General Guide to Low Voltage Motor Protection
This guide is a general recommendation and does not include the many variables that can exist for specific situations such
as special local codes, unusual temperature or other operating conditions, NEC demand factors, conductor derating, etc.
Code Requirements
The NEC or CEC requires that motor branch
circuits be protected against overloads
and short circuits. Overload protection
may be provided by fuses, overload relays
or motor thermal protectors. Short circuit
protection may be provided by fuses or
circuit breakers.
Overload Protection
The NEC or CEC allows fuses to be used
as the sole means of overload protection
for motor branch circuits. This approach is
often practical for single phase motors. If
the fuses is the sole means of protection,
the fuse ampere rating must not exceed
the values shown in Table 1.
Short Circuit Protection
The motor branch circuit fuses may be
sized as large as shown in Table 2 when an
overload relay or motor thermal protector
is included in the branch circuit. Time delay
fuse ratings may be increased to 225%
and non-time delay fuse ratings to 400%
(300% if over 600 amperes) if the ratings
shown in Table 2 will not carry motor
starting current. Some manufacturers’
motor starters may not be adequately
protected by the maximum fuse sizing
shown in Table 2. If this is the case, the
starter manufacturer is required by UL 508
to label the starter with the maximum
permissible fuse size. If so labeled, the
maximum value is not to be exceeded.
Where the percentages shown in Table 2
do not correspond to standard fuse ratings,
the next larger fuse rating may be used.
Standard fuse ratings in amperes:
15
45
100
225
500
1600
6000
20
50
110
250
600
2000
25
60
125
300
700
2500
30
70
150
350
800
3000
35
80
175
400
1000
4000
40
90
200
450
1200
5000
Time Delay vs. Non-Time Delay
Time delay fuses are the most useful fuses
for motor branch circuit application. A
time-delay fuse can be sized closer to
motor full load current, providing a degree
of overload protection, better short circuit
protection, and possible use of a smaller
disconnect switch.
What Ampere Rating?
The selection of fuse ampere rating is
a matter of experience and personal
preference. Some prefer to size time delay
fuses at 125% of motor full load amps. This
sizing will provide a degree of protection
for motors with a service factor of 1.15.
Sizing fuses at 125% of motor nameplate
amps in some applications may result in
nuisance fuse openings. Time delay fuses
sized at 125% may open at motor locked
rotor current before some NEMA Class 20
overload relays operate. Nuisance fuse
openings may result if Class RK1 or Class
115V Single Phase Motors
Table 1 - Maximum Fuse Rating for Overload Protection
Motor Service Factor or
Marked Temperature Rise
Service Factor of 1.15 or greater
Fuse Rating as % *
of Motor Full Load
125
Marked Temperature Rise
not exceeding 40°C
125
All Others
115
*These percentages are not to be exceeded.
Table 2 - Maximum Fuse Rating for Short Circuit Protection
Fuse Rating as % *
of Motor Full Load
FUSE TYPE
Type of Motor
Non-Time
Time
Delay
Delay
All Single Phase AC Motors
300
175
AC Polyphase Motors other than Would Rotor
Squirrel Cage
Other than Design E
300
175
Design E
300
175
300
175
Wound Rotor
Synchronous
150
150
Direct-Current (Constant Voltage)
150
150
*The non-time delay ratings apply to all class CC fuses.
J fuses are sized at 125% of motor full load current.
These fuses are more limiting than the RK5 and have
less short time current carrying capability. Sizing time
delay fuses between 125% and 150% of motor full load
current provides advantages. The fuse will coordinate
with NEMA Class 20 overload relays. Nuisance fuse
opening will virtually be eliminated and effective short
circuit protection will be maintained.
230V Single Phase Motors
HP
FLA*
Wire
Circuit
Breaker Size
RK5 Fuse
Heavy Load
HP
FLA*
Wire
Circuit
Breaker Size
RK5 Fuse
Heavy Load
0.25
5.8
14
15
12
1
2.9
14
15
5-6/10
0.33
7.2
14
15
15
1.5
3.6
14
15
7
0.5
9.8
14
20
17-1/2
2
4.9
14
15
9
0.75
13.8
14
25
25
3
6.9
14
15
15
1
16
14
30
30
5
8
14
15
15
1.5
20
12
40
35
7.5
10
14
20
17-1/2
2
24
10
50
40
10
12
14
25
25
3
34
8
70
60
15
17
12
35
30
5
56
4
90
100
20
28
10
60
50
7.5
80
3
110
150
25
40
8
80
70
10
100
—
—
175
30
50
6
90
90
*FLA shown per HP rating is an average and does not reflect a particular motor brand.
