MOTOR BRANCH CIRCUITS

1
MOTOR BRANCH CIRCUITS

• Sizing Short Circuit and Ground Fault Protection

2
Focus of Attention
3
Learning Objectives

• Size Fuses and Breakers for Motors when Starting
  Current is not a problem
• Size Fuses and Breakers for Motors when Starting
  Current is a problem
• Sizing Ground-fault and Short-Circuit Protection
  for Torque Motors
• Review Sizing Conductors and Conduit for Motor
  Installations

4
Where to Find Current Values

• 430.6 Ampacity and Motor Rating Determination.
• (A) General Motor Applications.
• (1) Table Values. Other than for motors built
  for low speeds (less than 1200 RPM) or high
  torques, and for multispeed motors, the values
  given in Table 430.247, Table 430.248, Table
  430.249, and Table 430.250 shall be used to
  determine the ampacity of conductors or ampere
  ratings of switches, branch-circuit short-circuit
  and ground-fault protection, instead of the
  actual current rating marked on the motor
  nameplate. Where a motor is marked in amperes,
  but not horsepower, the horsepower rating shall
  be assumed to be that corresponding to the value
  given in Table 430.247, Table 430.248, Table
  430.249, and Table 430.250, interpolated if
  necessary. Motors built for low speeds (less than
  1200 RPM) or high torques may have higher
  full-load currents, and multispeed motors will
  have full-load current varying with speed, in
  which case the nameplate current ratings shall be
  used.

5
(No Transcript)
6
(No Transcript)
7
Size Fuses and Breakers for Motors when Starting
Current is not a problem

• Step 1 Determine What Type of Motor.
• Step 2 Find FLC from Appropriate Chart
• Step 3 Refer to 430.52 for Correct
  Percentage

8
(No Transcript)
9
Size Fuses and Breakers for Motors when Starting
Current is not a problem

• Step 4 Multiply the FLC by the Proper
  Percentage
• Step 5 Unless you are Installing an
  Adjustable Trip Breaker, Round Up to the
  Next Standard Size OCPD

10
(No Transcript)
11
Why Fuses are so Large.
700 to 1000 of FLC
12

• What size OCPD would be required for a 20 HP
  460V 3? motor using time delay, and non-time
  delay fuses, and an inverse time circuit breaker.

13
(No Transcript)
14
Solution 1 Time Delay Fuses

• Table 430.250 20HP 460V 27A FLC

15
(No Transcript)
16
Solution 1 Time Delay Fuses

• Table 430.250 20HP 460V 27A FLC

27A FLC X 175
47.25A
17
430.52(C)(1)

• Exception No. 1 Where the values for
  branch-circuit short-circuit and ground-fault
  protective devices determined by Table 430.52 do
  not correspond to the standard sizes or ratings
  of fuses, nonadjustable circuit breakers, thermal
  protective devices, or possible settings of
  adjustable circuit breakers, a higher size,
  rating, or possible setting that does not exceed
  the next higher standard ampere rating shall be
  permitted.

18
(No Transcript)
19
Solution 1 Time Delay Fuses

• Table 430.250 20HP 460V 27A FLC

27A FLC X 175
47.25A
50 Amp Time Delay Fuse
20
Solution 2 Non-Time Delay Fuses

• Table 430.250 20HP 460V 27A FLC

21
(No Transcript)
22
Solution 2 Non-Time Delay Fuses

• Table 430.250 20HP 460V 27A FLC

27A FLC X 300
81A
23
(No Transcript)
24
Solution 2 Non-Time Delay Fuses

• Table 430.250 20HP 460V 27A FLC

27A FLC X 300
81A
90 Amp Non-Time Delay Fuse
25
Solution 3 Inverse Time Breaker

• Table 430.250 20HP 460V 27A FLC

26
(No Transcript)
27
Solution 3 Inverse Time Breaker

• Table 430.250 20HP 460V 27A FLC

27A FLC X 250
67.5A
28
(No Transcript)
29
Solution 3 Inverse Time Breaker

• Table 430.250 20HP 460V 27A FLC

27A FLC X 250
67.5A
70 Amp Inverse Time Breaker
30
Size Fuses and Breakers for Motors when Starting
Current is a problem

