Sizing a Circuit on Your iPhone: Wire Size, Voltage Drop and Conduit Fill in One Pass With Electrician Calculator Toolkit

The three questions that come up on almost every job are the same: what breaker and wire does this load need, will the voltage drop pass over that run, and what conduit does the bundle fit in. Each has a table in the code book, each table has a footnote, and each footnote is annoying to find with gloves on in a crawl space. This post walks one realistic circuit through all three on an iPhone using Electrician Calculator Toolkit, showing which NEC 2023 table each answer comes from, so you can check it against the book or explain it to the inspector.

The app is a $3.99 one-time purchase for iPhone (iOS 17 or later), works fully offline, and bundles eight tools: voltage drop, wire size with derating, conduit fill, box fill, load and breaker, Ohm's law, resistor colour codes and the reference tables. There is no Android version yet. It is a calculation aid, not a substitute for the adopted code in your jurisdiction or the authority having jurisdiction, and this post is not electrical advice.

The example circuit

A 240 V single-phase continuous load of 9,600 W, so 40 A, feeding equipment about 100 feet from the panel in copper THHN, run in EMT with two other circuits sharing the raceway in a hot attic. Realistic enough to hit every rule that matters: the continuous-load factor, ambient and conductor-count derating, voltage drop over a long run, and conduit fill with more than three conductors.

Step 1: Load and breaker (240.6(A) and the 125 percent rule)

Open Load & Breaker, enter 9,600 W, 240 V, single-phase, and mark the load continuous because it runs for three hours or more. The tool returns the load current (40 A), the design current at 125 percent (50 A), the next standard breaker rating from Table 240.6(A), and the minimum 75°C conductor for that current.

The 125 percent step is the one people skip when they size from the nameplate alone. A 40 A continuous load needs an overcurrent device and conductors rated for 50 A, which is why the breaker and the wire both come out a size larger than the nameplate suggests. The app shows both numbers on the same screen so the reasoning is visible rather than buried.

Step 2: Wire size with derating (Table 310.16, 310.15(B)(1), 310.15(C)(1))

Carry the 50 A into Wire Size. In a mild room with three or fewer current-carrying conductors, Table 310.16 at 75°C gives #8 AWG copper for 50 A, and that is the textbook answer to "what size wire for a 50 amp circuit". Our example is not a mild room. Set the termination rating (75°C for typical breaker and equipment terminals), enter the attic ambient temperature, and set the number of current-carrying conductors sharing the EMT to six, since three two-wire circuits are in it.

The tool applies the ambient correction from 310.15(B)(1) and the adjustment factor for more than three conductors from 310.15(C)(1), then shows the adjusted ampacity of every size around the answer rather than a single number. That table view is the useful part: you can see exactly how far #8 falls short once the factors are applied and whether #6 clears it with margin. For the small sizes, it also enforces the 240.4(D) caps so that #14, #12 and #10 are never shown protected above 15, 20 and 30 A no matter what the derated ampacity says.

• Termination rating caps you at the 60°C or 75°C column even if the insulation is 90°C; the 90°C column is only the starting point for derating.
• Ambient correction and conductor-count adjustment multiply together; a hot attic and a crowded conduit can push a circuit up two sizes.
• Equipment grounding conductors and most neutrals in balanced circuits do not count as current-carrying; the app asks for the count, you decide what counts.

Step 3: Voltage drop over the run (210.19(A) informational note)

Now the 100-foot one-way run. Open Voltage Drop, enter 240 V, 40 A (the actual load current, not the design current), copper, the conductor size you landed on, and 100 feet. The tool uses VD = 2 × K × I × L ÷ CM for single-phase and DC (1.732 instead of 2 for three-phase), with K = 12.9 for copper and 21.2 for aluminium, and returns the drop in volts and percent plus the voltage at the load.

To see why this step matters even after derating, run a smaller case: a 120 V, 20 A load on #12 copper at 100 feet. The formula gives 2 × 12.9 × 20 × 100 ÷ 6,530 circular mils, which is about 7.9 V, or 6.6 percent, more than double the 3 percent branch-circuit figure in the informational note to 210.19(A). #10 brings it to roughly 4.1 percent; #8 gets it under 3 percent. The app does this comparison for you, flagging anything above the 3 percent branch and 5 percent total informational limits and showing the smallest conductor that stays under 3 percent. You can flip to aluminium or add parallel sets to see the alternatives side by side.

Those 3 and 5 percent figures are informational in the NEC, not mandatory, and energy codes or the equipment manufacturer may impose their own. The tool labels them as such; the decision stays yours.

Step 4: Conduit fill (Chapter 9, Tables 1, 4 and 5)

Finally the raceway. In Conduit Fill, add the conductors: the two circuit conductors and grounding conductor for this circuit at the size you chose, plus the conductors of the two other circuits, each with its insulation type. THHN/THWN, XHHW and THW are supported, and you can mix sizes freely. The tool reads the 40 percent fill limit from Table 1, the raceway areas from Table 4, and the conductor areas from Table 5, and shows the minimum trade size for EMT, PVC Schedule 40, PVC Schedule 80, RMC and IMC side by side.

There is also a reverse mode, Max conductors, for the question that gets asked the other way round: how many of one conductor fit in a given conduit. The classic example is sixteen #12 THHN in a 3/4 inch EMT, which is what Tables 1, 4 and 5 give and what the tool reports.

Step 5: Box fill, if the run ends in a box you have to justify

Box Fill applies the 314.16(B) volume allowances: one per conductor originating outside the box, a single allowance for all internal clamps, one per support fitting, two per device yoke, and one for all equipment grounds together, each based on the largest conductor in the box. Enter what enters the box and compare the required cubic inches against the box you have in your hand; the tool gives a pass or fail against standard box volumes from the tables.

Why the table reference on every screen matters

Plenty of free single-purpose calculators will give you a wire size. Few tell you which table and which edition they used, and that is the number an inspector will ask about. Every result in Electrician Calculator Toolkit names the NEC 2023 table or section it came from, which does two things: it lets you verify any answer against the book in seconds, and it turns the app into a teaching tool for apprentices who need to learn where the numbers live, not just what they are. The Code Tables tool opens those references directly, including ampacity tables for copper and aluminium, standard breaker ratings, derating factors, box volumes and the US and IEC wire colour conventions.

When you still open the book

• Your jurisdiction is not on the 2023 edition. Many places adopt a code cycle late or with amendments. The tables rarely move much between editions, but the app is explicit that it follows 2023, so check which edition your inspector enforces.
• Motor, HVAC, welder and other special loads. Those articles have their own conductor and overcurrent rules that override the general method. Use the tool for the arithmetic, not for the rule.
• Feeders and services with demand factors. Article 220 load calculations are not what the Load & Breaker tool does; it sizes a known load.
• Anything outside AWG and kcmil. Lengths can be in metres and temperatures in Celsius, but conductor sizes follow the US tables. European installers working to IEC cross-sections in square millimetres will find the voltage drop and Ohm's law tools useful and the ampacity tables not directly applicable.

The quick version

Load & Breaker for the design current and breaker, Wire Size for the derated conductor, Voltage Drop for the run, Conduit Fill for the raceway, Box Fill for the termination. Five screens, each naming its table, under a minute once you have done it twice, and nothing uploaded or downloaded along the way. If that is the flow you already do on paper, Electrician Calculator Toolkit just makes it faster and shows its working.