Texas Electrician ExamTexas journeyman & master

Checked against primary sources 2026-08-24

Three devices on a motor circuit, doing three different jobs

One device protects the motor from working too hard. A second protects the circuit from a fault. A third protects nothing and simply opens the circuit. A question asking for the overcurrent device almost never wants the first one.

On this page
  1. The two failures are not the same failure
  2. Overload protection reads the nameplate
  3. The relief clause, and what it is not
  4. Short-circuit protection reads the table
  5. The disconnect is a third device and it protects nothing
  6. Reading the question
  7. The control circuit has its own rules
  8. What this page cites

The two failures are not the same failure

A motor circuit can fail in two ways that have almost nothing in common.

It can be overloaded. The motor is running, drawing more than it should, heating slowly. Nothing is broken. Left alone for long enough the windings cook. This can take minutes.

Or it can fault. A short circuit or a ground fault puts a very large current into the conductors immediately. This takes milliseconds.

A device fast enough to catch the second one would trip every time the motor starts, because starting current is also very large. A device patient enough to ride through starting would never notice the first one. So the code uses two devices, and Part IV says in its own opening that the short-circuit and ground-fault device protects the motor, the control apparatus and the conductors against faults but not against overload.

Overload protection reads the nameplate

Overload protection is watching this motor, so it is sized from this motor nameplate full-load current (NEC 430.32).

The percentage depends on what the nameplate says about the motor construction, and there are three cases.

Those two markings describe different things. A service factor above 1.0 means the manufacturer rates the motor to deliver more than its nameplate horsepower under defined conditions. A low temperature rise means the windings run further below what their insulation can take. Either one leaves thermal headroom, so either one earns the larger multiplier, and a motor with neither gets the smaller one.

The familiar percentages sit in 430.32(A) and they are for continuous-duty motors rated more than 1 horsepower. Motors of 1 horsepower or less and motors on other than continuous duty are handled in the subsections that follow, so check the size and the duty before you reach for the percentage.

Read the values in your own code book. They are short and worth knowing cold, because this is the step most likely to appear without a table in front of you.

The relief clause, and what it is not

Sometimes the device sized at the base percentage will not let the motor start, or will not carry the load. The code permits a higher size, with a hard ceiling on the trip current (NEC 430.32(C)).

The ceilings follow the same three cases and each one sits above its matching base value.

This is a ceiling on a permitted increase, not an alternative starting point. You size at the base percentage first. You only reach for the ceiling when the first selection actually fails to start the motor.

Answering with the ceiling value on a question that never says the motor failed to start is a wrong answer that looks like knowledge.

Short-circuit protection reads the table

The branch-circuit short-circuit and ground-fault device has a different job: carry the starting current without tripping, and clear a fault when one happens.

It is sized from the table current, not the nameplate, and the maximum percentage depends on two things at once: what kind of motor it is, and what kind of device you are using (NEC 430.52).

A non-time-delay fuse, a dual-element time-delay fuse, an instantaneous trip breaker and an inverse time breaker each get their own column, because each one tolerates inrush differently. A time-delay fuse rides through starting current by design and therefore needs a lower multiplier than a fuse that does not.

Find the row for your motor type first, then the column for your device type. Reading the right number from the wrong column is the standard mistake here, and both numbers are plausible.

The disconnect is a third device and it protects nothing

A disconnecting means exists so somebody can open the circuit and work on the motor without being killed by it. It carries no overcurrent function of its own, which is why it appears in a different part of the article from the other two (NEC 430.109 for what type of device qualifies, NEC 430.110 for its rating).

Its ampere rating starts at 115 percent of the motor full-load current (NEC 430.110(A)), and that full-load current is the table value, not the nameplate. A disconnect is a switch, and switches are one of the three things 430.6(A)(1) names as coming off the table.

So three devices, two source currents, and one of the three is not protection at all. Candidates carrying the 125 percent motor habit reach for it on the disconnect and lose the point to a figure that is not in the rule.

The controller is a fourth thing again, with its own rules in Part VII. It is what starts and stops the motor. On many installations one enclosure holds the controller, the overload relay and the disconnect together, which is precisely why the exam can ask about them separately and expect people to blur them.

Reading the question

Exam language is not always precise about which device it wants, and the reliable read has two stages.

First, does the table apply to this motor at all

Before you decide which device, decide which current the motor is entitled to. Low speed under 1200 RPM, high torque, and multispeed motors are excluded from the table by 430.6(A)(1) itself. So are equipment with a marked shaded-pole or permanent-split-capacitor fan or blower motor, and a listed motor-operated appliance marked with both horsepower and full-load current, under the exceptions to that subsection.

In any of those cases the marked current runs the whole branch circuit, conductors and disconnect and controller and short-circuit device and overload device together, and the table never enters the problem. Any heuristic that skips this step will confidently eliminate the correct answer.

Then, which device

The giveaway is which current the question bothered to give you, but only once you have confirmed the motor is one the table covers. Questions do not include numbers they do not intend you to use.

The control circuit has its own rules

A motor control circuit tapped from the load side of the branch-circuit device is protected under its own section, from a table that varies by conductor size and by whether the conductor leaves the enclosure (NEC 430.72).

It comes up less often than the other devices, and it is a fourth plausible answer whenever a question says control circuit. If the question is about the wire running to a start-stop station, that is the section it lives in.

It also reaches sideways into the transformer rules. A transformer supplying a motor control circuit is excused from the general transformer overcurrent table and follows 430.72(C) instead, which is a connection worth noticing before you meet it in a question.

What this page cites

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