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.