Voltage drop · Wire size · Short circuit · Conduit fill
Electrical calcs
with the working shown.
Fishtape is a calculator app for electricians. Enter the circuit and it finds the wire size, checks it against your code and shows every step on a sheet you can save as a PDF. It works offline too, in a basement or a plant room with no signal.
- A sheet with every result. Each step, the formula and the numbers that went into it.
- All the checks at once. Derating, terminations, breaker size and voltage drop, and it tells you which one set the size.
- Your code and edition. Stored on the phone, so it works without signal.
- No AI in the calculations. Plain formulas, so the same inputs always give the same answer.
Not out yet. Leave your email and we’ll send you the link on launch day.
What it does that free apps don’t
Plenty of apps give you a number. Fishtape also shows how it got there, so you can back it up when an inspector or an engineer asks.
It shows the working
Every result comes with a sheet: each step, the formula used and the numbers that went into it. Save it as a PDF, attach it to your paperwork or send it to whoever asked. It’s a record of what you calculated and how. It doesn’t certify the installation.
On every sheet
- The code and edition used, with the date and time of the calculation
- The job and the board or circuit, so you can still tell what it was a year later
- Each step with its formula and the numbers put into it
- Intermediate values at full precision, so you can see where rounding happens
- How the size was chosen: the smallest one inside your limit, and how many sizes were tried to find it
One PDF page per calculation. Anyone can open it, with or without the app.
One size that passes every check
A cable has to pass several checks at once, and any one of them can decide the size.
Fishtape runs all of them, gives you the smallest size that passes, and tells you which check decided it. On a long run it’s usually voltage drop. In a hot attic packed with other circuits it’s usually derating.
What gets checked
- Current capacity after derating for grouping, ambient temperature and installation method
- Voltage drop over the whole run, at the cable’s working temperature instead of 20 °C
- That the breaker or fuse in front of the circuit protects the cable
- Termination temperature limits and continuous load rules, where your code has them
- If a check is missing an input, it shows as not checked. It never passes by default.
Change one input and a different check may end up deciding the size.
The edition you work to
Codes change, and not everywhere at the same time. NEC 2026 is out, but many states still enforce 2023. The Canadian code has its own three-year cycle. Fishtape keeps each edition as a separate pack, so you pick the one your job falls under.
In the app now: NEC 2023, CEC 2024, IEC 60364, Portugal’s RTIEBT, India’s IS 732, Japan’s 電技解釈 and Korea’s KEC. The app picks the one for the country your phone is set to, and you can change it.
A job from last year still opens under the edition it was done to. Packs are stored on the phone, so they work with no signal.
How editions work
- Each edition is its own pack, with its amendments and the date it came into force
- A job keeps the edition it was calculated under, even after a newer one comes out
- Local amendments are kept separate from the base code, the way they’re published
- Switching editions recalculates the job from scratch
Where we can license an edition, the pack includes its tables. Where we can’t, the app tells you which few values to look up in your own copy and calculates the rest.
Plain formulas, no AI
Same answer every time
Every result comes from plain formulas. There’s no AI model anywhere in the calculation and we don’t plan to add one. A wrong cable size is a safety problem.
It won’t guess
If an input is outside the range a table covers, you get an error, not an extrapolated number. Metric and AWG sizes are kept apart and never converted without telling you.
Price
One price covers all of it: every calculator, the full sizing chain, saved jobs, PDF sheets, offline use, and the code packs. Pay by the month, or pay once and never again. Shown in euros; your app store charges in your own currency.
Free for 7 days from when you start. Cancel the monthly any time: your jobs and sheets stay, and the app goes back to asking you to type the figure in.
The code packs are included. Where a wiring code is public law, like Portugal’s, the capacity figure fills in from the pack, with the table and clause on your sheet. Where a code is copyrighted, the app names the table and you read the one figure from your own copy. The calculation is the same either way.
What’s in the app
- Voltage drop: single phase, three phase and DC, with conductor temperature, stranding and reactance
- Full wire sizing: ambient correction, grouping, termination limit, continuous load, breaker size and voltage drop, with any check it couldn’t make flagged
- Current capacity for your code and edition, filled in from the pack instead of your bookcode pack
- Short-circuit withstand, and the smallest size that survives the fault
- Conduit and tubing fill
- Ohm’s law and real power, single and three phase
- Sheets saved by job, searchable, and a whole job as one PDF
- Metric, AWG and kcmil sizes
- Protective conductor size, from the device’s let-through energy or from a fault current
- Transformer full-load and fault current from the nameplate
- All of it offline, on iPhone, iPad, Android and the web
- In English, Portuguese, Spanish, Japanese and Korean
Only the line marked code pack needs a figure from a national wiring code. Where that code is public, the pack carries it; where it is copyrighted, the app asks you for the figure and tells you which table it’s in. Everything else is pure calculation.
Try it now, free
Five calculators run free in your browser on the same engine as the app, with the full sheet. They need no code tables, only physics, so anyone can use them. Full wire sizing with derating is in the app. See it worked through on one circuit →
Voltage drop calculator →
Single phase, three phase and DC, in mm², AWG or kcmil. Shows every step and gives you a sheet to print.
Wire size by voltage drop →
The smallest size that keeps the drop under your limit, plus a list of what it doesn’t check.
Ohm’s law and power →
Volts, amps or ohms from the other two, plus real power for single and three phase.
Wire gauge chart →
AWG and kcmil sizes with diameter, mm², circular mils and resistance, worked out from the gauge formula.
Conduit fill →
How full your conduit is, for any conduit and any cable, in mm or inches, against the limit from your rules.
Get it on launch day
We’ll send one email when it’s out. Tell us which code you work to and that pack moves up the list.