|
| Defaults | Continuous load on and THHN, so the range got a 50 A breaker | Not continuous and NM-B by default, the switch next to the current. The range now reads 8 AWG on 40 A with a 10 AWG ground |
| Next step | Opened Voltage Drop on demo numbers | Carries the circuit: current, voltage, wire, run and drop limit to Voltage Drop and Max Wire Length, the wire and its ground to Conduit Fill |
| No working, so nothing to hand the inspector | Saved results keep "How it's calculated", and the PDF prints the steps under each result | |
| Words | Installation method, grouping, Zs, stabilizer on an American screen | NEC wording in onboarding and the paywall; no loop impedance as a next step in NEC mode |
| Kitchen | Receptacles without AFCI, one kitchen circuit | AFCI on kitchen, laundry and dishwasher circuits, two small-appliance circuits (210.12(A), 210.11(C)(1)) |
| Wire types | No SER or USE-2 | Added; NM-B above 2 AWG, which is not made, says to use SER or THHN in conduit |
Wording that reads as the wrong instruction
One step of the working said "voltage drop ≤ 3 % first met by 10 AWG". It was meant as a lower bound and read as "install 10 AWG". It now says "10 AWG or larger keeps the voltage drop within 3 %", followed by a last step that names the size to install: the larger of the ampacity size and the drop size.
Two more of the same kind came later:
- Voltage Drop used to pass 14 AWG on a 20 A circuit if the run was short. It now fails it, because the wire may carry 15 A at most (Table 310.16 at 75 °C and 240.4(D)), and its "smallest size" has to carry the load as well as meet the drop.
- When Wire Size upsizes NM-B for voltage drop, the ground inside the cable stays the size the cable is made with, which can be smaller than 250.122(B) asks for. The note now says so and offers THHN in conduit.
What App Review asked for
On 6 November Apple sent back Electrician Calc 1.0.0, and Days Until with it, under Guideline 2.1, Information Needed. Nothing in the apps was rejected. Apple wanted a screen recording from a physical iPhone and an explanation in the review notes, then a new submission.
The recording, one to two minutes, starts on the home screen and launches the app. It shows the free Wire Size calculator with a PASS result, saving it to a project, a locked Pro calculator, the paywall, a purchase with a sandbox Apple ID and the calculator unlocked after it.
The notes followed six headings, each a short paragraph:
- Screen recording: what it shows, in order.
- Purpose and audience: electricians, contractors and apprentices sizing circuits on site.
- How to use the main features: no account or sample files; what is free, what is Pro, where the paywall opens, which products can be bought with a sandbox account.
- External services: only Apple In-App Purchase through RevenueCat. No analytics, ads, AI or login; everything runs on the device.
- Regional differences: NEC or IEC by region, switchable at any time; the same features and prices everywhere.
- Regulated industry: a design aid, not a licensed electrical service; methods from published standards, no protected tables copied; the NFPA trademark note.
The last heading matters for a trade app. A calculator that names code sections looks like regulated advice, so the notes say plainly what the app is and is not, in the same words as the store description.
Try it
Free: Wire Size, Voltage Drop, Power & Ohm's Law, the reference tables and three projects with their PDF reports. Pro adds the Panel Planner with a printable panel directory, Conduit and Box Fill, Motor Circuit, Solar PV Strings and the rest of the 25 calculators. No ads, no account.
The iPhone app is on the App Store. The same wire-size and voltage-drop formulas also run free in the browser: wire size calculator, voltage drop calculator.
Calculations are a design aid. Check the final design against the code edition adopted where you work.