What Size Breaker (and Wire) Do I Need for This Load? Watts to Amps, the 125 Percent Rule and the Next Standard Breaker on iPhone With Electrician Calculator Toolkit

"What size breaker do I need?" The nameplate says watts, the panel wants amps, and the code wants a margin for anything that runs for hours. This post walks the chain from watts to amps to design current to breaker to conductor, does the arithmetic for two loads, and shows how the Load & Breaker tool in Electrician Calculator Toolkit for iPhone runs the same chain on one screen while naming the table each step came from.

Two boundaries. The app calls itself a calculation aid, not a substitute for the code, engineering judgement or the authority having jurisdiction, and this post is not electrical advice. Every number marked as arithmetic was worked by hand from the generic formulas, not read off the app's screen.

Step 1: watts to amps (the load current)

For a single-phase load, I = P ÷ V: watts divided by volts gives amps, and if the load is not purely resistive you divide by the power factor too, so I = P ÷ (V × PF). For a three-phase load, I = P ÷ (√3 × V × PF), where √3 is about 1.732 and V is the line-to-line voltage. A heater has PF = 1; a motor draws more current than its watts alone suggest.

The Load & Breaker tool asks for exactly these: watts, voltage, phase and power factor, and returns the load current.

Step 2: continuous or not, and the 125 percent rule

This is the step people skip. In general NEC practice a continuous load is one expected to run at its maximum current for three hours or more, and for such a load the overcurrent device and the conductors are sized at 125 percent of the load current, because a breaker and a wire at full rating for hours run hotter than the tables assume. A load that cycles is sized at its load current.

The result is the design current: 1.25 × load current for a continuous load, otherwise the load current itself. In the app you mark the load continuous with a switch, and the app applies the 125 percent rule when you do, showing both currents on the same screen.

Step 3: the next standard breaker rating

Breakers are not made in every amperage. The code lists a set of standard ratings, and when your design current lands between two of them the general rule is the next standard rating at or above it, subject to the small-conductor limits on the smallest wire sizes. The app's Code Tables tool includes that list, and the Load & Breaker result shows the next standard rating above your design current and names the table it came from, so when an inspector asks, you point at the table rather than at an app.

Step 4: the minimum 75°C conductor, then Voltage Drop

The last line of the result is the minimum conductor at a 75°C termination rating, the common case for breaker and equipment terminals. Treat it as the floor: a hot attic, a crowded conduit or a 60°C termination all move the wire up, which is what the separate Wire Size tool is for; the circuit sizing walkthrough covers derating.

Then, for anything beyond a short run, open Voltage Drop. Ampacity protects the wire; it says nothing about whether the equipment at the far end gets its volts. Enter the voltage, the actual load current (not the design current), material, size and one-way length, and the tool reports the drop in volts and percent and the smallest conductor under 3 percent. The breaker rarely changes here; the wire often does.

Worked example 1: a 1,500 W space heater on 120 V, continuous

• Load current: 1,500 ÷ 120 = 12.5 A (arithmetic; PF = 1 for a resistive heater).
• Design current, continuous: 1.25 × 12.5 = 15.6 A (arithmetic, rounded).
• Breaker: the arithmetic alone says 15.6 A is more than a 15 A breaker can be sized for; the next standard rating above it is what the app reads from its table, small-conductor limit applied.
• Conductor: the app shows the minimum 75°C conductor for that rating; confirm it in Wire Size if the run is hot or shares a raceway.

Sized on the nameplate alone, 12.5 A looks comfortable on a 15 A circuit. Marked continuous, it is not, and the breaker and wire both come out a size larger. In the app that is one switch.

Worked example 2: a 10 kW three-phase load on 208 V at PF 0.8

Nameplate: 10,000 W input, 208 V three-phase, power factor 0.8, treated as continuous. A motor-style load for the arithmetic only; see the caveat below.

• Denominator: 1.732 × 208 × 0.8 = 288.2 (arithmetic).
• Load current: 10,000 ÷ 288.2 = 34.7 A (arithmetic, rounded).
• Design current, continuous: 1.25 × 34.7 = 43.4 A (arithmetic, rounded).
• Breaker and conductor: the app reads the next standard rating above 43.4 A and the minimum 75°C conductor from its tables; the lines above are the input, not the app's screen.

Notice what the power factor did: at PF 1 the load current would be 10,000 ÷ (1.732 × 208) = 27.8 A, so leaving PF at 1 for a 0.8 load under-sizes the circuit by a quarter. The caveat: a real motor is not sized by this general method. Motor loads, HVAC equipment and similar have their own code rules for conductors and overcurrent protection that take precedence; use the tool for a general load, and for a motor only as a sanity check.

When not to trust a calculator, including this one

• The app's own disclaimer applies. Reference values follow the 2023 NEC; it is a calculation aid, not a substitute for the code, engineering judgement or the authority having jurisdiction, and your jurisdiction may enforce an earlier edition.
• Feeders and services with demand factors are a different calculation; Load & Breaker sizes a known load.
• Equipment instructions win. A maximum overcurrent device or minimum circuit ampacity on the nameplate governs, whatever the arithmetic says.
• Garbage in. A wrong voltage, a guessed power factor or an all-night load marked non-continuous gives a tidy, wrong answer. The app shows its working; it cannot check your inputs.

The code table behind every result

Each result names the table or section it came from, and the Code Tables tool opens those references directly: ampacity tables for copper and aluminium, standard breaker ratings, derating factors, box fill volumes and US and IEC wire colours. For an apprentice that teaches where the numbers live; for an electrician it is the fastest answer to "which table?" with gloves on.

As of November 2026 the app is a $3.99 one-time purchase on the App Store, iPhone only, 2.3 MB, iOS 17 or later, with Family Sharing and 19 languages. It works with no signal because every table is inside the app; no account, no subscription, no ads, no analytics, and the privacy label reads Data Not Collected. Load & Breaker is one of eight tools; the box fill worked example covers the check at the other end of the run.

FAQ

• What is the 125 percent rule for continuous loads? In general NEC practice a load that runs at its maximum for three hours or more is continuous, and the breaker and conductors are sized at 125 percent of its current. The app applies it when you mark the load continuous.
• Does the app pick the breaker for me? It shows the next standard rating above your design current and names the table. Whether that is the right breaker still depends on equipment instructions, special-load rules and local code.
• Is there an Android version? Not at the time of writing; the app page will link to Google Play if one ships.