The requirements for motor rating determination and for sizing the conductors that supply motor-related equipment are in subsection 430.6. This subsection starts off by essentially telling you to select your conductors the same way you would for any other circuit.

What it doesn’t tell you is that getting the ampacity right is the tricky part. Article 430 provides detailed instructions, later.

Once you know the correct ampacity for a given motor circuit conductor, then you can turn to the ampacity tables in 310.15(B). As with other conductors, you also have the option of calculating the ampacity per 310.15(C).

To determine the motor rating, you first need to decide what type of application this is. You have four choices:

1. General motor application.
2. Torque motor.
3. AC adjustable motor.
4. Valve actuator motor assembly.

If it’s not one of the last three, it automatically defaults to the first one. Let’s look at those requirements.

Base motor overload protection on the motor nameplate current rating [430.7(A)(2)]. For all motors except low-speed, high-torque, or multi-speed, don’t use the actual current rating on the motor to determine the ampacity of conductors. This same restriction applies to determining the ampere ratings of switches and overcurrent protection devices. Instead, use the NEC’s Table 430.247, 430.248, 430.249, or 230.50 as applicable.

For:

• Torque motors, use the locked-rotor current.
• AC adjustable motors, use the maximum operating current on the motor nameplate.
• Valve actuator, use the name-plate full load current.

Source: Mark Lamendola | Mindconnection

After presenting definitions, Article 430 provides requirements for part-winding motors [430.4]. Why would you install a part-winding motor versus another type?

The simplest way to wire a motor is across the line. This applies full voltage to the motor upon starting, but motor inrush current can be significant.

One way to solve the inrush problem is by installing a device that applies the line voltage over a short time curve rather than instantly. A soft-start or electronic drive will do this.

Another way is to energize one part of the motor windings first. This provides a low starting torque that’s typically insufficient to turn the motor and it will quickly overheat if the other part isn’t energized within 2 or 3 seconds.

Part-winding starting reduces the amount of inrush current (typically by 25 to 40%) and corresponding voltage dip. However, there is a cost – the motor presents a higher initial impedance.

The language in 430.4 may be confusing because the second and third paragraphs refer to each set of windings as “one-half.” This assumes the standard arrangement where each part equals one-half of the windings. Verify that your part-winding motor follows this standard. Sometimes it’s a one-third/two-thirds arrangement and you will need to adjust accordingly.

A part-winding motor must have branch-circuit, short-circuit, and ground-fault protection for each motor-winding connection. Additionally, you might need separate overload devices for each one.

Don’t confuse part-winding with multiple voltage. The latter is for permitting connection to various power supplies.

Source: Mark Lamendola | Mindconnection

Because it’s so large, Article 430 can seem overwhelming. But don’t let its size distract you. If you open your NEC to Article 430, you’ll notice Figure 430.1. This lays out the contents of Article 430 in the sequence of what you need to do to install a motor system.

Rather than wade through all of Article 430 to find requirements that might apply to whatever step you’re on, you can use Figure 1 to hone in on the requirements that do apply.

For certain applications, other articles may also apply [Table 430.5]. For example, hermetic motors are used in air conditioning and refrigeration systems. If you’re installing such a motor, you must apply Article 430 and Article 440.

The requirements of Chapters 1, 2, and 3 also apply to motor systems except where contradicted (amended) by Article 430. For instance, you still have to protect conductors from over current [240.3], but you wouldn’t follow the rule for the next size up to do so [240.6(A)].

With motor circuits, overload protection differs from how you’d normally do it. The two normal jobs of the overload protection device (OCPD) are split into two separate functions handled separately.

Normally, the branch circuit OCPD protects against overload and faults. But motor branch circuit OCPDs don’t handle overcurrent protection. They protect against short-circuit and ground-fault protection [Article 430, Part IV].

To protect motors, motor control apparatus, and motor branch circuit conductors against over current, delve into Part 3 for what applies to your application.

Source: Mark Lamendola | Mindconnection

Before we discuss how motor requirements differ from the requirements for all other circuits, let’s look at why they differ. The shaft of an electric motor rotates because of the magnetic fields induced in the motor windings when electric power is applied. When the motor’s running, those windings present impedance (opposition) to current flow because of counter EMF developed during rotation.

When the motor starts, it isn’t yet rotating so there’s no counter EMF. The only impedance to the source current is the impedance of the winding wire. If you look up the ohms per foot of copper or aluminum conductors of the size used for motor windings, you’ll see it’s just about zero.

This means that when a motor starts up, it momentarily presents a dead short to the supply. This moment doesn’t last long because the motor starts rotating almost instantly. However, the motor isn’t instantly up to its running speed. This period between start and running speed is a high current draw period for the motor. The current is typically five or six times what the motor draws while running. We call this the inrush current. And we must allow for it so that our circuit protection devices don’t open during the period between startup and running.

This is why the requirements for motor circuits differ from those of all other circuits. It’s also why we have Article 430. In all other circuits, a single device provides overload protection and fault protection. Not normally so with motors.

Source: Mark Lamendola | Mindconnection

Article 430 provides the NEC requirements for installing motors, motor circuits, and related protection devices. You’ll find additional requirements for a special type of motor, the hermetic motor (used in chillers and air conditioners), in Article 440. Other Articles may also apply; see Table 430.5.

A notable characteristic of Article 430 is its sheer size. It’s the biggest article in the NEC. But look a little closer and you’ll find something that will help you quickly make sense of it all – Table 430.1.

This table isn’t a design guide; the NEC focuses on hazard prevention [90.1]. But it will help you more efficiently apply the NEC requirements for your motor installation. It outlines the major steps in the logical sequence of completion, and shows you which of the 12 Parts of Article 430 applies to each one.

Another very helpful feature of Article 430 consists of the three Full Load Current (FLC) tables at its very end. These eliminate the need to perform multiple calculations when determining conductor ampacity. We’ll look at FLC more closely in a future issue.

When applying Article 430, keep in mind that it doesn’t work in isolation. As with any other NEC Article, the first four Chapters of the NEC apply except as amended by this Article. In our next issue, we’ll see why we need the amendments made by Article 430 (and why it’s so big).

Source: Mark Lamendola | Mindconnection