Motor short-circuit and ground-fault protection

The breaker Dana chose for the compressor surprised Tyler. A 25-horsepower motor drawing 34 amperes, on conductors sized for 42.5 amperes, and Dana had written in a 90-ampere breaker. "Won't that let the wire burn?" he asked. It is the right question, and the answer is the heart of motor protection. A motor needs two different kinds of protection that the code keeps separate. The branch-circuit short-circuit and ground-fault protective device clears a fault quickly but must not trip on the motor's large starting current. The overload device, covered in Lesson 4, protects the motor and its conductors from sustained moderate overcurrent. Because the overload device protects the conductors against overload, the short-circuit device can be much larger than the conductor ampacity.

This lesson covers Part IV of Article 430: the purpose of the branch-circuit short-circuit and ground-fault protective device, Table 430.52 and its device types, how to round to standard sizes, the exception for motors that will not start, protection for several motors on one branch circuit, and feeder protection under 430.62.

Why starting current changes the rules

When an induction motor starts, it draws locked-rotor current, typically six to eight times its full-load current, for a second or more until it comes up to speed. An ordinary breaker or fuse sized close to the running current would trip every time the motor started. The code's solution is to divide the work. The overload relay in the controller, set close to the motor's running current, handles overloads with a time delay long enough to ride through a normal start. The branch-circuit device is sized well above running current and handles only short circuits and ground faults, which produce currents many times larger than starting current and must be cleared fast. Section 430.51 states the general purpose, and 430.52 sets the maximum ratings.

Table 430.52

Table 430.52 lists, for different motor types, the maximum rating or setting of each kind of short-circuit and ground-fault protective device as a percentage of the motor's full-load current from the table. The device types are a nontime-delay fuse, a dual-element or time-delay fuse, an instantaneous-trip breaker and an inverse-time breaker. For the common case of a single-phase or polyphase squirrel-cage motor other than a high-efficiency Design B motor, this course uses the following illustrative percentages, which are of the kind the table gives.

The percentages differ because the devices respond to starting current differently. A time-delay fuse rides through inrush on its own, so it can be sized closest to running current. An inverse-time breaker, the ordinary thermal-magnetic breaker, needs more headroom. An instantaneous-trip breaker, sometimes called a motor circuit protector, has no thermal element and must be set above the inrush peak; it is permitted only as part of a listed combination controller. The table also has rows for wound-rotor, DC and synchronous motors, and for high-efficiency Design B motors, which draw a higher inrush and are allowed higher instantaneous settings.

Rounding to a standard size

The percentage seldom lands on a standard rating. Section 430.52(C)(1) handles this with an exception: where the calculated maximum does not correspond to a standard size or setting of a fuse or breaker, the next higher standard size, rating or setting is permitted. The standard sizes are listed in 240.6. This is opposite to most general-circuit rules, where you round down, and it is one of the most tested details in Article 430.

This is why Tyler's worry was unfounded. The 90-ampere breaker will not protect the 42.5-ampere conductors against a modest overload, but it does not have to. The overload relay in the starter, set near the motor's 32-ampere nameplate current, opens the circuit long before the conductors overheat. The breaker's job is to clear a short circuit or ground fault, where the current would be hundreds or thousands of amperes and the breaker trips almost instantly. Section 240.4(G) points to Article 430 for this reason: motor circuit conductors are protected according to the motor rules, not the general 240.4 rules.

When the motor will not start

Some motors with heavy starting loads, such as compressors that start against pressure, will trip a device sized at the table percentage. The code anticipates this in 430.52(C)(1) Exception No. 2, which permits a larger device, up to stated limits, when the value from the table is not sufficient for the starting current of the motor. In this course's illustrative values, the limit for a time-delay fuse is 225% of full-load current, and the limit for an inverse-time breaker is 400% for motors of 100 amperes or less and 300% for larger motors. A nontime-delay fuse may go up to a higher stated limit as well.

Two points matter on exams. First, the second exception is used only when the motor actually fails to start; a question that simply asks for the maximum rating wants the table percentage with next-size-up rounding, not the exception limit. Second, the exception limits are maximums that cannot be rounded up: if 400% gives 136 amperes, you must use a device of 136 amperes or less, which means 125 amperes.