• Step 1 Determine What Type of Motor.
• Step 2 Find FLC from Appropriate Chart
• Step 3 Refer to 430.52(C)(1) Exp.2 for
  Correct Percentage

31
430.52(C)(1)

• Exception No. 2 Where the rating specified in
  Table 430.52, as modified by Exception No. 1, is
  not sufficient for the starting current of the
  motor
• (a) The rating of a nontime-delay fuse not
  exceeding 600 amperes or a time-delay Class CC
  fuse shall be permitted to be increased but shall
  in no case exceed 400 percent of the full-load
  current.
• (b) The rating of a time-delay (dual-element)
  fuse shall be permitted to be increased but shall
  in no case exceed 225 percent of the full-load
  current.
• (c) The rating of an inverse time circuit
  breaker shall be permitted to be increased but
  shall in no case exceed 400 percent for full-load
  currents of 100 amperes or less or 300 percent
  for full-load currents greater than 100 amperes.
• (d) The rating of a fuse of 6016000 ampere
  classification shall be permitted to be increased
  but shall in no case exceed 300 percent of the
  full-load current.

32
Size Fuses and Breakers for Motors when Starting
Current is a problem

• Step 4 Multiply the FLC by the Proper
  Percentage
• Step 5 Unless you are Installing an
  Adjustable Trip Breaker, Round Down to the
  Next Standard Size OCPD

33

• What size OCPD would be required for a 50 HP
  460V 3? motor using non-time delay fuses, with
  starting current being a problem.

34
(No Transcript)
35

• Table 430.250 50HP 460V 65A FLC

36
430.52(C)(1)

• Exception No. 2
• (a) The rating of a nontime-delay fuse not
  exceeding 600 amperes or a time-delay Class CC
  fuse shall be permitted to be increased but shall
  in no case exceed 400 percent of the full-load
  current.

37

• Table 430.250 50HP 460V 65A FLC

65A FLC X 400
260A
38
(No Transcript)
39

• Table 430.250 50HP 460V 65A FLC

65A FLC X 400
260A
250 Amp Non-Time Delay Fuse
40

• What size OCPD would be required for a 50 HP
  460V 3? motor using time delay fuses, with
  starting current being a problem.

41
(No Transcript)
42

• Table 430.250 50HP 460V 65A FLC

43
430.52(C)(1)

• Exception No. 2
• (b) The rating of a time-delay (dual-element)
  fuse shall be permitted to be increased but shall
  in no case exceed 225 percent of the full-load
  current.

44

• Table 430.250 50HP 460V 65A FLC

65A FLC X 225
146.25A
45
(No Transcript)
46

• Table 430.250 50HP 460V 65A FLC

65A FLC X 225
146.25A
125 Amp Time Delay Fuse
47
Torque Motors

• 430.52 Rating or Setting for Individual Motor
  Circuit.
• (D) Torque Motors. Torque motor branch circuits
  shall be protected at the motor nameplate current
  rating in accordance with 240.4(B).

48
Torque Motors

• 240.4 Protection of Conductors.
• (B) Devices Rated 800 Amperes or Less. The next
  higher standard overcurrent device rating (above
  the ampacity of the conductors being protected)
  shall be permitted to be used, provided all of
  the following conditions are met
• (1) The conductors being protected are not part
  of a multioutlet branch circuit supplying
  receptacles for cord-and-plug-connected portable
  loads.
• (2) The ampacity of the conductors does not
  correspond with the standard ampere rating of a
  fuse or a circuit breaker without overload trip
  adjustments above its rating (but that shall be
  permitted to have other trip or rating
  adjustments).
• (3) The next higher standard rating selected
  does not exceed 800 amperes.

