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MOTOR BRANCH CIRCUITS

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MOTOR BRANCH CIRCUITS Sizing Short Circuit and Ground Fault Protection Focus of Attention Learning Objectives Size Fuses and Breakers for Motors when Starting Current ... – PowerPoint PPT presentation

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Title: 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.

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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

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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

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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.

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Solution 1 Time Delay Fuses

Table 430.250 20HP 460V 27A FLC

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Solution 1 Time Delay Fuses

Table 430.250 20HP 460V 27A FLC

27A FLC X 175
47.25A
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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.

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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

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Solution 2 Non-Time Delay Fuses

Table 430.250 20HP 460V 27A FLC

27A FLC X 300
81A
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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

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Solution 3 Inverse Time Breaker

Table 430.250 20HP 460V 27A FLC

27A FLC X 250
67.5A
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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

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

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Table 430.250 50HP 460V 65A FLC

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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.

Table 430.250 50HP 460V 65A FLC

65A FLC X 400
260A
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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.

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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.

Table 430.250 50HP 460V 65A FLC

65A FLC X 225
146.25A
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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.

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.

91 Amps LRC
91 X 125

113.75A
Find Proper Wire Size
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91 Amps LRC

Use 115 amps with 2 THW Copper Find Next Higher
Fuses Size
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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

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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

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Calculate conductor sizes for multiple motors

5 hp, 208 volt, 3-phase

16.7 X 125 20.875
Determine Correction Factors

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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

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.