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Study guides · NEC Article 430
Motor Conductor Overload Calculation, Step by Step (NEC Article 430)
Motor questions scare people because Article 430 looks huge. It is not. Almost every exam motor problem is the same three calculations, in the same order, and every one of them starts with a single decision: table or nameplate. Get that decision right and the math is just multiplication. Open your Code book to Article 430 and work through it once, slowly. After that, these are some of the fastest points on the test.
- Step 1 — The one rule: tables vs. nameplate (430.6)
- Step 2 — Get the table full-load current
- Step 3 — Branch-circuit conductors (430.22)
- Step 4 — Overload protection (430.32)
- Step 5 — Short-circuit and ground-fault protection (Table 430.52)
- Step 6 — A feeder with more than one motor (430.24)
- The traps the exam writers use
Step 1 — The One Rule: Tables vs. Nameplate
Section 430.6(A) splits every motor calculation in two, and the exam tests whether you know which side you are on:
- Conductors and short-circuit / ground-fault protection: size them from the tables, Table 430.247 for DC, Table 430.248 for single-phase, and Table 430.250 for three-phase. Never the nameplate.
- Overload protection: size it from the nameplate current, per 430.6(A)(2) and 430.32. The nameplate is for overload, and nothing else.
Write that on a tab and stick it at Article 430: tables for wire and breaker, nameplate for overload. Half the wrong answers on motor questions come from grabbing the nameplate number and running a conductor calculation with it. The exam gives you a nameplate value in almost every stem for exactly that reason.
Step 2 — Get the Table Full-Load Current
Read horsepower down the left column, then read across to your voltage. That sounds obvious, but the wrong-column grab is the most common motor mistake on the exam. Know these rows, because they show up constantly:
| Motor | Table FLC |
|---|---|
| 1-1/2 HP, 208V, 3-phase (Table 430.250) | 6.6A |
| 3 HP, 230V, 3-phase (Table 430.250) | 9.6A |
| 7-1/2 HP, 230V, 3-phase (Table 430.250) | 22A |
| 7-1/2 HP, 460V, 3-phase (Table 430.250) | 11A |
| 10 HP, 230V, 3-phase (Table 430.250) | 28A |
| 15 HP, 460V, 3-phase (Table 430.250) | 21A |
| 25 HP, 460V, 3-phase (Table 430.250) | 34A |
| 2 HP, 120V, single-phase (Table 430.248) | 24A |
| 5 HP, 230V, single-phase (Table 430.248) | 28A |
Two patterns help you sanity-check your lookup. First, in Table 430.250 the 460V column is half the 230V column: 7-1/2 HP is 22A at 230V and 11A at 460V, and 15 HP is 42A at 230V and 21A at 460V. If your 460V answer is not roughly half the 230V value, you read the wrong column. Second, watch the 25 HP row: at 460V it is 34A, not the 68A sitting in the 230V column. And notice the trap built into the last two tables: a 5 HP single-phase motor at 230V and a 10 HP three-phase motor at 230V both come out to 28A. Same number, different table. The exam loves that coincidence.
Step 3 — Branch-Circuit Conductors (430.22)
Section 430.22 is one sentence of math: branch-circuit conductors for a single motor need an ampacity of at least 125% of the table full-load current. Motors run for hours and start hard, so the wire gets a quarter extra. That is the whole rule.
Worked example: A 10 HP, 230V, three-phase motor. What is the minimum branch-circuit conductor ampacity?
Table 430.250 gives 28A. Multiply by 1.25: 28A x 1.25 = 35A. Your conductor needs an ampacity of at least 35 amps. Table first, then times 1.25, every time.
Run one more so the pattern sticks. A 7-1/2 HP, 230V, three-phase motor has a table FLC of 22A. Conductors: 22A x 1.25 = 27.5A minimum ampacity. Notice what you did not use: anything printed on the motor. If the stem mentions a nameplate current, it is there to pull you off the table. Ignore it for this step.
Step 4 — Overload Protection (430.32)
Now, and only now, you switch to the nameplate. The overload device protects the motor itself from cooking during a sustained overload, so it is sized to the actual motor in front of you, per 430.32(A)(1):
- Service factor 1.15 or higher, or temperature rise 40°C or less: up to 125% of nameplate current.
- All other motors: up to 115% of nameplate current.
Worked example 1: A motor with a marked service factor of 1.15 and a nameplate FLC of 28A. Maximum overload setting: 28A x 1.25 = 35A. A marked service factor earns you the 125%.
