Texas Electrician ExamTexas journeyman & master

Why a motor circuit needs two kinds of protection

One device protects the motor from working too hard. A different device protects the circuit from a fault. They are sized from different currents, and a question asking for the overcurrent device usually wants the second 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. Reading the question
  6. One more device you may be asked about
  7. 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.

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.

Service factor and temperature rise are two different markings describing the same underlying property, which is how much margin the manufacturer built in. A motor with either marking is permitted the larger multiplier. A motor with neither is not.

Read the percentages in 430.32 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 the next 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.

Reading the question

Exam language is not always precise about which device it wants. A few reliable signals:

The giveaway is which current the question bothered to give you. Questions do not include numbers they do not intend you to use.

One more device you may be asked about

A motor control circuit tapped from the load side of the branch-circuit device has its own overcurrent rules, taken 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 two, but it is a third answer that sounds correct when the question says control circuit. If the question is about the wire running to a start-stop station, that is the section it lives in.

What this page cites

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Every rule here is cited to the section it comes from, and corrections are logged in public with the date. This is study material rather than electrical advice, and it does not replace your own code book or your inspector.