Short Circuit Current Ratings (SCCR) for UL508A and NEC
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:
Please see the following statements of:
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.
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
The NEC 2011 has the same requirements.
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 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: