Code Requirements for Motor Protection
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.
Figure 1. Motor Branch Circuit
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 with small single phase motors. If the fuse is the sole means of protection, the fuse ampere rating must not exceed the values shown in Table 1.
|motor Service Factor or MArked Temperature Rise||Fuse Rating as %* Motor Full Load|
|Service factor of 1,15 or greater||125|
|Marked Temperature Rise not exceeding 40°C||125|
*These percentages are not to be exceeded
Table 1. Maximum Fuse Rating for Overload Protection
Most integral horsepower 3 phase motors are controlled by a motor starter which includes an overload relay. Since the overload relay provides overload protection for the motor branch circuit, the fuses may be sized for short circuit protection.
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.
Table 2. Maximum Fuse Rating for Short Circuit Protection
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 a maximum permissible fuse size. If so labeled, this maximum value is not 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 are:
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000
What fuse type and ampere rating is best for a given application? The answer depends upon the application and objective to be met.
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