# Motor branch circuit calculations NJATC code calcs 2008

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 Author: johnbowens ID: 59843 Filename: Motor branch circuit calculations NJATC code calcs 2008 Updated: 2012-01-14 23:33:59 Tags: motors NEC code electrical Folders: Description: motor calculations and the NEC 2011 Show Answers:

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1. The motor overload relays are for what type of protection of the motor? NJATC, Code Calculations, NJATC, Code Calculations: 2008, PP 120
running protection is the type of protection provided by this in a motor.
2. What type of motors use the nameplate rating FLC (full load current)? NJATC, Code Calculations, PP 121

What RPM limit defines a low speed motor?
Torque & multi-speed motors used this rating.

less than 1200 RPM defines this motor.
3. According to section 430.6(B) of the NEC, as applied to torque motors, the FLC (full load current) is what? Taken from where? NJATC, Code Calculations, PP 121
locked rotor current is the FLC of this type of motor. Taken from the nameplate.
4. Because a multi-speed motor has 2 windings, any one winding is used at any one time, so the multi-speed motor branch circuit is rated at what percentage and why? NJATC, Code Calculations, PP 121
125 percent, is the branch circuit rating in this type of motor, because it is a continuous load.
5. A multi-speed motor has how many windings? NJATC, Code Calculations, PP 123

Because of this, any one winding is used at any one time, so the multi-speed motor branch circuit is rated at what percentage and why?
2 windings are found in this type of motor.

at 125% because it becomes a continuous load
6. Periodic duty motors use what to calculate FLC? NJATC, Code Calculations, PP 125
T430.22E is used with this type of motor.
7. Locked rotor current is used in conjunction with what type of motor? NJATC, Code Calculations: 2008, PP126
torque
8. As it applies to a torque motor, what happens to prevent damage once the motor has, for example, closed a valve? NJATC, Code Calculations: 2008, PP 126
The circuit is opened by means of a torque or limit switch before the locked-rotor current can damage the motor. NJATC, Code Calculations: 2008, PP 126
9. Wye start, delta-run motors are used for what? NJATC, Code Calculations: 2008, PP 127

This permits how many conductors between the controller and the motor of the wye-start winding and at what calculation % of FLC? NJATC, Code Calculations: 2008, PP 127

At what % rating for line conductors to the controller?
reduced-voltage starting, resulting in a lower starting current.

6 72% for the 2011 NEC

• 125%
10. How many sets of conductors are there to calculate for the branch circuit conductors of a wound-rotor motor? NJATC, Code Calculations: 2008, PP 130
There are two sets of conductors to calculate for the branch-circuit conductors of this type of motor: The primary branch circuit conductors to the stator and the secondary conductors connected to the rotor through a bank of resistors.
11. there are (are not) line electrical connections to the secondary or rotor of a squirrel cage induction motor? pp 130

this motor is also known as what?
are not

• Wound-rotor induction motor
12. When calculating branch circuit ratings of more than one motor per 430.24 of the NEC, on one branch circuit, what is the % of the FLC used? pp 133

Is the largest FLC always the largest HP rating?
125% of the largest FLC and 100% of all others.

No
13. The purpose of motor branch-circuit short circuit and ground-fault protection is to protect what? NJATC, Code Calculations, pp140 (NC: motor branch circuits)
the circuit conductors, motors, and motor-controller equipment against overcurrent due to short circuits and ground faults.
14. A more common term for inverse time circuit breaker is what? NJATC, Code Calculations: 2008, PP 141
Thermal magnetic circuit breaker is another term for this.
15. Is the use of nonstandard rated fuses or circuit breakers permitted for overcurrent and short circuit protection? NJATC, Code Calculations: 2008, PP 141
This is allowed to be used for overcurrent and short circuit protection with a motor.
16. Motor branch circuit short circuit and GFP uses what 3 types of fuses? NJATC, Code Calculations: 2008, PP 140
• non-time delay
• time delay
• Class CC
17. Motor GFP and short circuit protection use what 3 types of circuit breakers? NJATC, Code Calculations: 2008, PP 140

the calculations differ only when what type is used?
• fixed or inverse time
• instantaneous trip circuit breakers

instantaneous trip CB's
18. For motor branch circuit ratings on short-circuit and ground fault protection the nameplate is (is not) used for FLC (full load current) rating? NJATC, Code Calculations: 2008, PP 141
This is not used for short-circuit and ground fault protetion for motor FLC?
19. Starting current is a.k.a.? NJATC, Code Calculations: 2008, PP 141
locked rotor current (LRC) is also known as this.
20. Where the maximum branch circuit and ground fault protection ratings are shown in the manufactures overload relay table for use with a motor controller or otherwise marked on the equipment, What must be done? NJATC, Code Calculations: 2008, PP 143
The calculated standard size must not exceed the listed manufacturers rating when this occurs with a motor.
21. A general rule of thumb puts the locked-rotor current for other than a Design B energy-efficient motor at ________ times the motor full-load current. NJATC, Code Calculations: 2008, PP 141

And for Design B?
6 times locked rotor current.

8 or more for the second type.
22. When sizing a breaker or fuse to motor short-circuit or GFP protection versus overload protection, what is considered the best installation? NJATC, Code Calculations: 2008, PP 146
sizing the breaker or fuse to just protecting the equipment is considered the best installation. I.E. not rounding up.
23. For torque motors the conductor size is based on what? NJATC, Code Calculations: 2008, PP 158
That name plate full load current of the motor is used for branch circuit calculations with this type of motor, which is the lock rotor current of the motor.
24. Instantaneous trip circuit breakers are covered separately in 430.52(c)(3) of the NEC and can be set to trip at what range? NJATC, Code Calculations: 2008, PP 153

What does this mean for rounding?
any range within its rating may be used for this type of circuit breaker

no rounding is necessary, for this type of circuit breaker as well.
25. A service factor on a motor nameplate is 1.15, meaning what? NJATC, Code Calculations: 2008, PP 160
This means that the motor is designed to operate at 115% of FLC without damage.
26. Motor overload protection requires the use of what to determine FLC? NJATC, Code Calculations: 2008, PP 160

If this is not available then what must be done in what two seperate cases?
nameplate FLC is used for this calculation.

Overload protection should be reduced to 115% when there is no nameplate FLC, and also when a temperature rise greater than 40 celcius / 104 F is present.
27. Are the overload calculations the same for design B, C, and D motors? NJATC, Code Calculations: 2008, PP 160
The overload calculations are the same for these design motors.
28. Selecting the higher class overload relay may preclude the need for selecting a higher trip current for what reason? And what are the classes of overload relays? NJATC, Code Calculations: 2008, PP 162
There are three classes of overload relays: Class 10, class 20, and class 30. A class 20 overload will carry the same starting current As a class 10 for a longer period of time. A class 30 will carry the same starting current for a longer period of time than a class 20.

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