Electrical & HVAC Calculators With Code Shown
Free home electrical and HVAC calculators with the math shown: voltage drop, wire size, breakers, BTUs, mini splits, generators, and solar sizing.
Most sizing calculators give you a number and no reason to believe it. Every tool here cites the code table or sizing chart it uses, shows the arithmetic, and says what the number does not cover -- the conduit-fill derating, the Manual J that a rule of thumb stands in for, the local amendment.
Wiring and circuits covers wire gauge, breakers, voltage drop and Ohm's law. Heating and cooling sizes air conditioners, heaters and mini splits. Power and energy covers generators, what things cost to run, and solar.
Wiring & circuits
Voltage drop in volts and percent for a copper or aluminum run, single- or three-phase or DC, using NEC Ch. 9 resistance values and the 3%/5% rule.
Wire gauge & breakerThe wire gauge and breaker size for a load, from NEC ampacity tables at 75°C, the 125% continuous-load rule, and voltage drop over your run length.
Conduit fill (EMT)How many wires fit in EMT conduit, from NEC Chapter 9 Table 4/5, with the 53%/31%/40% fill limits and the smallest trade size that fits.
Wire gauge chartAWG wire sizes 14 through 500 kcmil with real NEC 310.16 ampacity at 60/75/90°C for copper and aluminum, plus typical circuits for each gauge.
Conduit fill chartHow many THHN/THWN-2 conductors of each AWG fit in each EMT trade size, from NEC Chapter 9 Tables 1, 4 and 5 at the 40% (3-or-more) fill limit.
Box fill (NEC 314.16)How much box volume your wires, grounds and devices actually need, from NEC 314.16(B), against your box's real stamped volume.
Ohm's law: V, I, R, PEnter any two of volts, amps, ohms and watts to get the other two, plus AC watts-to-amps conversions with power factor for single-phase and three-phase.
Amps ↔ wattsConvert amps to watts and back for DC, AC single- or three-phase loads, with power factor, VA, and a table of common breaker ratings in watts.
Heating & cooling
How many BTUs a room or house needs, from the ENERGY STAR chart adjusted for ceiling height, sun, occupants, insulation and climate, in BTU/h and tons.
Mini split sizingThe mini split size (9K-24K BTU and up) per zone, with a multi-zone total, from room area, sun, ceiling height, climate, and draw at your SEER2.
Power & energy
What size generator you need: running watts summed, the largest motor's starting surge added, and a standard generator size with headroom.
Cost to run an applianceWhat any appliance costs to run per day, month and year from its wattage, hours of use and your rate, with kWh shown and a table of common household loads.
Solar panels neededHow many solar panels offset your annual usage, from peak sun hours and the PVWatts derate, with system size, roof area, cost and payback.
Frequently Asked Questions
Where do the numbers come from?
Each calculator names its source on the page: NEC Chapter 9 Table 8 for conductor resistance, NEC 310.16 for ampacity, 240.4(D) for small-conductor breaker limits, the ENERGY STAR sizing chart for cooling BTUs, manufacturer tables for generator watts, EIA for electricity rates, and NREL PVWatts for solar losses. Nothing here is invented; the arithmetic is shown so you can check it.
Can I use these instead of an electrician?
No. They tell you what the general case in the national code says so you can plan a job and understand a quote. Local amendments, conduit fill, temperature, terminal ratings and the inspector's judgement all apply, and mistakes in electrical work are dangerous. Use the numbers to have a better conversation with the person doing the work.
Do I need an account, and is anything saved?
No account, nothing saved. Every calculation runs in your browser and nothing you type is sent anywhere.