Motor Size

Does anyone have an idea what motor size needs to be included in the study? We have several large 480 MCC's that have a wide variety of motor sizes ranging from 3.5 HP up to several hundred. I have modeled the whole MCC in SKM, but which ones should I not include. Obviously maybe not the 3.5 hp motors, but what is a good stopping point. Plus, if we do have motor data and fuse/wire data for that MCC, why not put it in anyway? Thoughts on this?

Jack

Group them.

I would group the smaller motors <50 hp and leave separate the larger motors. With smaller motors there may be redunant motors so you can also put in a diversity factor and if they are not fully loaded, a load factor.

Some of your larger motors may have VFD's on them and if they are non-regen, then they don't contribute to the fault.

Grouping the motors sounds like a good idea. This may sound like a silly question, but when you guys say group the motors, are you talking about adding HP/Fuse sizes/wire lengths together?

Most of the MCC's contain motor overloads. Do we have to take that into account when performing arc flash study?

Jknox,

When grouping the motors, I add the hp, pick an appropriate wire/fuse size since the main purpose is to get the short circuit contribution from the individual motors. Does your software have an integrate feature for arc flash analysis?

Usually the overloads will not affect the arc flash as they are too slow to have any effect on the arc flash.

I am using SKM to do DAPPER, CAPTOR AND arc flash.

If you are talking of a MCC, and implementing the IEEE analysis method, then you rate the MCC section as one entity. It sounds as if you are rating the load side of the starter, downstream of the heaters. But when you open the bucket door, you also have exposure to the line side of the fuse disconnect or breaker. That line side has to be included. Since the line side is connected directly to the vertical bus without any other overcurrent device, the bus becomes the rating for all the buckets.

If I have a MCC that is MLO I rate the whole MCC lineup to the same rating. If the lineup had an incoming MCB, then I would label the MCB compartment to the line side rating, and the remainder of the MCC to the load side rating. But I would also implement special training so the techs no the difference between working inside a bucket versus taking off the back cover of a MCC (which is rarely if ever done).

Motor Contribution

It was generaly accepted the A/C induction motors under 75 HP do not contribute enough to available currents to make a difference in the results. I would only include in the analysis motors of 75 Hp and greater and are likely to be running at a time of an Arc Flash. For instance a hydraulic system that has two motor/pumps one the main and the other the backup, only include one of them.
Synchronous motor have a greater effect on Arc-Flash the A/C induction motors.

IEEE 1584:The study must take into account all sources, including utilities, standby and power generators, and large motors—those 37 kW and larger that contribute energy to short circuits.

37kW=50 HP.

I personnaly group <50 HP motors.