Size Every Circuit
To Code.
Nine NEC calculators and seven reference tables
Wire size, voltage drop, conduit fill, ampacity, box fill, motor circuits, grounding and bends — every result cites the article it comes from and shows the steps behind it. 100% offline, no signal required.
Key Features
Everything an electrician checks before the conductor goes in the pipe
Wire Size Calculator
Conductor size from load, run length, ambient temperature and bundling — checked against ampacity and voltage drop at once, and it tells you which limit decided the answer.
Voltage Drop
Drop in volts and percent across a run, plus the smallest conductor that still meets your target — single-phase or three-phase, copper or aluminium.
Conduit & Box Fill
Raceway fill against the Chapter 9 limits with the next trade size when you are over, and box volume tallied item by item the way an inspector counts it.
Ampacity with Derating
Table 310.16 value, ambient correction, bundling adjustment and the 110.14(C) termination limit — shown as four separate lines, not one opaque number.
Motor Circuit & Grounding
Motor conductors at 125%, the overload window and the short-circuit device from Tables 430.248/430.250 — plus equipment grounding and grounding electrode sizing.
NEC Reference Tables
Ampacity, conductor properties and DC resistance, raceway and conductor areas, box fill volumes, equipment grounding and motor full-load currents — searchable, with copper and aluminium side by side.
How It Works
Three steps from load current to a conductor you can defend
Enter the Load
Load current in amps and the one-way run length in feet or metres. Mark it continuous if it runs three hours or more.
Set Conditions & Derating
Copper or aluminium, insulation rating, ambient temperature, how many current-carrying conductors share the raceway, and your voltage-drop target.
Get the Conductor + Article
The conductor size, the derating chain that produced it, which limit decided it, and the NEC article behind each step.
Derating Is Where Branch Circuits Go Wrong
One 8 AWG copper conductor with 90 °C insulation, in a 40 °C ambient, bundled with six current-carrying conductors. Four steps sit between the table value and the number you can actually use — and VoltRef Pro shows every one of them.
| Step | Code reference | Ampacity |
|---|---|---|
| Table 310.16 | 8 AWG copper, 90 °C column | 55 A |
| Ambient correction | 40 °C ambient — Table 310.15(B)(1), × 0.91 | 50.1 A |
| Bundling adjustment | 6 current-carrying conductors — Table 310.15(C)(1), × 0.80 | 40.1 A |
| Termination limit | 110.14(C) — 60 °C terminals at 100 A or less | 40 A |
The conductor starts at 55 A and lands at 40 A. Pick 90 °C insulation and the terminals still cap you at the 60 °C column — which is why the Wire Size result also tells you which of the four limits decided the conductor.
Every result cites its article
Table 310.16, Chapter 9 Table 4/5/8, 314.16(B), 250.122, 430.248/430.250 — the reference travels with the answer.
No answer beats a wrong answer
Out-of-range inputs return a warning instead of a clamped number. The app will not invent a conductor that the tables do not cover.
Reference, not authority
A planning aid for licensed electricians — local amendments and your authority having jurisdiction govern the installation.
Why VoltRef Pro
Built for the conditions electrical work actually happens in
100% Offline
Every calculator and table runs on the device. Basements, shafts, slab pours — no signal needed.
Shows Its Work
Table value, ambient correction, bundling adjustment and termination limit as separate lines — with the article behind each.
Glove-Friendly UI
Large touch targets and a high-contrast dark interface that stays readable in direct sun.
No Silent Guesses
An input the tables do not cover returns a warning, not a clamped number. You always know when the code has no answer.
Calculation History
Recent runs are saved on the device so a figure can be checked again later, and the ones you reuse can be starred. Pro removes the limit.
Free Wire Size Calculator
Wire Size and Ohm’s Law are free and unlimited — no account, no sign-up, no trial needed.
Frequently Asked Questions
Does VoltRef Pro work offline?
Yes. Every calculator and every reference table runs locally on your iPhone. No signal is needed in a basement, a shaft, or during a slab pour.
What's included for free and what needs Pro?
Wire Size and Ohm’s Law are free and unlimited — no account, no sign-up. Pro unlocks the remaining seven calculators (Voltage Drop, Conduit Fill, Ampacity, Box Fill, Motor Circuit, Grounding, Conduit Bending), the full reference library and unlimited calculation history.
How does the subscription work?
VoltRef Pro is free to download. Pro is available weekly ($1.99) or annually ($19.99). Prices are shown in US dollars and may vary by region; the price that applies to you, along with any introductory offer, is shown in the app before you subscribe. Subscriptions renew automatically through your Apple account and can be cancelled any time in App Store subscription settings.
Which code edition do the calculations follow?
Table values and calculation procedures follow the National Electrical Code as published — Table 310.16 ampacities, the 310.15(B)(1) and 310.15(C)(1) correction and adjustment factors, the 110.14(C) termination limits, Chapter 9 Tables 4, 5 and 8, Table 314.16(B), Table 250.122 and Tables 430.248/430.250. VoltRef Pro is not affiliated with, authorised by, or endorsed by the NFPA.
Are the results valid for an inspection?
Treat them as a planning and estimation aid, not as an authority. Local amendments, the authority having jurisdiction and the equipment listing govern the actual installation, and a conductor that satisfies the tables can still fail a specific job condition. Always verify against the current code book before you wire. VoltRef Pro is a reference tool for licensed electricians and does not replace training, licensure or inspection.
Imperial or metric?
Both. Run lengths accept feet or metres and the built-in converter handles the length, area, current and power conversions used in electrical work. Conductor sizes are AWG/kcmil, as the NEC tables publish them.
Contact Us
Questions, a table you need added, or a bug to report? We read every message.