Diagram A 2. protection, select fuse ratings for correct type of motor from Motor Protection Table Columns headed, “Back-up Running Protection” or “With Overload Relays.” Fuse ratings selected from these columns coordinate with most UL Class 10 and 20 overload relays which covers over 90% of motor Transformer primary protection — Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. Overcurrent protection rating and location are shown in Diagram A. 1. Overcurrent Protection Fusing and circuit breaker protection. The motor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. As a licensed professional engineer, he presented seminars on electrical overcurrent protection for more than 40 years. If the branch circuit feeding the transformer has overcurrent protection to meet the individual protection requirements in Example 1, then individual transformer protection is not required. The three major categories or types of overcurrent are overload, short-circuit, and ground-fault. E3705.5.4 Air-Conditioning and Heat Pump Equipment. (NEC 430-52b) Yes, your eqipment does have to be sized to carry the starting current. of Overcurrent Protection from the mid-1960s until his passing in 2015. The overcurrent protection for these transformers is fairly straightforward. The use of overcurrent protection (OCP) is a common practice in designing electrical circuits, and several common methods currently exist, such as; circuit breakers, fuses, and ground fault circuit interrupts. Primary protection only is required if the transformer is single-phase and the secondary has only two wires. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). Among his many accolades, he was awarded the Outstanding Educator Award by the IEEE as a … Figures 4 and 5 illustrate two examples using a transformer with a 90 A-rated primary and a 180 A-rated secondary. Diagram B 3. The maximum overcurrent-protection-device rating shall not exceed the conductor allowable ampacity determined by the application of the correction and adjustment factors in accordance with Sections E3705.2 and E3705.3. Note: The wire size in this table is based on 75°C terminations and insulation. 50G Ground Instantaneous Overcurrent 50N Neutral Instantaneous Overcurrent 50P Phase Instantaneous Overcurrent 50_2 Negative Sequence Instantaneous Overcurrent 50/27 Accidental Energization 50/74 Ct Trouble 50/87 Instantaneous Differential 50EF End Fault Protection 50IG Isolated Ground Instantaneous Overcurrent 50LR Acceleration Time See Maximum Allowable Ampacities for Conductors in Raceway, Cable or Earth (30°C) for other insulation ratings.. Motor Wire Sizing. 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