49
Torque Motors

• Step 1 Determine nameplate LRC
• Step 2 Multiply by 125 Per 430.22
• Step 3 Pull Ampacity of Proper Wire Size
• Step 4 Size OCPD to Standard Size Above.

50

• What size OCPD would be required for a 5 HP 240V
  3? motor with a locked rotor current of 91A,
  using time delay fuses, and THW copper.

51

• 91 Amps LRC
• 91 X 125

113.75A
Find Proper Wire Size
52
(No Transcript)
53

• 91 Amps LRC

Use 115 amps with 2 THW Copper Find Next Higher
Fuses Size
54
(No Transcript)
55

• 91 Amps LRC

Use 115 amps with 2 THW Copper 125A Time Delay
Fuse
56
Sizing Conductors and Conduit

• Step 1 Find FLC from tables
• Step 2 Calculate in accordance with 430.22 and
  24

57
Single Motor Branch Circuit

• 430.22 Single Motor.
• (A) General. Conductors that supply a single
  motor used in a continuous duty application shall
  have an ampacity of not less than 125 percent of
  the motors full-load current rating as
  determined by 430.6(A)(1).

58
Multi-Motor Branch Circuit

• 430.24 Several Motors or a Motor(s) and Other
  Load(s).
• Conductors supplying several motors, or a
  motor(s) and other load(s), shall have an
  ampacity not less than 125 percent of the
  full-load current rating of the highest rated
  motor plus the sum of the full-load current
  ratings of all the other motors in the group, as
  determined by 430.6(A), plus the ampacity
  required for the other loads.

59
Sizing Conductors and Conduit

• Step 3 Divide by Temperature Correction
  Factors if Necessary
• Step 4 - Divide by Number of Conductors in
  Conduit Correction Factors if Necessary

60
(No Transcript)
61
Calculate conductor sizes for multiple motors

• Two separate branch circuits feeding two 5 hp,
  208 volt, 3-phase squirrel-cage motors are to be
  fed in the same conduit, in an ambient
  temperature of 110 F. What is the minimum size
  THW copper conductor permitted for the
  branch-circuit conductors? And what is the
  minimum size EMT?

62
Calculate conductor sizes for multiple motors

• 5 hp, 208 volt, 3-phase

63
(No Transcript)
64
Calculate conductor sizes for multiple motors

• 5 hp, 208 volt, 3-phase

• 16.7 X 125 20.875
• Determine Correction Factors

65
(No Transcript)
66
(No Transcript)
67
Calculate conductor sizes for multiple motors

• 5 hp, 208 volt, 3-phase
• 16.7 X 125 20.875
• Determine Correction Factors

• 20.875 .82 25.457
• 25.457 80 31.82

68

• 240.4 Protection of Conductors.
• (D) Small Conductors. Unless specifically
  permitted in 240.4(E) or 240.4(G), the
  overcurrent protection shall not exceed 15
  amperes for 14 AWG, 20 amperes for 12 AWG, and 30
  amperes for 10 AWG copper or 15 amperes for 12
  AWG and 25 amperes for 10 AWG aluminum and
  copper-clad aluminum after any correction factors
  for ambient temperature and number of conductors
  have been applied.

69
(No Transcript)
70
(No Transcript)
71
Calculate conductor sizes for multiple motors

• 5 hp, 208 volt, 3-phase
• 16.7 X 125 20.875
• Determine Correction Factors

• 20.875 .82 25.457
• 25.457 80 31.82

• 10 THW Copper

72
(No Transcript)
73
Calculate conductor sizes for multiple motors

• 5 hp, 208 volt, 3-phase
• 16.7 X 125 20.875
• Determine Correction Factors

• 20.875 .82 25.457
• 25.457 80 31.82

• 10 THW Copper

• ¾ EMT