By Mike Holt, NEC Consultant

Calculations for motor circuits are different from those of other circuits, because of how motor circuits must handle overcurrent.

When motors start across the line, they typically draw five times the running current. This temporary “inrush” is an overload condition. To handle inrush while still providing adequate overcurrent protection, we divide overcurrent protection into two parts:

1. Short-circuit and ground-fault protection. Usually, this is via a fuse or circuit breaker sized large enough to let the motor start. But if it’s large enough to let the motor start, it’s too large to provide overload protection.

2. Overload protection. This protects the motor and wiring at a value close to the actual running current of the motor, but with sufficient time delay to allow the motor to start.

Which current?

When performing motor calculations, should you use the Motor Full Load Current (FLC) (Code) or the Full Load Amperes (FLA) (Actual)? The wrong answer can result in overcurrent protection that inhibits operation or simply fails to protect.

• The FLC is in NEC Tables 430.247, 430.248, and 430.250 [430.6(A)(1)].
• The FLA is the motor nameplate rating [430.6(A)(2)]. This is the current the motor draws while producing its rated horsepower at its rated voltage (based on its rated efficiency and power factor).

The actual current drawn by the motor depends on the load on the motor and on the actual operating voltage at the motor terminals. If the load increases, the current increases. If the motor operates at a voltage below its nameplate rating, the operating current increases.

Use the FLC to determine the:

• Conductor ampacity [430.22].
• Circuit short-circuit and ground-fault overcurrent device size [430.52 and 430.62].
• Ampere rating of disconnecting switches [430.110].

Use the FLA to determine the:

• Conductor ampacity as covered in 430.22(E) for other than continuous duty) [430.22(E)].
• Branch-circuit short-circuit and ground-fault overcurrent device size.

You must use the FLA for:

• Motors built to operate at less than 1,200 RPM.
• High torque motors with higher FLCs.
• Multispeed motors (full-load current varyies with speed).

And if you have a listed motor-operated appliance, you must use the motor FLC marked on the appliance nameplate (instead of the horsepower rating on the appliance nameplate) to determine the ampacity or rating of the disconnecting means, the branch-circuit conductors, the controller, and the branch-circuit short-circuit and ground-fault protection.

Highest rated

When selecting the feeder conductors or the feeder short-circuit ground-fault protection device, the highest rated motor is the one with the highest FLC (not the highest horsepower) [430.17].

▶ Highest Rated Motor Example

Question: Which of the following is the highest rated motor?

(a) 10 hp, 208V, three-phase      (b) 3 hp, 208V, single-phase

(c) 3 hp, 120V, single-phase       (d) any of these

Answer: (c) 3 hp, 120V, single-phase, rated 34A FLC

10 Hp, 208V, Three-Phase = 30.80A [Table 430.250]

3 Hp, 208V, Single-Phase = 18.70A [Table 430.248]

Sizing branch-circuit conductors

When working with motors that are Design B, C, or D, you can size the conductors to the 75 DegrC ampacity column of Table 310.15(B)(16) [110.14(C)(1)(a)(4)].

The ampacity of branch-circuit conductors to a single motor used for continuous duty must be at least 125 percent of the motor FLC listed in Tables 430.247 through 430.250 [430.6(A)(1) and 430.22].

When selecting motor current from one of these tables, note the last sentence above each table; you can use the ampacity columns for several common system voltages without any adjustment. But you must select the actual conductor size from Table 310.15(B)(16) per the terminal temperature rating (60DegrC or 75DegrC) of the equipment [110.14(C)(1)].

Motor applications are continuous duty unless the nature of the control (or apparatus that the motor drives) is designed such that the motor won’t operate continuously with load [Table 430.22(E) Note]. When a motor isn’t continuous duty because of this type of application, size the conductors using the percentages of Table 430.22(E).

And if a motor must stop during the course of performing its function (e.g. elevator motor), that’s a good sign it’s intermittent duty.

▶ Conductor Sized For Continuous Duty Application Example

Question: What size conductors are required for a 1 hp, single-phase, 115V motor, terminals rated 60DegrC?