Worked example 2: A motor with no marked service factor, a 50°C temperature rise, and a nameplate FLC of 20A. Maximum overload setting: 20A x 1.15 = 23A. No service factor marked means 115%, no debate.
One escape hatch: if the overload sized under 430.32(A) will not let the motor start, 430.32(C) permits the next higher size or trip rating. You size it by the book first, then go one step up only if the motor cannot start. You do not get to jump straight to a bigger number because it feels safer.
Lookup tip: tab three places before test day, Table 430.248, Table 430.250, and Table 430.52, and put a sticky note at 430.32 that reads "nameplate x 1.25 if SF 1.15+, otherwise x 1.15." Every motor calculation on the exam starts within a page or two of those tabs.
Step 5 — Short-Circuit and Ground-Fault Protection (Table 430.52)
This is the step that looks wrong until you understand it. The breaker or fuse protecting a motor branch circuit is sized way above the running current, because its job is not overload protection (that is Step 4). Its job is to clear a short circuit or ground fault, and it has to hold through starting current to do it. Table 430.52 gives the maximum percentages of table FLC:
| Device | Maximum, % of FLC |
|---|---|
| Inverse-time breaker | 250% |
| Nontime-delay fuse | 300% |
| Dual-element (time-delay) fuse | 175% |
| Instantaneous-trip breaker | 800% |
Worked example 1: The same 28A FLC motor on an inverse-time breaker: 28A x 2.50 = 70A. Seventy amps is a standard size under 240.6, so a 70A breaker is your answer. Yes, a 70A breaker on a 28A motor is correct. The overloads from Step 4 protect the motor; the breaker protects against faults.
Worked example 2: The same motor on a dual-element, time-delay fuse: 28A x 1.75 = 49A. Forty-nine amps is not a standard size, so 430.52(C)(1) Exception 1 permits the next standard size up: 50A. When the math lands between standard sizes for this device, you round up, not down.
If the Table 430.52 size still will not let the motor start, 430.52(C)(1) Exception 2 lets you increase it, capped at 400% of FLC for motors rated 100A or less (300% over 100A). That cap is the number the exam wants when a stem says the breaker "trips on starting current."
Step 6 — A Feeder With More Than One Motor (430.24)
When one feeder supplies several motors, 430.24 changes the conductor math slightly: take 125% of the largest motor's FLC, plus 100% of the rest. Only the biggest motor gets the quarter extra.
Worked example: Two motors on one feeder, with table full-load currents of 28A and 15.2A. Minimum feeder conductor ampacity: (28A x 1.25) + 15.2A = 35A + 15.2A = 50.2A.
The trap here is multiplying everything by 1.25, or picking the smaller motor as "the largest." Find the biggest FLC first, give it the 125%, then add the rest straight.
Two supporting rules ride along with these calculations on the exam. The motor disconnect must be located within sight of the motor, per 430.102(B), and "in sight" in Article 100 means visible and not more than 50 feet away. And per 430.83(A)(1), the controller's horsepower rating cannot be less than the motor's horsepower rating. Neither one is a calculation, but both are cheap questions that show up next to the calculation questions.
The Traps the Exam Writers Use
- Nameplate for conductors. The stem hands you a nameplate current, you use it for the wire, you miss. Conductors come from the tables. Always.
- Wrong voltage column. 25 HP at 460V is 34A. The 68A next to it is the 230V column. Read across to your voltage before you read the number.
- Same number, different table. 5 HP single-phase at 230V and 10 HP three-phase at 230V are both 28A, from Tables 430.248 and 430.250. If you opened the wrong table and still got 28A, you will never know you were wrong until the score comes back.
- 115% vs. 125% overloads. Service factor 1.15 or higher, or 40°C rise or less, gets 125% of nameplate. Everything else gets 115%. The stem always tells you which motor you have.
- Rounding the fuse down. A 49A result for a dual-element fuse goes up to the next standard size, 50A, under 430.52(C)(1) Exception 1. For short-circuit protection, between sizes means up.
- Panicking at the big breaker. A 70A inverse-time breaker on a 28A motor is not a typo. Starting current demands it, and the overload device does the motor's actual protection.
That is the whole method: table for wire and breaker, nameplate for overload, 125% on the conductors. It is the same discipline as box fill and voltage drop: one method, repeated until it is automatic.
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