NEC Article 430 Motor Calculations Explained for the Master Electrician Exam

Article 430 is one of the most tested sections on the Texas Master Electrician exam — and one of the most confusing for people who haven't worked much with motor circuits.

This post breaks it down into the pieces that actually show up on the exam.

Why Article 430 Is Different

Most of the NEC deals with branch circuits, feeders, and services. Article 430 is its own world. Motor circuits have different rules for conductor sizing, overcurrent protection, and short-circuit protection — and the differences matter on the exam.

The key concept to understand upfront: motors have two separate protection systems. One protects the motor itself from overheating (overload protection). The other protects the conductors and equipment from short circuits and ground faults (branch-circuit short-circuit and ground-fault protection). These are not the same thing, and the NEC treats them separately.

Conductor Sizing for Motor Circuits

Under NEC 430.22, the conductors supplying a single motor must have an ampacity of at least 125% of the motor's full-load current (FLC).

The full-load current comes from the NEC tables — specifically Tables 430.247 through 430.250 — not from the motor nameplate. This is a common exam trap. The code says to use the table values for conductor sizing and overcurrent protection calculations, not the nameplate ampere rating.

Example: A 10 HP, 230V, single-phase motor has a table FLC of 50 amperes (Table 430.248). The minimum conductor ampacity is 50 × 1.25 = 62.5 amperes. You'd select the next standard conductor size that meets or exceeds that.

Overload Protection

NEC 430.32 covers overload protection. For motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less, the overload device must be set at no more than 125% of the motor's nameplate full-load current.

For all other motors, the maximum is 115%.

If the overload device trips and won't allow the motor to start, you can increase it — but there are limits. For motors with SF ≥ 1.15, you can go up to 140%. For others, up to 130%.

Branch-Circuit Short-Circuit and Ground-Fault Protection

This is where Article 430 gets its reputation for being complicated.

NEC 430.52 sets the maximum ratings for branch-circuit protective devices (fuses and breakers) based on the type of device and the motor type. Table 430.52 is the key table here.

For an inverse time circuit breaker protecting a single-phase motor, the maximum rating is 250% of the motor's FLC. For a dual-element (time-delay) fuse, it's 175%.

These are maximums. The actual device can be smaller — but it can't exceed these values unless the motor won't start, in which case 430.52(C)(1) allows you to go higher, up to specific limits.

The Exam Approach

When you see a motor calculation question on the exam, work through it in this order:

First, identify the motor type and voltage, then find the FLC from the correct table. Second, calculate the conductor ampacity at 125% of FLC. Third, determine the overload protection based on nameplate current. Fourth, calculate the maximum branch-circuit protective device rating using Table 430.52.

Tab Table 430.52 and Tables 430.247–430.250 in your code book before the exam. You will use them.

Motor calculations look intimidating until you've worked through a few. After that, they're some of the most straightforward points you can pick up on the exam.