sizing ocpd for vfd

Does the system include a variable-frequency drive (VFD)? You now have a three-step process for sizing the conductors: Step 1.

How you proceed depends on whether these conductors are for: If you have load diversity, you can apply a feeder demand factor [430.26]. How would you know to go there? But if you have motor circuits, remember that the inrush current of motors changes things: If you understand the branch circuit conductor and OCPD sizing steps, it’s just a matter of modifying them a bit for feeders and/or motors. The size you come up with doesn't fit a standard breaker. Is it sized based upon 430-52(c)(1) or 430-62(a)? Single motor. Let’s assume the motor is on its own branch circuit. The meaning of this emerges in 430.52(C). Then see if any of subsections (A) through (I) apply to your installation. 4. Noncontinuous? In Part III, you’ll find the requirements for providing thermal protection to the motor; that’s outside the scope of this discussion. But to size the OCPD, you use the FLC. Sizing VFD sizing should always be based on motor current and voltage, not hp. The motor can be started XL by using the bypass. If, for some reason, you can’t get this from the nameplate or the motor data sheet, use the applicable NEC table (e.g., Table 430.250). All rights reserved. Your other ocp must protect the motor circuit conductors and the motor from for fault current and the starter (assume 3 phase) must provide the running overcurrent protection. Why you size motor circuit conductors and protection differently than you do for circuits of other types of loads. Using the 53A FLC of our example with an inverse time breaker, you multiply the FLC by 2.5 for a maximum rating of 132.5A. In Trade Since: 1980Registered: Nov 2001Total Posts: 200, [This message has been edited by (edited November 08, 2001).]. I looked in several books and have found conflicting information for this type of installation. Next, turn to 430.53. For the conductor requirements on that feeder, turn to Part II. Both inverter and bypass contactor are connected on the … How do you size the OCPD for that circuit? The circuit conductors to the drive HMCP have been sized for 125% of the drive input current per 430-22(a) and 430-2. Normally, the OCPD also handles overloads. With motors, you have an additional device that does the overload protection job normally done by the OCPD.

Multiply the FLC of the largest motor by 125%. If I size the switchboard breaker to match the conductor size as computed by 430-22(a)(exc 2), the motor will not start in the bypass mode.

In such a case, split the run; see Annex D3(a) for an example of how to do this.

Does it have harmonics mitigation? Then, multiply your FLC by the percentage of FLC required by the chart. [This message has been edited by (edited November 08, 2001). I've read section 430-2. The point is however, you do NOT size anything for the motor any longer. Although this task may seem simple, it can be dangerous if not performed correctly. The steps for feeders modify the steps for branch circuits, as shown in Fig. But what if your ceiling temperature is, say, 160°F (71°C)? Because of inrush, motor circuits handle those functions separately — that is, the job of protecting the conductors and the load gets split up for motor circuits.


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