Voltbox is the field toolbox for electricians, apprentices, and serious DIYers — a
fast, offline set of calculators and references built by someone who's actually run
pipe and pulled wire. No sign-in, no clutter, no ads on your calculations. Just the
numbers you need, on the job, in your pocket.
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WHAT'S INSIDE
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⚡ WIRE & BREAKER SIZING (new)
Enter a load in amps — or watts and voltage — and Voltbox returns the minimum
conductor, the standard breaker (NEC 240.6), the largest breaker that conductor may
legally carry, and the equipment ground from Table 250.122. A breaker-to-conductor
quick chart for 60 °C and 75 °C terminations and a common-circuits reference come
built in.
🔌 MULTIMETER GUIDE (new)
Learn to use your meter the right way, written around the Fluke meters most techs
actually carry — the 115, 116, 117 and 87V, plus the clamps. Safety and CAT ratings,
the live–dead–live check, input jacks, symbol legend, voltage, continuity and
resistance, ghost voltage and low-Z, and the mistakes that cost fuses. Advanced
sections cover current measurement, capacitance, diode, frequency, temperature,
microamp flame-rod testing, and a full field-procedure playbook.
📐 CONDUIT BENDING
90° stub, back-to-back, offset, three-bend saddle, four-bend saddle, and rolling
offset. Enter your numbers and get exactly where to mark and how far to bend — marks
shown as tape-measure fractions with the decimal underneath.
🧮 OHM'S LAW
Enter any two of volts, amps, ohms, or watts and Voltbox solves the other two, with
the formulas shown so you can check the work.
🧵 CONDUIT FILL
Add your conductors by type, size, and quantity, pick the conduit, and get the
smallest trade size that stays within NEC Chapter 9 fill percentages.
🎨 RESISTOR COLOR CODE
A 4, 5, or 6-band picker with a live-drawn resistor and computed value, tolerance,
and temperature coefficient.
📊 WIRE AMPACITY
NEC 310.16 copper and aluminum ampacities at 60/75/90 °C with the small-conductor
rules built in.
📉 VOLTAGE DROP
Single- and three-phase voltage drop in volts and percent, with a 3% pass/fail check.
📦 BOX FILL
NEC 314.16 box-fill volume from conductors, devices, clamps, and grounds.
🔋 POWER & MOTOR
kVA to volts to amps (single and three-phase), plus a horsepower–kilowatt converter.
🌎 GROUNDING
Equipment grounding conductor (250.122) and grounding electrode conductor (250.66)
sizing.
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WHY VOLTBOX
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• Works 100% offline — in the panel, the attic, the crawlspace, no signal needed.
• No account and no personal data collection. Your calculations never leave the phone.
• Clean, readable, one-handed layout that works with gloves on.
• Light and dark themes that follow your system setting.
• Built and maintained by a working-trade developer, not a content mill.
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FREE & PRO
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Most of Voltbox is free, including Ohm's law, resistor codes, conduit fill, ampacity,
grounding, the everyday conduit bends, the Wire & Breaker sizer at standard
conditions, and the core of the Multimeter Guide.
Voltbox Pro is a one-time purchase — no subscription. It unlocks the advanced conduit
bends, Voltage Drop, Box Fill, Power & Motor, the Wire & Breaker derating engine
(ambient temperature, raceway bundling, termination ratings), and the advanced half
of the Multimeter Guide — and it removes the single banner ad.
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Voltbox is an estimating and reference aid. Always verify against the edition of the
National Electrical Code (NFPA 70) adopted in your jurisdiction, and follow your
employer's safe-work practices and NFPA 70E. Work de-energized whenever possible.
NEC® and NFPA 70® are registered trademarks of the National Fire Protection
Association. Fluke® is a registered trademark of Fluke Corporation. TapForge Studios
is not affiliated with, sponsored by, or endorsed by the NFPA or Fluke.
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