Several motors on one branch circuit

Section 430.53 allows several motors on one branch circuit under specific conditions. The simplest case is small motors: motors of not more than 1 horsepower each, with full-load current of not more than 6 amperes, may share a branch circuit protected at not more than 20 amperes at 120 volts or 15 amperes at 600 volts or less, provided several other conditions are met. Larger groups are allowed when the smallest motor's overload and controller are listed for group installation and the branch-circuit device is sized so it protects the smallest motor's components. Group installations are common in packaged machinery; on exams they appear mostly as a question about the small-motor limits.

Feeder protection

When a feeder supplies several motors, its short-circuit and ground-fault protection follows 430.62. In general terms, the feeder device may not be larger than the largest rating or setting of the branch-circuit device on any of the motors, plus the sum of the full-load currents of the other motors on the feeder. Unlike the branch circuit, the feeder rule has no next-size-up permission: the result is a maximum, so you round down to a standard size if it falls between ratings.

Ray checked the result against the feeder conductor from Lesson 2. The conductors needed an ampacity of at least 119.5 amperes, or 133 amperes with the lighting. A 150-ampere breaker on conductors with an ampacity of, say, 130 amperes would look wrong under ordinary rules, but 430.62 and 240.4(G) permit it for motor feeders, because the motor overloads protect against overload and the feeder breaker only needs to clear faults. In practice Ray often sizes motor feeder conductors up to the breaker anyway, which leaves room for future motors.

When the feeder also supplies non-motor loads, 430.63 allows the device to be sized from the motor calculation above plus the non-motor loads. The same rule of not rounding up applies to the total.

STANDARD = [15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125,
            150, 175, 200, 225, 250, 300, 350, 400]

def next_up(x):
    return min(s for s in STANDARD if s >= x)

def round_down(x):
    return max(s for s in STANDARD if s <= x)

PCT = {"nontime-delay fuse": 3.00, "time-delay fuse": 1.75, "instantaneous breaker": 8.00, "inverse-time breaker": 2.50}
flc = 34
for device, pct in PCT.items():
    print(f"{device:22} {pct * flc:6.1f} A  ->  {next_up(pct * flc)} A" if pct < 8 else f"{device:22} {pct * flc:6.1f} A (adjustable setting)")

print("Exception limit for an inverse-time breaker:", 4 * flc, "A ->", round_down(4 * flc), "A")
print("Feeder breaker:", 90 + 21 * 3 + 14, "A ->", round_down(90 + 21 * 3 + 14), "A")

nontime-delay fuse      102.0 A  ->  110 A
time-delay fuse          59.5 A  ->  60 A
instantaneous breaker   272.0 A (adjustable setting)
inverse-time breaker     85.0 A  ->  90 A
Exception limit for an inverse-time breaker: 136 A -> 125 A
Feeder breaker: 167 A -> 150 A

Exam strategy

Three questions sort out almost every motor protection problem. Is this a branch circuit or a feeder? For a branch circuit, use the Table 430.52 percentage for the stated device and round up to the next standard size; for a feeder, add the largest branch device to the other motors' currents and round down. Does the question say the motor will not start? Only then use the exception limits, and never round them up. Which current goes in? Always the table full-load current, never the nameplate. Keep the standard sizes from 240.6 tabbed, because nearly every answer depends on them.

Combination controllers and motor circuit protectors

Many modern motor circuits use a combination starter, a single listed assembly with a disconnect, short-circuit protection and a controller with overloads in one enclosure. When the short-circuit device is an instantaneous-trip breaker, often called a motor circuit protector, the code allows it only as part of a listed combination controller, because its setting must be coordinated with the overload relay and the contactor that the manufacturer tested with it. The instantaneous setting is adjustable, and Table 430.52 sets the maximum percentage. In practice the installer sets the dial just above the motor's inrush so it does not trip on start, following the manufacturer's selection table, and never above the table limit except under the specific exceptions for high-efficiency motors.

Self-protected combination controllers, sometimes called Type E or Type F starters, go a step further: one listed device provides short-circuit protection, overload protection, control and disconnecting means. On exams, the most common question about these devices asks whether an instantaneous-trip breaker may be used alone as branch-circuit protection. The answer is no; it must be part of a listed combination controller.