NEC Article 430 Motor Calculations on the Electrician Exam
Motor questions are some of the most reliable points on a journeyman or master exam, once you know which number goes where. Almost every wrong answer comes from one mistake: using the motor nameplate current where the code wants the table current, or the other way around.
Study aid only, not legal or design advice. Confirm every value in the edition your state has adopted and in your own code book. Table numbering in Article 430 has shifted between editions (for example, the old Table 430.52 became Table 430.52(C)(1) in the 2023 edition), so tab the table in your own book.
Rule one: table current or nameplate current?
430.6(A)(1) says to use the full-load current from the tables (Table 430.247 for DC, 430.248 for single-phase, 430.249 for two-phase, 430.250 for three-phase) when you size conductors, the branch-circuit short-circuit and ground-fault protection, and other ampacity-based items. 430.6(A)(2) says to use the nameplate current for the separate overload protection.
| What you are sizing | Use | Where |
|---|---|---|
| Branch-circuit conductors | Table FLC | 430.22 |
| Breaker or fuse (short-circuit and ground-fault) | Table FLC | 430.52 |
| Overload protection | Nameplate FLA | 430.32 |
| Feeder conductors (several motors) | Table FLC | 430.24 |
Branch-circuit conductors: 125%
Under 430.22, the conductors for a single continuous-duty motor must have an ampacity of at least 125% of the table full-load current. Then pick the conductor from Table 310.16, watching your termination temperature under 110.14(C).
Breaker or fuse: the Table 430.52 percentages
The branch-circuit short-circuit and ground-fault device is sized as a percentage of the table FLC. For the common motor types (single-phase, and polyphase other than wound-rotor), the familiar values are:
| Device | Max % of table FLC |
|---|---|
| Nontime-delay fuse | 300% |
| Dual-element (time-delay) fuse | 175% |
| Instantaneous-trip breaker | 800% (higher for some Design B energy-efficient motors) |
| Inverse-time breaker | 250% |
If the result is not a standard size from 240.6, the first exception to 430.52 lets you go up to the next standard size. A further exception allows a higher setting if the motor will not start (for example, an inverse-time breaker up to 400% for FLC of 100 A or less). Read both exceptions in your book; exam questions often ask for the "maximum" and expect the basic percentage plus the next-size-up step, not the starting exception.
This is why 240.4(D) does not limit motor circuits. 240.4(G) sends you to Article 430, where the overload device protects the conductors from overload and the breaker only has to handle faults.
Overloads: nameplate, 125% or 115%
430.32(A)(1) sizes a separate overload device from the nameplate current:
- Service factor of 1.15 or more: 125%.
- Marked temperature rise of 40°C or less: 125%.
- All other motors: 115%.
If that setting will not let the motor start, 430.32(C) allows up to 140% and 130% for the same groups.
Worked example: one 10 hp motor
A 10 hp, 230 V, three-phase induction motor. Nameplate 26 A, service factor 1.15. Copper THWN, 75°C terminations. Inverse-time breaker.
- Table 430.250 FLC: 28 A (not the 26 A nameplate).
- Conductors (430.22): 28 × 1.25 = 35 A. Table 310.16, 75°C copper: 10 AWG is rated 35 A, so 10 AWG. If the terminations were limited to 60°C, 10 AWG drops to 30 A and you would need 8 AWG.
- Breaker (430.52): 28 × 2.50 = 70 A, a standard size, so 70 A. With dual-element fuses instead: 28 × 1.75 = 49 A, next standard size 50 A.
- Overload (430.32): 26 × 1.25 = 32.5 A maximum, from the nameplate.
Notice that a 70 A breaker on 10 AWG copper is correct here. That is the answer people talk themselves out of because it "looks wrong."
Several motors on one feeder
430.24: feeder conductors must carry at least 125% of the largest motor's FLC, plus 100% of the FLC of the other motors, plus other loads (continuous non-motor loads at 125%, noncontinuous at 100%).
Example: 10 hp (28 A), 5 hp (15.2 A) and 3 hp (9.6 A) motors, all 230 V three-phase. 28 × 1.25 = 35 A, plus 15.2 + 9.6 = 59.8 A minimum feeder ampacity.
For the feeder breaker, 430.62(A) works differently: take the largest branch-circuit protective device rating in the group, add the FLC of the other motors, and do not exceed that total. There is no next-size-up step here, so if the result is not a standard size, go down.
Exam traps to watch for
- Using nameplate amps for conductors or breakers, or table amps for overloads.
- Applying 240.4(D) to a motor branch circuit.
- Rounding up on the feeder breaker under 430.62 (you round down).
- Reading the wrong voltage column. A 208 V motor uses the 208 V column (30.8 A for 10 hp), not 230 V.
- Forgetting the 125% on the largest motor only, not on every motor, in a feeder calculation.
How to tab it
Tab 430.6, Table 430.52 and Table 430.250. In the margin of 430.6, write "Table: wire + breaker. Nameplate: overloads." That one line answers half the motor questions on the test.
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