Short answer: For standard motor circuit problems, take full-load current from the NEC motor tables, not the nameplate, to size conductors and short-circuit protection. Conductors are sized at 125% of that current. Overloads use the nameplate current, usually at 115% or 125% depending on the motor. In the standard cases in this article, an inverse-time breaker may be up to 250% of table current, and the next standard size up is allowed when the result does not land on one. Read the Article 430 exceptions for other cases.
Motor questions mix three different percentages, and one wrong pick costs the point. The PSI outline gives Motors and Generators 5 items in Texas Journeyman NEC Knowledge and 2 in Calculations. The Texas Journeyman Practice Pack has step-by-step solutions for 299 calculation questions, each showing which table and which percentage applies. US$39 once.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the method in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC. Motor circuits on the job are sized by the people responsible for the design; this page is for learning the method.
Article 430 map
Article 430 is long because a motor circuit has several separate parts, and each part has its own rule. Texas uses the 2026 NEC for installations and exams from September 1, 2026 (TDLR rule adoption notice, PSI Candidate Information Bulletin).
| Part of the circuit | What it protects or does | Current you use | Where to look |
|---|---|---|---|
| Branch-circuit conductors | Carry the motor current | 125% of table full-load current | 430.22 |
| Overload protection | Protects the motor from running too hot | Nameplate full-load amps | 430.32 |
| Short-circuit and ground-fault protection | Breaker or fuses for faults | Table full-load current × the percentage in the table | Table 430.52(C)(1) |
| Feeder conductors (several motors) | Carry more than one motor | 125% of the largest plus the rest | 430.24 |
| Feeder protection | Feeder breaker or fuses | Largest branch device plus the other motors | 430.62(A) |
| Control circuit | Start, stop and interlock wiring | Control-circuit rules | 430.72 |
| Disconnecting means | Lets you shut off motor and controller | In sight of both, with exceptions | 430.102 |
The first rule to learn: for conductors and short-circuit protection, use the full-load current from the NEC tables (430.6(A)). Use the nameplate current only for overloads. Single-phase motor currents are in Table 430.248 and three-phase in Table 430.250.
Values used in the examples
Check each one in your own 2026 NEC.
| Item | Value |
|---|---|
| 10 hp, 460 V, three-phase, table full-load current | 14 A |
| 15 hp, 460 V, three-phase, table full-load current | 21 A |
| 3 hp, 230 V, single-phase, table full-load current | 17 A |
| Overloads, service factor 1.15 or more | 125% of nameplate |
| Overloads, all other motors (simplified) | 115% of nameplate |
| Inverse-time breaker | 250% of table current |
| Dual-element (time-delay) fuse | 175% of table current |
When the percentage gives a size that is not a standard rating, you may go up to the next standard size (430.52(C)(1)).
Worked example 1: one 10 hp motor
A 10 hp, 460 V, three-phase motor. Nameplate full-load amps 13.2 A, service factor 1.15. Terminals are rated 75°C.
- Table full-load current: 14 A. (Not the 13.2 A on the nameplate.)
- Conductor ampacity: 14 × 1.25 = 17.5 A minimum. At the 75°C column, 14 AWG copper (20 A) meets this. Article 430 sets motor-circuit protection, so the general 15 A limit for 14 AWG does not apply (240.4(G)). Many shops still run 12 AWG as a company standard.
- Overload: nameplate 13.2 × 1.25 = 16.5 A maximum setting.
- Inverse-time breaker: 14 × 2.50 = 35 A. 35 A is a standard size, so 35 A.
- Dual-element fuse instead: 14 × 1.75 = 24.5 A. Next standard size up: 25 A.
Check: reverse each step. 17.5 ÷ 1.25 = 14. 16.5 ÷ 1.25 = 13.2. 35 ÷ 2.5 = 14. 24.5 ÷ 1.75 = 14. All match.
Worked example 2: one 15 hp motor
A 15 hp, 460 V, three-phase motor.
- Table full-load current: 21 A.
