To prevent the motor burning, we have to prepare a way to protect it from overcurrent (overload, short circuit, or ground-fault). Before we go into further discussion, there should not be confused with a motor protection and circuit protection. Motor protection is a safety system tehubung motor with the motor power circuit. While the protection circuit is a safety system for power series rather than the motor itself.
Please refer to the NEC 430.1 pictures to help you see clearly the difference of the two. There are described the requirements for the motor overload protection and the requirements of Chapter III short-circuit protection and ground-fault in Chapters IV and V.Table or nameplate? To determine the CRC (or ampacity capability Flow Conductivity) minimum from power supply conductor motor, you should know clearly how much current will flow when the first traction motor. But there also exist various kinds of currents in motor applications (see "Current Motor Basics" on page 80). Full load current / full-load current (FLC) or ampere full load / full-load amperes (FLA) Which we use to calculate your motor?
- If you calculate the motor overload protection separately for the motor torque, locked rotor current use value on the nameplate [430.6 (B)]
- If there is a variable-frequency drive (inverter) as motor controllers, use the maximum operating current marked on the nameplate (motor or control). If the value is not found on the nameplate, use 150% of the value found in the table NEC [430.6 (C)].
- Motor with High Torque (generally made and to operate under the speed of 1.200 rpm) typically have a larger value of FLC compared with multispeed motors. For motor like this, use the current rating stamped on nameplate [430.6 (A) (1)].
- For a listed motor-operated appliance, use the FLC marked on the nameplate of the appliance (rather than the horsepower rating) to determine the ampacity (or rating) of the disconnecting means, branch-circuit conductors, controller, and branch-circuit short -circuit and ground-fault protection [430.6 (A) (1) Ex 3].
Overload protection. Overload protection shall be in accordance with Chapter III 430. Overload protection device size is based on the rating indicated on the motor nameplate (this serves to protect the motor windings due to the current damage incurred by the locked-rotor or the rotor jams / drag / jamming) [430.31].
You can use a single overcurrent device, sized per 430.32 requirements, to protect a motor from overload, short circuit, and ground faults
Branch-circuit conductor size. Branch-circuit conductors to a single motor must have an ampacity of not less than 125% of the FLC as listed in Tables 430.247 through 430.250 [430.6(A)(1), 430.22(A)].
When selecting motor current from one of these tables, note that the last sentence above each table allows you to use the ampacity columns for a range of system voltages without any adjustment. Select the conductor size from Table 310.16 according to the terminal temperature rating (60ºC or 75ºC) of the equipment [110.14(C)].
THHN/THWN is a common conductor insulation type that can be used in a dry location at the THHN 90ºC ampacity, or in a wet location at the 75ºC ampacity for the THWN insulation type. Regardless of the conductor insulation type, size the conductor per 110.14(C).
In 110.14(C)(1)(a), we read that equipment terminals are rated 60ºC for equipment rated 100A or less (unless marked 75ºC). Today, most equipment terminals are rated at 75ºC. Look for that specification, so you can use the 75ºC column if your conductors are also rated for 75ºC. If this is the case, you may save considerable money on your project. If you can’t find that specification, use the rules of 110.14(C).
Test your knowledge by answering this question: What size branch-circuit conductors are required for a 7½-hp, 3-phase, 230V motor (Fig. 1 on page 76)?
The motor FLC from Table 430.248 is 22A. The conductor is sized no less than 125% of motor FLC: 22A 3 1.25 = 27.50A. As per Table 310.16, a 10 AWG conductor is rated 30A at 75ºC.
The minimum size conductor permitted for building wiring is 14 AWG [310.5]; however, some local codes and many industrial facilities require branch-circuit conductors to be 12 AWG or larger.
Feeder conductor size. Perform feeder conductor size calculations the same way as for branch circuits, but use the different ampacity rules provided in 430.24. Conductors that supply several motors must have an ampacity of not less than:
(1) 125% of the highest rated motor FLC [430.17], plus
(2) The sum of the FLCs of the other motors (on the same line). Find the FLC in the NEC Tables [430.6(A)(1)].
The highest rated motor is the motor with the highest FLC [430.17]. Determine the “other motors in the group” value by balancing the motor FLCs on the feeder being sized, then select the line that has the highest rated motor on it (Fig. 2 on page 78).
Branch-circuit short-circuit and ground-fault protection. Each motor branch circuit must be protected against short circuit and ground faults by an overcurrent device sized no greater than the percentages listed in Table 430.52. The motor branch-circuit short-circuit and ground-fault protective device must be capable of carrying the motor’s starting current, and it must comply with 430.52(B) and 430.52(C).
A branch-circuit short-circuit and ground-fault protective device protects the motor, the motor control apparatus, and the conductors against short circuits or ground faults, but not against overload [430.51] (Fig. 3 on page 78).
It bothers many electrical practitioners to see a 14 AWG conductor protected by a 30A circuit breaker, but branch-circuit conductors are protected against overloads by the overload device (Fig. 4). That device is sized between 115% and 125% of the motor nameplate current rating [430.32]. See 240.4(G) for details.
Where the branch-circuit motor short-circuit and ground-fault protective device values derived from Table 430.52 don’t correspond with the standard overcurrent device ratings listed in 240.6(A), you can use the next higher overcurrent device rating. The “next size up protection” rule for branch circuits [430.52(C)(1) Ex 1] doesn’t apply to the motor feeder overcurrent device rating (Part II).
Keeping it straight. Articles 430 and 250 are the largest of the NEC Articles, and arguably the most misapplied. Something else these two Articles have in common but not with the other Articles is a “Figure 1” you can use as a guide.
In the case of Art. 430, this figure is a simple representation of the motor system with the correct Part of Art. 430 noted for each area of application. At the beginning of this article, we said that using Figure 430.1 will help you to not confuse motor protection with circuit protection when in actuality it can do much more. Spend some time working with it, and you’ll see how useful it really is.
If you base each motor project on Figure 430.1, you will reduce — if not eliminate — Art. 430 application errors.
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