What this lesson covers
Motors make up a large share of the Calculations portion of the Texas master exam: 6 of the 30 scored calculation questions and 7 of the 70 scored NEC Knowledge questions are on motors and generators. The questions look different from each other, but they all rest on one idea. A motor has three different currents, and each one has its own job. Mix them up and every later step is wrong. This lesson gives you a four-step habit: table current, conductors, protection, then feeder.
We cite the 2026 NEC by section number in our own words. Keep your own copy open beside this page.
Step 1: pick the right current
| Current | Where it comes from | What it is used for |
|---|---|---|
| Table full-load current | Table 430.248 (single-phase) or Table 430.250 (three-phase) | Conductors, short-circuit and ground-fault protection, disconnect (430.6(A)(1)) |
| Nameplate current | The motor's nameplate | Overload protection only (430.6(A)) |
| Locked-rotor current | Starting data | Nothing you size on this exam |
Find the horsepower row and the voltage column in the table. A 20 hp, 460 V, three-phase motor is 27 A. A 15 hp motor is 21 A. A 3 hp motor is 4.8 A. Do not read the nameplate for anything except the overload.
Step 2: size the conductors
One motor takes 125% of the table current (430.22). Then check the termination temperature, as for any conductor.
20 hp, table current 27 A: 27 × 1.25 = 33.75 A. With 60°C terminations, 8 AWG copper (40 A) passes.
Step 3: size the protection
Three separate devices, three separate percentages.
Short-circuit and ground-fault device (430.52). A percentage of the table current, by device type: inverse-time breaker 250%, dual-element fuse 175%, nontime-delay fuse 300%, instantaneous-trip breaker 800%. If the result is not a standard rating, the next higher standard rating is allowed (the exception to 430.52(C)(1)). If the device trips when the motor starts, the exceptions allow a higher value up to a stated cap.
Overload device (430.32). A percentage of the nameplate current: 125% for a motor with a service factor of 1.15 or more or a temperature rise of 40°C or less, and 115% for other motors.
Disconnect (430.110(A)). At least 115% of the table current.
Step 4: size the feeder when there are several motors
Conductors (Article 430). 125% of the largest motor's table current, plus the table current of every other motor on the feeder.
Protection (430.62). The rating of the largest branch-circuit short-circuit device on the feeder, plus the table current of every other motor. The feeder device may not exceed that total, so you round down to a standard size.
Worked examples
Example 1: one motor. A 20 hp, 460 V, three-phase motor. Nameplate current 25.5 A, service factor 1.15. Table current 27 A.
- Conductors: 27 × 1.25 = 33.75 A.
- Inverse-time breaker: 27 × 2.5 = 67.5 A. That is not a standard size, so the next higher is 70 A.
- Dual-element fuse: 27 × 1.75 = 47.25 A. The next higher standard size is 50 A.
- Overload: 25.5 × 1.25 = 31.9 A.
- Disconnect: 27 × 1.15 = 31.05 A or more.
Example 2: a feeder. The feeder supplies the 20 hp motor above (27 A, 70 A breaker), a 15 hp motor (21 A) and a 3 hp motor (4.8 A), all 460 V, three-phase.
- Conductors: 27 × 1.25 = 33.75, plus 21, plus 4.8 = 59.55 A. 6 AWG copper at 75°C is 65 A, which meets it.
- Protection: 70 + 21 + 4.8 = 95.8 A. The feeder device may not exceed this, so choose 90 A, the largest standard size at or below it.
Common traps
- Table or nameplate. Overload is the only step that uses the nameplate. If you used 25.5 A for the breaker, you used the wrong current.
- 125% on the largest motor only. On a feeder, the other motors go in at 100%.
- Round up on the branch, down on the feeder. The branch-circuit device may go to the next higher standard size. The feeder device may not exceed the calculated value.
- Voltage column. A 208 V motor reads the 208 V column. Reading the 230 V column gives a wrong current.
- Exceptions only when asked. The higher starting percentages apply only when the question says the first value will not carry the start.
Check yourself
Try the ten questions in the free practice set for this lesson. The same four-step habit works on each one.
Where to look in the NEC
- Table 430.248, Table 430.250 and 430.6(A)
- 430.22 and Article 430 for conductors
- 430.52 and its table for short-circuit and ground-fault protection; 430.32 for overload
- 430.62 for feeder protection; 430.110(A) for the disconnect
- Table 310.16 and 240.6(A); Article 445 and 445.13 for generators