SCCR Calculations

[lgabriel]

New to SCCR calculations and have downloaded several tutorials which are helpful but sometimes conflicting and I would like to post a simple question

Looking for SCCR rating for the following combinations:

480Vac 3pole fusing (Branch circuit feeding 4-6 motors per 20A fuse) CC Type fusing rated at 200kA, connected to motor protector rated at 65ka
connected to motor contactor, rated at 5ka and then to motor

480Vac 3pole fusing 20A (Branch circuit Type J) rated at 200kA, connected to motor contactor, rated at 5ka,connected to VFD (Variable Frequency Drive) rated at 100ka with UL508C label, and then to motor.

Thanks in advance

 

[W6SJK]

Your question is the subject of a 3 to 5 day training course so you won't get enough info here. Thumbnail answer: The short circuit current rating is determined by calculating the available fault current and x/r ratio at the point of the electrical circuit in question. You start with a given available fault current at some upstream point in the system, usually the main switchboard, along with fault contributions (backfeed) from large motors (usually > 10HP, if any). Then you add the impedance of the wiring and transformers in the "circuit" all the way from that main swbd to the point in question, that will lower the current. So you need the available utility fault current, wire sizes and lengths, conductor type, conduit type (metallic or non-metallic) and transformer kva and impedance. Confused? I don't blame you.

 

[jim dungar]

Typically, you will use any "listed series" combinations that are available to you.

For example the manufacturer of the 5KA motor contactor probably has a series rating with generic up stream fuses or its own motor protective device.

I know of no VFD manufacture that has a series rating with a 'motor protector and contactor combination'.