There are two distinct levels of protection philosophy provided in this "Optimal Branch Circuit Protection". LPS-RK_SP, LPN-RK_SP, FRS-R and FRN-R fuses are sized for motor running “back-up” protection and provide superb short circuit protection at the same time. LPJ_SP, TCF, and LP-CC fuses are sized a little larger but are even more current limiting, providing an even greater degree of short circuit protection for the motor circuit.

All the fuses selected from "Optimal Branch Circuit Protection" provide short circuit and ground-fault protection for motor branch circuits (430.52), but typically are not the maximum allowed. Fuses sized in accordance with "Optimal Branch Circuit Protection" must be used in conjunction with properly sized motor overload protection such as overload relays or solid state motor controllers (430.32). This fuse sizing is normally large enough to allow the overload protective device to operate on overloads without opening the fuse. Yet for many cases, this fuse amp rating selection is smaller than the maximums allowed per "Branch Circuit Protection, Max. General" or "Branch Circuit Protection, Max. General" (430.52). In some cases, this smaller amp rating selection may provide the benefits of a smaller size disconnect and better short circuit protection. If a motor has a long starting time, high starting current profile or is cycled frequently, it may be necessary to use "Branch Circuit Protection, Max. General" or "Branch Circuit Protection, Max. General".

The LPS-RK_SP, LPN-RK_SP, FRS-R and FRN-R fuses sized per "Optimal Branch Circuit Protection" provide short circuit and ground-fault protection for motor branch circuits (430.52) as discussed in the previous paragraph. In addition, these dual-element fuses exhibit longer time-delay characteristics and can therefore be sized to provide back-up motor overload protection. The fuse sizing in "Optimal Branch Circuit Protection" for LPS-RK_SP, LPN-RK_SP, FRS-R and FRN-R fuses provides a degree of motor and circuit overload protection to back-up the normal motor overload protective device. Note: This level of protection requires a well-designed, true dual-element fuse. The Fusetron Fuses, FRS-R and FRN-R, and Low-Peak Fuses, LPS-RK_SP and LPN-RK_SP, are the industry leading dual-element fuses with excellent over-load time-delay characteristics and current-limiting short circuit ability. The Low-Peak Dual-Element Fuses have better current-limiting ability than Fusetron Dual-Element Fuses.

The amp ratings in "Optimal Branch Circuit Protection" are determined by using "Full load amps" motor ampacity values and the following:
LPJ_SP & TCF: 150% or the next larger Bussmann amp rating if 150% does not correspond to a Bussmann fuse amp rating.
LP-CC 1⁄2 to 15A: 200% (150% for DC) or the next larger Bussmann size if 200% (150% for DC) does not correspond to a Bussmann fuse amp rating.
LP-CC 20 to 30A: 300% (150% for DC) or the next larger Bussmann size if 300% (150% for DC) does not correspond to a Bussmann fuse amp rating.
LPS-RK_SP and LPN-RK_SP: 130% or the next larger Bussmann amp rating if 130% does not correspond to a Bussmann fuse amp rating.
FRS-R and FRN-R: 125% or the next larger Bussmann amp rating if 125% does not correspond to a Bussmann fuse amp rating.

Fuses selected from "Branch Circuit Protection, Max. General Applications" are intended to provide short circuit and ground-fault protection for motor branch circuits. Fuses sized in accordance with "Branch Circuit Protection, Max. General Applications" must be used in conjunction with properly sized motor overload protection such as overload relays or solid state motor controllers (430.32). "Branch Circuit Protection, Max. General Applications" fuse sizing provides the maximum NEC® 430.52 amp ratings for general purpose applications. It takes into account 430.52(C)(1) Exception No. 1, which allows the next standard amp rating fuse (per standard fuse amp ratings in 240.6) to be used if the maximum percentage in Table 430.52 does not correspond to a standard fuse amp rating. If this "Branch Circuit Protection, Max. General Applications" fuse sizing does not allow the motor to start, then "Branch Circuit Protection, Max. Heavy Start" may provide a larger amp rating.

The amp ratings in "Branch Circuit Protection, Max. General Applications" are deter-mined by using "Full load amps" motor ampacity values and the following:
LPJ_SP, TCF, LPS-RK_SP, LPN-RK_SP, FRS-R, FRN-R and KRP-C_SP: 175% (150% for DC motors) or the next larger 240.6 standard fuse amp rating if 175% (150% for DC motors) does not correspond to a standard fuse amp rating.
LP-CC: 300% (150% for DC motors) or the next larger 240.6 standard fuse amp rating if 300% (150% for DC motors) does not correspond to a standard fuse amp rating.
Sizes shown for the LP-CC can also be used for non-time delay fuses such as JKS, KTN-R, KTS-R, JJN, JJS, and KTK-R.

When the amp rating shown in "Branch Circuit Protection, Max. General Applications" is not sufficient to start a motor, a larger amp rating is often available by utilizing 430.52(C)(1) Exception No. 2. The amp ratings in "Branch Circuit Protection, Max. Heavy Start" are the larger of the amp rating allowed by 430.52(C)(1) Exception No. 1, or 430.52(C)(1) Exception No. 2. These amp ratings will often be required when acceleration times are greater than 5 seconds, when plugging or jogging applications exist, or where there are high inrush currents (such as energy efficient Design B motors). (In a few cases, the amp rating in "Branch Circuit Protection, Max. Heavy Start" may be smaller than the maximum permitted due to the limitation of the fuse type, such as LP-CC, Class CC fuses that are only available in ratings up to 30 amps. In these cases, if the amp rating shown is not sufficient to start the motor, select a different family of fuses that meet the requirements.) The amp ratings in "Branch Circuit Protection, Max. Heavy Start" are determined by using Full load amps motor ampacity values and the following:

LPJ_SP, TCF, LPS-RK_SP, LPN-RK_SP, FRS-R, and FRN-R: 225% or the next smaller Bussmann amp rating if 225% does not correspond to a Bussmann fuse amp rating.
LP-CC: 400% or the next smaller Bussmann amp rating if 400% does not correspond to a Bussmann fuse amp rating.
KRP-C_SP: 300% or the next smaller Bussmann amp rating, if 300% does not correspond to a Bussmann amp rating. Sizes shown for the LP-CC can also be used for non-time delay fuses such as
FCF, JKS, KTN-R, KTS-R, JJN, JJS, AND KTK-R.