Short Circuit Current Ratings (SCCR) for UL508A and NEC

If you have a VFD Controller in which the FLA online side of the drive is higher than the FLC for the HP of the connected motor, should you use the FLA of the Drive or the FLC of motor per the table?

The application of drives often leads to confusion with the rules for sizing the branch circuit protection device.

VFD’s usually provide full protection for the motor and conductors on the load side of the VFD. The VFD itself is considered as a “load” which requires special protection. There is not a conducting path through the VFD. However, there is such a path in a contactor. Thus, specific rated devices, based upon the UL certificate, shall be used to protect the VFD only.

In general for all VFD’s and Servo drive systems the following applies:

  • The sizing/ratingof the branch circuit protection device follows solely the input current of the drive multiplied with 115%, not the FLA of the motor.

Please see the following statements of:

  • NFPA79,
    19.3.1: Servo Drive System Supply Conductors. Circuit conductors supplying servo drive systems shall be sized to have an ampacity not less than 115 percent of the rated input of the equipment.

  • UL508A
    31.3.2 The branch circuit protection for a single-motor circuit provided with a variable-speed drive shall
    be of the type and size specified by the manufacturer’s instructions provided with the drive. When the
    instructions do not specify the type and size, a branch-circuit fuse or inverse-time circuit breaker shall be
    used and shall be sized based upon the input current rating of the drive multiplied by the percentage from
    Table 31.1.

  • NEC:
    The NEC 2011 has the same requirements.

Per UL 508A guidance, the MSP to be set to the motor FLA shall be capable of being set at 115% of the motor FLA. Does this mean that the adjustment must range to 115% of the motor FLA?

Motors require sometimes an individual protection, depending on the SF (Service Factor), or the max. Ambient temperature (40 degree Celsius).

The NEC code describes it in detail: Separate Overload Device: A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

  • Motors with a marked service factor 1.15 or greater: 125%

  • Motors with a marked temperature rise 40 degrees Celsius or less: 125%

  • All other motors: 115%

Motors without a service factor smaller 1.15 and not marked with max temperature rise 40 degree Celsius must be protected against overload at 115% of the Full load amp rating (nameplate rating). The default ratio between dial current vs. tripping current is 125% (please see nameplate of MSP or Overload relays). That means, to adjust the overload relay, a correction factor of 115/125 = 0.92 need to be applied. The UL508A has this confusing requirement that an overload relay shall be capable to be adjusted at 115%, in case a motor which requires, this is in use.

Example: 100 Amp motor, with SF smaller 1.15 and not marked with max. Temperature rise of 40 degree Celsius. --> the overload relay shall be adjusted at 92 Amps. (100 Amps * 0.92 correction factor).

The motor has:

  • A SF of 1.15

  • Is marked with 40 degree Celsius maximum temperature.

  • Adjust the FLA at the overload relay OR it will trip at 125% as required in the NEC

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