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Looking to install a 50HP VFD for a Chilled Water Pump.

50HP 3-phase 65amps @460v 65amps @ 125% = 81.25amps or 90amp … Why you size motor circuit conductors and protection differently than you do for circuits of other types of loads . This is caused by the input diodes conducting only when the input voltage is higher than the DC bus.

I have three 230V 1.5 HP motors with a 5.6A FLA each, to be driven from a single VFD. This section is sufficient for electromechanical motor starting equipment but may not always be suitable when power conversion equipment, such as a VFD, is used.
The selection & sizing of these fuses is critical for semiconductor protection in the event of a fault.

Select your motor conductors as if they were being fed by any standard motor OLR (e.g. based on the VFD protection settings).

In their calculations, the motor conductors are a #1. Okay, you can now proceed with sizing the conductors and protection for the motor. Ignore the line side of the VFD. See primary fuse chart at the bottom of this article to quickly determine your fuse size. "Example to determine motor conductors and protection".

Overcurrent protection must be provided for every conductor based on its ampacity from 310.15 at the point where it receives its supply, unless it fits one of the situations we have discussed [240.4].

I'm a little old school and always protect my motors at 175% assuming I'm using a thermally protected breaker or time delay fuses. What I need to know is, can I use 125% due to the fact it will not be set up for full voltage start? Fuses are less likely to have nuisance outages than circuit breakers, and are a better option for circuits that have a high-fault current. You can then round to the next fuse size which would be 10 Amps. An overcurrent protection device is required in the AC supply to the VFD.


Following the example, they calculate the overcurrent fuse size for time delay fuses at 174Amps (150 Amp standard size) There is a section 430.52-5 which allows you to use different fuses for power electronic devices. Circuit breakers can …

What size feeder overcurrent protection device and conductor (THHN) is required for a 184A continuous load on a panelboard (75°C terminals). An OCPD must be capable of opening a circuit when an overcurrent situation occurs, and must also have an interrupting rating sufficient to avoid damage in fault conditions [110.9].

Mark Lamendola. This allows the motor to operate at up to 15 percent above its normal rating without causing the overload protection to trip. The purpose of this device is twofold: To provide thermal protection for the VFD cables etc between the location of the overcurrent protective device (usually at the origin of the relevant supply sub-circuit) and the VFD.

Sizing Circuit Protection and Conductors — Part 3. In most VFD applications the VFD itself provides overload protection for the motor.

The VFD manufacturer's recommendations must be followed in installing or replacing fuses for the variable frequency drive to assure fast operation of fuses in case of a fault. Sometimes some manufacturers recommend a quicker fuse to protect the electronics rather than the dual element time delay.

Step 1 - Size the overcurrent device in accordance with 215.3.

An example that has me perplexed at the moment seems to be falling in a grey area for me, as my ability to interpret NEC is lacking. Using the Bussman SPD Motor Circuit Protection Tables … Overcurrent protection using appropriate type of protective devices and determination of the short-circuit current rating of a\൮ electrical equipment according to codes’ and standards’ requirements has always been somewhat challenging & confusing topic!\爀對䘀漀爀 琀栀攀 䰀嘀 搀爀椀瘀攀猀Ⰰ 䤀 栀愀瘀 … This has worked fine for years. Step 3 Branch Circuit Overcurrent Protection Sizing - 430-152 Select type of motor and type of over current protection device from the chart and multiply values given by the FLC of the motor. Apr 16th, 2013.

Hello all, I have a question about properly fusing VFDs. VFD/Motor Overcurrent Protection??? If the calculated value

For a single motor on an individual overcurrent device, multiply the full load ampacity of the motor by 1.25 and then size the conductors the next size larger than the resultant value.

Overcurrent Protection Either circuit breakers or fuses can be used as the overcurrent protection for a VFD. QUESTION: I'm concerned about sizing motor circuit wiring and overcurrent protection. This is predominantly a measure to prevent injury and property damage from melting and fire.

However, fuses can drop out one leg of a three-phase circuit and cause damage to a motor if not properly equipped with phase-loss detection.

Confirm the VFD provides short circuit,ground fault and running overload protection to the motor (all UL Listed ones do). Fuses are less likely to have nuisance outages than circuit breakers, and are a better option for circuits that have a high-fault current. VFD overcurrent protection toygasm4u (Electrical) (OP) 1 May 08 11:02.