*FLA shown per HP rating is an average and does not reflect a particular motor brand.
Data supplied herein is intended as a general reference tool, is based on information available at time of publication, and is subject to change. Elektrim Motors is not responsible for the accuracy
or completeness of information contained herein. Users should evaluate and independently verify correctness and suitability of data herein for each individual intended application. Failure to
do so could result in damage to the product, system in which it is installed and/or other property damage or personal injury.
11/14 1.0
208V Three Phase Motors
230V Three Phase Motors
HP
FLA*
Wire
Circuit
Breaker Size
RK5 Fuse
Heavy Load
HP
FLA*
Wire
Circuit
Breaker Size
RK5 Fuse
Heavy Load
1
4.8
14
15
9
1
4.2
14
15
8
1.5
6.9
14
15
12
1.5
6
14
15
12
2
7.8
14
15
15
2
6.8
14
15
12
3
11
14
20
20
3
9.6
14
20
17-1/2
5
17.5
12
35
30
5
15.2
14
30
30
7.5
25.3
10
50
45
7.5
22
10
45
40
10
32.2
8
60
60
10
28
10
60
50
15
48.3
6
90
90
15
42
6
80
80
20
62.1
4
100
110
20
54
4
90
100
25
78.2
3
110
150
25
68
4
100
125
30
92
2
125
175
30
80
3
110
150
40
120
1/0
175
200
40
104
1
150
200
50
150
3/0
200
300
50
130
2/0
200
250
60
177
4/0
250
300
60
154
3/0
225
300
75
221
300
300
400
75
192
250
250
350
100
285
500
400
500
100
248
350
350
450
125
359
2-4/0
600
600
125
312
2-3/0
450
600
150
414
2-300
600
—
150
360
2-4/0
600
600
200
552
2-500
800
—
200
480
2-350
800
—
*FLA shown per HP rating is an average and does not reflect a particular motor brand.
*FLA shown per HP rating is an average and does not reflect a particular motor brand.
460V Three Phase Motors
575V Three Phase Motors
HP
FLA*
Wire
Circuit
Breaker Size
RK5 Fuse
Heavy Load
HP
FLA*
Wire
Circuit
Breaker Size
RK5 Fuse
Heavy Load
1
2.1
14
15
4
1
1.7
14
15
3
1.5
3
14
15
5-6/10
1.5
2.4
14
15
4-1/2
2
3.4
14
15
6
2
2.7
14
15
5
3
4.8
14
15
9
3
3.9
14
15
7
5
7.6
14
15
15
5
6.1
14
15
12
7.5
11
14
20
20
7.5
9
14
15
17-1/2
10
14
14
25
25
10
11
14
20
20
15
21
10
40
40
15
17
12
35
30
20
27
10
60
50
20
22
10
45
40
25
34
8
70
60
25
27
10
60
50
30
40
8
80
70
30
32
8
60
60
40
52
6
90
100
40
41
6
80
75
50
65
4
100
125
50
52
6
90
100
60
77
3
110
150
60
62
4
100
110
75
96
1
125
175
75
77
3
110
150
100
124
2/0
200
225
100
99
1
150
175
125
156
3/0
225
300
125
125
2/0
200
225
150
150
4/0
250
350
150
144
3/0
200
300
200
240
350
350
450
200
192
250
350
*FLA shown per HP rating is an average and does not reflect a particular motor brand.
*FLA shown per HP rating is an average and does not reflect a particular motor brand.
Data supplied herein is intended as a general reference tool, is based on information available at time of publication, and is subject to change. Elektrim Motors is not responsible for the accuracy
or completeness of information contained herein. Users should evaluate and independently verify correctness and suitability of data herein for each individual intended application. Failure to
do so could result in damage to the product, system in which it is installed and/or other property damage or personal injury.
11/14 1.0