MOTOR CIRCUIT
CALCULATION
MOTOR CIRCUIT CALCULATION
Step 6
Feeder
Overcurrent
Device
Step 4
Branch Circuit
Overcurrent
Device
Step 5
Feeder Wire
Size
Step 3
Branch Circuit
Wire
Step 2
Overload
Protection
MOTOR
MOTOR
Step 1 F.L.C
STEP 1
“F.L.C (Full Load Current)”
• Table 4.30.14.1 for Direct Current.
• Table 4.30.14.2 for Two Phase AC motor.
• Table 4.30.14.3 for Two Phase AC motor (4-Wire)
• Table 4.30.14.4 for Three Phase AC motor.
STEP 2
“Overload Protection”
• Motor running overload protection.
Thermal Protector called a “heater”.
• PEC provision 4.30.3 not more than 600V
• PEC Provision 4.30.9 more than 600V
• If motor nameplate is given, use it, if not
use FLC from tables
Types of Overload Protection
• Thermal Protection
• Thermal Protection
Types of Overload Protection
• Separate Overload Device
(Overload Relay)
Types of Overload Protection
A fuse shall be inserted
in each ungrounded
conductor and also in
grounded condcutor
• Separate Overload Device
(Fuse)
STEP 3
“Branch Circuit Wire”
Refer to PEC 4.30.2.2 (Motor Circuit Conductor)
 Shall not be less than FLC x 125%= required
ampacity
Refer to Table 3.10.1.16
STEP 4
“Branch Circuit Overcurrent Device”
• Branch circuit overcurrent device shall be selected from
Table 4.30.4.2
• First select the type of motor (1Ø,3Ø,AC,DC,wound
rotor,etc.)
• Next Select the type of protective device(non-time delay
fuse,dual element fuse,inverse time breaker)
• Select the percentage from the proper column and
multiply it times the FLC of the motor.
• Use 2.40 to select the standard size overcurrent device.
• When the value does not match a standard, the code
permits the next higher STANDARD size for branch
circuit OC device.
Types of Overcurrent Devices
• Non-Time Delay Fuse
• Dual-Element time delay-fuse
• Instantaneous trip circuit Break
• Inverse time circuit breaker
STEP 5
“Feeder Wire Size”
• PEC provision 4.30.2.4
• Multiply the largest rated motor in FLC by
125% and add the FLC of all other motors
connected to the same feeder conductor
for the total amps.
STEP 6
“Feeder Overcurrent Device”
• Select the largest branch circuit overcurrent
device and add all the other motor FLC
connected on the same feeder conductor to find
the amperage required to select the feeder fuse
or CB.
• The Code does NOT permit going to a higher
size if the values does not match a standard size
from 2.40.6
• Drop DOWN to the next smaller standard size.
Code permits going up only on branch circuit.
Example
• A 230V,1 HP, Single Phase motor with SF
1.15.
a.) Determine overload protection?
b.) Determine the size of branch circuit using
THW wire?
c.) Determine the maximum rating of
branch-circuit and ground-fault device. Use
inverse-time circuit breaker?