If your motor operates at over 600V, use Part XI instead. As I understand it the drive has very good and no doubt properly sized ocp (fast acting to protect internal SS devices). ], Name: Bill PoindexterEmail: [email protected]: New MexicoTitle: Inspector
Not coincidentally, the requirements are in Art. Part II applies to motors operating at under 600V, such as typical 480V industrial motors. In such a case, split the run; see Annex D3(a) for an example of how to do this. To see why not to disregard the motor instead, review the calculations we just did. Does the system include a variable-frequency drive (VFD)?

In the 60°C column, the ambient temperatures end at 55°C. © 2020 Endeavor Business Media, LLC.

As with any circuit, analyze the loads before sizing motor OCPDs and conductors. 4.

The conductors must be able to handle and dissipate the short-term increase in heat from that starting current. Determine if the trip is due to a fault or from overload. For example, part-winding motors have additional requirements. Thanks again for posting the question. The disconnect at the cabinet is an HMCP. If you read on just a bit, you’ll see that Exception No. The first requirement is that you must use fuses or inverse time circuit breakers. All rights reserved. The power anomalies resulting from across the line starting can damage loads on other feeders, not just the motor system. In this example, the calculation is 136A so you can use a 150A breaker. Hint: Firewall Failure, 7 Things You Probably Didn’t Know About Election Day, NECA 2020: Mike Eby’s Product Picks — Part 3, Top 10 Highest-Paying Cities for Electricians, Rosendin Exec Shares Personal Survival Story, Tips for Storing Electric Motors the Right Way, NEMA Publishes Motors and Generators Revision, How to Select Motors for Hazardous Locations. You find this order of calculation in the examples of Appendix D. But with motors, your second step is to size the conductors [Table 430.1]. But to size the OCPD, you use the FLC. 5. Part II applies to motors operating at under 600V, such as typical 480V industrial motors. Hint: Firewall Failure, 7 Things You Probably Didn’t Know About Election Day, NECA 2020: Mike Eby’s Product Picks — Part 3, Top 10 Highest-Paying Cities for Electricians, Rosendin Exec Shares Personal Survival Story, Stumped by the Code? Apply 430.27. In this example, the calculation is 136A so you can use a 150A breaker. register | my profile | faq | archive search, Name: William HigginsEmail: [email protected]: KansasTitle: Building wiring engineer The drives ocp will protect the drive (supplemental protection). The answer is in 430.52. To make things simple, let’s assume you need to size the conductors for a single, continuous-duty 40HP motor. Do any of these operate in a mutually exclusive fashion?
Doing your calculations in the same sequence shown in all the Annex examples will provide you with several advantages.

How do you size the OCPD? The conductors from the drive to the motor have been sized for 125% of the motor FLA per the NEC tables.

How you proceed depends on whether these conductors are for: If you have load diversity, you can apply a feeder demand factor [430.26]. Does it have harmonics mitigation? So all you need is the correct OCPD for the drive itself as the others have said. [This message has been edited by [email protected] (edited November 08, 2001). Use the FLC from the motor nameplate. Are both the HMCP at the drive cabinet and the switchboard breaker considered the branch circuit OCPD? You now have a three-step process for sizing the conductors: Step 1. Multiply the FLC of the largest motor by 125%. Apply 430.22. In Trade Since: 1963Registered: Jan 2001Total Posts: 66. Hope I got the picture clear, and did not make a big ugly mistake. First, analyze the loads. Apply 430.29. 430 Part II. Noncontinuous? For this step, Table 430.1 directs you to Part IV. This isn’t a standard OCPD size [240.6(A)]. The overcurrent protection device (OCPD) must accommodate inrush current while still protecting the conductors. Step 3. 224: Not Secured in Place, Moving Violations Video No. Since 430.52(C)(1) says the OCPD can’t exceed this calculated value, do you use the next size down? The point is however, you do NOT size anything for the motor any longer. 226: Guessing Game, What’s Wrong Here? This doesn’t mean you size the OCPD for the starting current, however. a 30A circuit breaker can safely carry the exact 30A current but NEC suggests 80% … You may have voltage drop or power factor problems that lead to excess starting current. In Trade Since: 1980Registered: Nov 2001Total Posts: 200, [This message has been edited by [email protected] (edited November 08, 2001).]. For feeder OCPD requirements, turn to Part V. The key here is the “don’t exceed the rating” requirement of 430.52(C)(1) applies only to the largest load. If it turns out that the breaker trips every time you try to start the motor, then what?


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