NEC motor calculations: Article 430 step by step

Where to turn

• Table 430.250 gives full-load current for three-phase motors by horsepower and voltage. Table 430.248 is the single-phase table.
• 430.6(A)(1) says to use those tables, not the nameplate, for conductor size, protection and disconnect ratings.
• 430.22 sizes branch-circuit conductors for one motor.
• Table 430.52 gives the maximum rating of the short-circuit and ground-fault device as a percentage of table current, by device type.
• 430.32 sizes overload protection from the nameplate current.
• 430.110 covers the disconnecting means rating.
• Use 240.6(A) for standard device ratings.

The method

1. Take full-load current from the table for the horsepower and voltage.
2. Conductors: multiply by 125% and pick from Table 310.16 at the right terminal column.
3. Short-circuit and ground-fault device: multiply the table current by the Table 430.52 percentage for your device type. Take the largest standard size not over the result. If the question directs you to the 430.52(C) exception, the next higher standard size is permitted.
4. Overload: use the nameplate current, with the percentage that depends on the service factor or temperature rise.
5. Disconnect: at least 115% of the table current, and horsepower rated where the rule requires it.

Worked example

A 10-horsepower, 208-volt, three-phase Design B squirrel-cage motor. The nameplate reads 29.0 amperes, service factor 1.15.

• Table current: 30.8 A.
• Conductors: 30.8 × 1.25 = 38.5 A. At the 60°C column, 8 AWG copper carries 40 A, which is enough. Answer: 8 AWG copper.
• Inverse time breaker: 250% × 30.8 = 77 A. By the exam's default, exceptions are off, so take the largest standard size not over 77 A: 70 A.
• Dual-element fuses: 175% × 30.8 = 53.9 A, so the largest standard size not over it is 50 A.
• If the question directs you to the 430.52(C) exception, the next standard size up (80 A, 60 A) is permitted.
• Overload: the nameplate is used, so 29.0 × 1.25 = 36.25 A.
• Disconnect: 30.8 × 1.15 = 35.4 A minimum.

Common traps

• Using the nameplate for everything. Only the overload uses it. Everything else uses the table.
• Using the wrong table. Single-phase and three-phase motors have separate tables, and voltage columns matter.
• Rounding up when you should not. Exceptions are off unless the question directs you to one, so do not round up to the next size by habit. Where the question does direct you to 430.52(C), the permission is for the short-circuit and ground-fault device only. The overload has its own rule for when the setting may be raised.
• Applying 125% twice. Conductors use 125% of table current. The overload uses 125% of the nameplate when the service factor or temperature rise qualifies. They are different calculations.
• Wrong device column. Dual-element fuses, nontime delay fuses, inverse time breakers and instantaneous breakers have different percentages.
• Forgetting special cases. Adjustable-speed drives, multiple motors on a feeder and hermetic refrigerant motors have their own parts of Article 430.
• Edition drift. Confirm the paragraph letters and any percentage you rely on against the 2026 NEC.

Working it with the book open

Tab Table 430.250, Table 430.248, Table 430.52, 430.22, 430.32 and 430.110, plus Table 310.16. Start every motor problem by writing the table current and the nameplate current at the top of your scratch paper, labeled, so you reach for the right one at each step. Then work the steps in the order above and box each result. A question usually asks for only one of them, so read the last sentence first.

How to practice

Pick ten motors and work all five outputs for each: conductors, short-circuit and ground-fault device, overload and disconnect. Put the answers in one row so you see how each current value is used. Then test yourself by naming which current each step needs before you open the book.

Related guides: conductor ampacity and voltage drop.

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• Electrician exam information, Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.