(a) 14 AWG      (b) 12 AWG

(c) 10 AWG      (d) 8 AWG

Answer: (b) 12 AWG

Motor FLC [Table 430.248]:

1 hp, 115V, FLC = 16A

The conductor is sized no less than 125 percent of the motor FLC:

16A x 1.25 = 20A, rated 20A at 60 DegrC

Table 310.15(B)(16)

▶ Motor Branch-Circuit Conductors for Duty-Cycle Service Example

Question: What size branch-circuit conductors are required for a 7½ hp, three-phase, 230V motor with a nameplate FLA of 20A, rated for 5-minute service, used for intermittent duty, terminals rated 75DegrC?

(a) 14 AWG      (b) 12 AWG

(c) 10 AWG      (d) 8 AWG

Answer: (a) 14 AWG

Use the motor nameplate FLA for duty-cycle service.

Conductor size must be at least 85 percent of the motor FLA [Table 430.22(E)]:

20A x 0.85 = 17A, rated 20A at 75¼

Table 310.15(B)(16)

The minimum size conductor permitted for building wiring is 14 AWG [Table 310.106(A)]; however, some local codes and many industrial facilities require 12 AWG as the smallest branch-circuit conductor.

Instead of using the FLC Tables, use the FLA from the motor nameplate with Table 430.22(E) [430.6(A)(1)].

Overcurrent

When current exceeds the rated current of the equipment or conductors, you have overcurrent. Short circuits, ground faults, and overloads are conditions that produce it.

• Short circuits and ground faults involve relatively large current changes and happen quickly. Because they are very low impedance, they typically carry several times the desired current (e.g., 200A on a 20A circuit).
• Overloads involve relatively small current changes and (usually) happen slowly. Think of overloads as too many straws on the camel’s back. An example is 22A on a 20A circuit.

Because of the nature of these conditions:

• Short-circuit or ground-fault protection is designed for fast current rise, short duration, and fast response time.
• Overload protection is designed for slow current rise, long duration, and slow response time.

Branch-circuit short-circuit and ground-fault protection devices (e.g., circuit breakers) protect the motor, the motor control apparatus, and the conductors against short circuits or ground faults. But they don’t protect against an overload [430.51]; for that, you need overload protection devices (OPDs)

The QT on OPD

Some causes of overload are:

• Power problems (e.g., low voltage, low power factor, voltage imbalance).
• Motor damage (winding problems, bearing failure) or locked rotor.
• Motor too small for the application.
• Single-phasing (one conductor of a three-phase circuit fails, leaving only two phases to run motor).
• Problems in the connected load ( e.g., gearbox low on lubricant).

Issues like these are why motor circuits need overload protection, not just a breaker big enough to let them start.

OPDs protect the motor, the motor control equipment, and the motor branch-circuit conductors from excessive heating due to motor overload [430.31]. They provide this protection at a value close to the motor running current, but with sufficient delay to allow the motor to start. That is, the delay is enough that the OPDs stay closed during the normal window of high starting current.

OPDs come in many forms. If your OPDs are fuses, you must install a fuse for each ungrounded conductor [430.36 and 430.55].

Commonly, OPDs are “heaters” in a magnetic starter. These are metal strips that heat up when the current exceeds a certain value, but if the current doesn’t stay high long enough nothing happens. However, give it enough time and it will melt open. The time it takes to open is inversely related to the amount of excess current.

Some OPDs are programmable; you can enter the nameplate FLA via a computer or other man-to-machine interface.

Things aren’t what they seem

When you size conductors, short-circuit protection, and ground-fault protection per Article 430, the fuse or circuit breaker may seem much larger than it should be for the conductors selected.

But the overcurrent protection rules of Article 240 don’t apply to motors. If a motor installation complies with the requirements of Article 430, there’s a good chance it won’t “look right.” Don’t let this throw you. Just review your calculations to ensure you used the correct current (FLA vs. FLC) and the correct tables for the application.