Tuesday, March 23, 2021
I am asked frequently what does "Design A" or "Design B" motor indicate about a motor? The NEC references these design letters in 110.14(C) and other places in the code. This can be somewhat confusing because these are NEMA letter designations and there are two types of NEMA letter designations used for motors. The first is an efficiency rating and this is the rating that is referred to in 110.14(C) of the NEC.
NEMA has established four different designs - A, B, C and D for electrical induction motors
Different motors with the same nominal horsepower may have different start currents, torque curves, speeds, and other variables. When selecting a particular motor for an intended task all engineering parameters must be taken into account.
The four NEMA (National Electrical Manufacturers Association) designs have unique speed-torque-slip relationships - making them suited for different types of applications.
NEMA Design A
maximum 5% slip, high to medium starting current, normal locked rotor torque, normal breakdown torque, suited for a broad variety of applications - like fans and pumps
NEMA Design B
maximum 5% slip, low starting current, high locked rotor torque, normal breakdown torque, suited for a broad variety of applications with normal starting torques - common in HVAC application with fans, blowers, and pumps
NEMA Design C
maximum 5% slip, low starting current, high locked rotor torque, normal breakdown torque, suited for equipment with high inertia and high starting torques at the start - like positive displacement pumps, conveyors
NEMA Design D
maximum 5-13% slip, low starting current, very high locked rotor torque, suited for equipment with very high inertia starts - like cranes, hoists, etc.
NEMA also has code letters, not to be confused with the Design Efficiency Letter Codes, for designating the amount of locked rotor or start up current a motor will pull as the voltage is applied to the motor. The locked rotor current is the current the motor will pull as it starts to spin until it reaches its full rotation speed. As the motor starts spinning it will also induce a voltage in the opposite polarity into the coils which is called counter electromotive force. The effect of this CEMF is that it counters the current in the motor and brings it down to the actual running current of the motor. Different motors designs will have different amounts of locked rotor current at start-up, and this is designated by the motors code letter as shown below.
NEMA - The National Electrical Manufacturer's Association - who sets the design standards for motors, has established a NEMA Code letter designation to classify motors by the ratio of locked rotor kVA per horsepower. This table is also found in the NEC as Table 403.7. To determine the kVA at locked rotor: Multiply the value in the table by the horsepower of the motor. To determine the current, divide the answer obtained by the voltage of the motor.
NEMA Code Letter kVA/HP with locked rotor Approximate Mid-Range Value
A 0 - 3.14 1.6
B 3.15 - 3.55 3.3
C 3.55 - 3.99 3.8
D 4.0 - 4.49 4.3
E 4.5 - 4.99 4.7
F 5.0 - 5.59 5.3
G 5.6 - 6.29 5.9
H 6.3 - 7.09 6.7
J 7.1 - 7.99 7.5
K 8.0 - 8.99 8.5
L 9.0 - 9.99 9.5
M 10.0 - 11.19 10.6
N 11.2 - 12.49 11.8
P 12.5 - 13.99 13.2
R 14.0 - 15.99 15.0
S 16.0 - 17.99
T 18.0 - 19.99
U 20.0 - 22.39
V 22.4 - and up
The starting kVA required by a motor starting at full voltage is determined from the nameplate on the motor or from the manufacturer.
In general, it is accepted that small motors require higher starting kVA than larger motors. Standard 3 phase motors often have these locked rotor codes:
less than 1 hp: Locked Rotor Code L, 9.0 - 9.99 kVA1 1/2 to 2 hp: Locked Rotor Code L or M, 9.0 - 11.19 kVA3 hp: Locked Rotor Code K, 8.0 - 8.99 kVA5 hp: Locked Rotor Code J, 7.1 - 7.99 kVA7.5 to 10 hp: Locked Rotor Code H, 6.3 - 7.09 kVA more than 15 hp: Locked Rotor Code G, 5.6 - 6.29 kVA
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