NEC Article 430 Motor Calculations: Why Four Different Numbers Come From One Motor
Motor calculations fail more journeyman candidates than any other NEC math — because one motor produces four different numbers, each from a different rule. Conductors, overload, branch protection, and feeders, with a worked 25 hp example.
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Why Article 430 Fails People Who Know Their Code
Article 430 is on every list of most-tested NEC articles, and it is where candidates who genuinely understand the code still get the wrong answer. The reason is structural: every other calculation on the exam takes one load and produces one number. A motor produces four — branch-circuit conductors, overload, branch-circuit short-circuit and ground-fault protection, and the motor's contribution to the feeder. Each comes from a different section, uses a different percentage, and — the part that costs people the question — they do not all start from the same current. Get the starting current wrong and all four answers fail together, so one misunderstanding costs several questions. The good news: Article 430 is mechanical once you see the structure — four rules in a fixed order, asked the same way nearly every time.
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The Trap: Nameplate FLA Is Not Table FLC
This is the trap the exam is built around. A motor has two current ratings that are not the same number. The nameplate full-load amps (FLA) is stamped on the motor by the manufacturer. The full-load current (FLC) is the value in NEC Table 430.250 for three-phase motors, or Table 430.248 for single-phase, looked up by horsepower and voltage. They rarely match, and the exam will deliberately give you both. Section 430.6(A)(1) settles it: for conductors, branch-circuit protection, and feeders, use the TABLE value (FLC) — not the nameplate. The nameplate FLA is used for exactly one thing: sizing the overload protection, per 430.32. The mnemonic: overload reads the nameplate; everything else reads the table. Test writers include the nameplate specifically because it is the plausible wrong answer — a distractor is usually calculated from it, so your arithmetic will 'check out' against an option on the page.
Rule 1 — Branch-Circuit Conductors (430.22): 125% of Table FLC
Conductors supplying a single continuous-duty motor must have an ampacity of at least 125% of the motor's full-load current from the table (430.22). Note what this percentage is NOT: it is not the 125% continuous-load factor from Article 210, and it does not stack with it — a motor falls under Article 430's own rule. Two things trip people. First, the ampacity you compare against is normally the 75°C column of Table 310.16, because 110.14(C) limits you to the termination temperature rating, and terminations are typically 75°C even when THHN insulation is rated 90°C. Reaching for the 90°C column is a classic distractor. Second, conductor sizing has no 'next size up' rule — pick a conductor whose ampacity meets or exceeds the calculated value.
Rule 2 — Overload Protection (430.32): 115% or 125% of Nameplate
Overload protection guards the motor against running over its rating for a long time — a jammed load, a failing bearing, a lost phase. This is the one rule that uses the nameplate FLA. For continuous-duty motors rated more than 1 hp, 430.32(A)(1) gives two multipliers: 125% if the marked service factor is 1.15 or greater, OR if the marked temperature rise is 40°C or less; 115% for everything else. Read carefully — the service factor and temperature rise are supplied precisely so you must pick between them. The conceptual point the exam tests separately: overload and short-circuit protection are different devices doing different jobs. Overload sits tight around the motor's running current and is slow, tolerating inrush and tripping on sustained overcurrent. Short-circuit protection sits far above the running current and is fast. That is why the next number is so much larger, and why the instinct that 'protection should be close to the motor's current' produces a wrong answer.
Rule 3 — Branch-Circuit Short-Circuit and Ground-Fault Protection (430.52)
This is the number that looks wrong the first time you calculate it. Branch-circuit protection is sized from Table 430.52 as a percentage of the TABLE FLC, and the percentages are large because the device must pass the motor's starting inrush without tripping — a motor can pull six to eight times its running current on startup. For most AC motors: non-time-delay fuse 300%, dual-element (time-delay) fuse 175%, instantaneous-trip breaker 800%, inverse-time breaker 250%. The device type is always stated and changes the answer completely. Then the step people forget: 430.52(C)(1) Exception No. 1 permits the NEXT HIGHER standard size from 240.6(A) when the calculated value does not land on one — the opposite of the rounding habit most candidates bring from other calculations, and deliberately tested. Standard sizes worth knowing cold: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200.
A Worked Example: 25 hp, 460 V, Three-Phase
Take a 25 hp, 460 V, three-phase motor with a nameplate FLA of 32 A, a service factor of 1.15, protected by an inverse-time breaker. Step 1 — FLC: Table 430.250 gives 34 A at 25 hp / 460 V. Notice 34 A does not equal the 32 A nameplate; that gap is the whole exam question. Step 2 — conductors (430.22): 34 × 1.25 = 42.5 A. From the 75°C column of Table 310.16, 8 AWG copper carries 50 A, which covers it. Step 3 — overload (430.32): nameplate, not table, service factor 1.15, so 32 × 1.25 = 40 A maximum. Step 4 — branch protection (430.52): inverse-time breaker at 250% of the table FLC, so 34 × 2.5 = 85 A. 85 A is not a standard size, so the next-higher exception applies: a 90 A breaker. One motor, four answers — 8 AWG conductors, 40 A overload, 90 A breaker — with the 32 A nameplate used exactly once. Practice the conductor and ampacity steps with our [wire size calculator](/tools/wire-size) and [voltage drop calculator](/tools/voltage-drop).
Rule 4 — Feeders for Multiple Motors (430.24 and 430.62)
When several motors share a feeder, two different rules apply to conductors and protection, and swapping them is a common error. Feeder conductors (430.24) must carry 125% of the LARGEST motor's full-load current plus 100% of the others' — only the biggest motor gets the 125%, because only one motor is assumed to start at a time. Feeder protection (430.62) works differently: the largest branch-circuit protective device rating in the group, plus the sum of the remaining motors' full-load currents. That rule reuses the device size from Rule 3, which is why Article 430 questions are often chained — an error at the branch-circuit step propagates into the feeder answer. Unlike branch-circuit protection, the feeder result is generally NOT rounded up, since doing so would undermine the protection. The contrast to remember: conductors add 125% to the largest motor; protection starts from the largest DEVICE.
How to Actually Study This
Article 430 rewards drilling over reading: the failure mode is not comprehension, it is picking the wrong starting number under time pressure. Tab Tables 430.248, 430.250, and 430.52 in your code book — on an open-book exam these are pure lookup, and candidates lose the question to the clock, not the concept. For every practice question, write down which current you are using and why before you multiply: 'table, because conductors' or 'nameplate, because overload'. That one line of discipline eliminates most Article 430 errors. One honest note on the code cycle: the 2026 NEC moved load calculations out of Article 220 into a new Article 120, but adoption is state-by-state and many states will still test the 2020 or 2023 edition in 2026 — confirm your exam's edition before memorizing section numbers, and see our [2026 NEC changes guide](/blog/nec-2026-code-changes-electrician-exam). Drill motor questions free at [VoltExam's electrician practice test](/free-electrician-practice-test), or get one a day at [question of the day](/question-of-the-day).
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