Skip to main content


When selecting the proper fuse for short circuit protection in motor starting applications, it is important to not only ensure that the fuse will not nuisance open during motor start up times, but also that the fuse will coordinate as required with overload relays. When sizing fuses between 125% and 150% of the motor nameplate current, several advantages, including ease of coordination with an overload device, a smaller disconnect, and increased short circuit protection from a lower fuse rating, can be achieved. However, if sizing at this level prevents the motor from starting, it may then be necessary to increase the fuse ampere rating and it then becomes important to know the NEC sizing limitations.

As of June 1, 2016, the US Department of Energy has mandated that newly manufactured electric motors will need to meet NEMA Premium® efficiency standards. As motor efficiencies increase, motor locked rotor currents can also be expected to increase. In addition to this, with across-the-line starting applications, it is critical to understand not only the locked rotor current, but also the starting time that can be expected.

With previous efficiencies, typically motor locked rotor currents between 300% and 600% of motor nameplate currents were common. However, with the new efficiency standards, locked rotor currents for NEMA Design B, C, and D motors can reach between 600% and 700% of nameplate currents and are restricted to maximum levels per the NEMA design standards. With NEMA Design E motors, these levels can be expected to be as high as 1000% of the rated current. Design A motors have no standardized maximums for locked rotor currents, but can be very high depending on the motor kVA code value. Special attention should be paid to the motor nameplate values when sizing motor protection fuses.

For Premium Efficiency motors, sizing fuses between 125% and 150% of the rated current may not be sufficient to allow the motor to start due to the potential magnitude of locked rotor currents. In addition to this, if the expected start time of the motor is over 5 seconds, this may be too long for this size fuse to handle without opening. Section 430.52(C)(1), Exception 1 in the NEC allows for Time-Delay Class R and J fuses to be sized at 175% of the rated motor current up to the next standard fuse size.

If sizing at 175% still does not allow for the motor to start, section 430.52(C)(1), Exception 2 in the NEC permits an absolute maximum fuse size of 225% of the motor rated current. In these cases, depending on the value determined from these multiplication factors, fuse sizes between Exceptions 1 and 2 may be exactly the same. Where Exception 1 permits rounding up to the next standard size, fuses sized to Exception 2 may not exceed the mentioned 225% value in any way.

For Time-Delay Class CC fuses, similar exceptions in the NEC also apply. Section 430.52(C)(1), Exception 1 allows for a fuse size of 300% up to the next standard rating. Section 430.52(C)(1), Exception 2 permits a fuse size not exceeding 400% of the motor rated current, should 300% sizing still not allow the motor to start.


Popular posts from this blog

Banner Engineering DF-G3 Discrete Fiber Amplifier Features World-Class Long Range Sensing Capability

Banner Engineering introduces its DF-G3 discrete long-range fiber amplifier with dual digital displays for use with plastic and glass fiber optic assemblies. Featuring increased sensing power, the DF-G3 can sense more than 3 meters (10 ft.) with opposed mode fibers or more than 1 meter (3 ft.) with diffuse mode fibers. The extra power provides increased detection reliability for dark targets at long range and enhanced detection sensitivity when using specialty fiber assemblies for large area and small part detection applications. The DF-G3 is available with a single discrete output or two dual discrete outputs. The dual discrete outputs can be independently taught to trigger at different intensity values, which is ideal for correct part-in-place or error-proofing, bottle down, and edge guiding applications. “We developed the DF-G3 fiber amplifier to meet our customers’ need to precisely detect smaller targets at longer ranges” said Dennis Smith, Senior Marketing Manager, Banner E

IDEC Releases New Line of High Efficiency Power Supplies

IDEC Corporation announces the PS5R-V line of DIN-rail power supplies, offering their customers high-efficiency in a compact form factor at competitive prices. These power supplies suit a wide range of needs in the industrial marketplace, and carry all of the required certifications necessary for use in these demanding applications. This next generation of the industry standard PS5R power supply family has updated features and specifications to meet current and future needs. The PS5R-V line of power supplies includes 10W, 15W, 30W, 60W and 120W versions, with additional versions coming soon. These power supplies have a very compact form factor with overall dimensions reduced by up to 25% from previous generations. The reduced form factors combine with DIN-rail mounting to free up valuable control panel space and reduce installation costs. Operating temperature ranges up to -25 to +75 degrees C offer more versatility. These extended operating temperature ranges often allow these p


TURCK is now offering the option to completely shield M8 picofast ® cordsets by connecting a braided shield directly to the coupling nut. This provides a cost-effective and reliable connection with 100 percent cable and connector shielding, which protects against electromagnetic and radio frequency interference. The 3 and 4-pin cordsets are available with flexlife ® PVC or PUR jacketed cable. All connectors deliver IEC IP 67 protection. The M8 picofast line, like most TURCK cordsets, provides male or female, straight and right angle connectors, standard and custom lengths, and pigtails or extensions. Fully encapsulated mating receptacles are also offered, completing the system.