- Conductor ampacity: 21 × 1.25 = 26.25 A minimum.
- Inverse-time breaker: 21 × 2.50 = 52.5 A. Not a standard size. Next standard size up: 60 A.
- Dual-element fuse: 21 × 1.75 = 36.75 A. Next standard size up: 40 A.
Check: 50 A is the standard size just below 52.5, and 52.5 is more than 50, so the rounded-up answer must be 60. 35 A is below 36.75, so the fuse answer must be 40.
Worked example 3: a feeder for both motors
The 10 hp and 15 hp motors share one feeder.
- Feeder conductor ampacity: 125% of the largest motor plus 100% of the other: (21 × 1.25) + 14 = 26.25 + 14 = 40.25 A minimum.
- Feeder breaker: not more than the largest branch-circuit device plus the full-load current of the other motor: 60 + 14 = 74 A.
- 74 A is not a standard size, and this rule does not let you round up. Use the next standard size down: 70 A.
Check: add in another order. 14 + 26.25 = 40.25. And 70 is the largest standard size that is not more than 74.
The three-wire start-stop circuit
The most common control circuit you will wire is the three-wire start-stop station.
- The stop button is normally closed and is wired in series, so pressing it breaks the circuit.
- The start button is normally open. A holding (seal-in) contact on the starter is wired in parallel with it, so the coil stays energized after you let go.
- The overload contacts are in series with the coil, so an overload trip drops the starter out.
- If power fails, the coil drops out and the motor stays off until someone presses start again. A two-wire control, like a float switch, restarts on its own when power returns.
Before tracing a starter circuit, follow the lockout/tagout procedure of your employer, and have a qualified person verify the absence of voltage with a test instrument. Do not assume a circuit is safe because a switch is off. See the NFPA 70E and lockout article before you work on any starter.
Check yourself
Our own practice questions. Answers below each.
- 3 hp, 230 V, single-phase motor. Minimum conductor ampacity? Answer: 17 × 1.25 = 21.25 A.
- Same motor, inverse-time breaker? Answer: 17 × 2.5 = 42.5 A, not standard. Next size up: 45 A.
- A motor nameplate reads 20 A, service factor 1.0, no temperature-rise marking. Maximum overload setting under the simplified rule? Answer: 20 × 1.15 = 23 A.
- Which current do you use for the overload: table or nameplate? Answer: Nameplate.
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Formulas and control diagrams in your pocket
Ugly’s Electrical References, 2026 Edition. The publisher’s description lists formulas, NEMA configurations, conduit bending and fill, ampacity, and transformer and control circuit wiring diagrams. It is a field reference, not a substitute for the NEC, and spiral-bound books are not allowed in the Texas exam room.
FAQ
Do you use nameplate or table current for motor wire size?
Table current. For branch-circuit conductors and short-circuit protection, the NEC requires the full-load current from its motor tables. Nameplate current is used for sizing overload protection.
How do you size a breaker for a motor?
Multiply the table full-load current by the percentage for the device type in the NEC motor protection table. For an inverse-time breaker that is 250%. If the result is not a standard size, you may use the next standard size up.
Why is motor wire sized at 125%?
The NEC requires branch-circuit conductors for a single continuous-duty motor to have an ampacity of at least 125% of the motor’s table full-load current, which gives margin for running at full load.
What is the difference between overload and short-circuit protection for a motor?
Overload protection trips on sustained current above the motor rating and protects the motor from overheating. Short-circuit and ground-fault protection, the breaker or fuses, opens quickly on faults and is sized much larger so the motor can start.
Sources checked October 2026
- TDLR rule adoption notice, 2026 NEC: https://www.tdlr.texas.gov/news/rulemaking/2026/09/01/commission-adopts-rules-12/
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- TDLR exam page (spiral-bound books not allowed): https://www.tdlr.texas.gov/electricians/elecexam.htm
- NFPA 70 (NEC), 2026 edition, Article 430 and its motor tables, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
