motors and short circuit current

[sceepe]

When determining the available short circuit current at a point in a distribution system, do you apply the contribution of your motors to all points in the system or just the points on the same distribution path as the point you are calculating.

In other words would the motors fed from one sub panel source current to fault on a different sub panel?

Before you point me to Bussman's EPR, it's not in there?

Lastly, it would seem to me that the short circuit current sourced from a motor would decrease the farther you get from the motor (just as it does the farther you get from the POCO xfmr). This would make the point to point method a lot more difficult because you have to start at both ends and work in both directions.

 

[ron]

Re: motors and short circuit current

Performing a point to point calculation when also considering motor contribution at different points is very difficult.
You DO consider motor contribution coming from all directions. It makes the hand calculation very tedious, as each place that you calculate (simulating a fault at that point in the distribution system), has contribution from different directions, and you need to do multiple calcs simultaneously. This also occurs if there is closed transition between the backup gens and the utility, in which there is contributions from different directions. I refer to these as path or branch currents, which add to become the calculated current at one point.
The further you are from the motor, the less it contributes, as you mention, similar reasons as distance from the utility reduces contribution.

 

[steve66]

Re: motors and short circuit current

Unless you have a lot of large motors, the easiest thing to do is just add up the contributions from all the motors and leave that much safety margin in the entire system.

For example, assume I calculated the available fault current (from the utiltiy) at the main panel to be 20,000 amps. At a subpanel it is down to 14,000 amps, and at a panelboard it is 6,000 amps. If I had 50HP of motor load at 208V, I might assume the fault current from the motors would be about 6 times full load current. That would be about 900 amps.

So I just add the 900 amps to each available fault current. The main panel would need a breaker rated for 20,900 amps, the subpanel 14900, and the panelboard at 6,900.

Notice in this example, the motors don't contribuite enough to require the next AIC rating for a breaker. In other words, the main would still be a 22KAIC breaker, the subpanel a 18KAIC, and the panelboard would be 10KAIC.

Do the rough calc. first, and if it seems like it makes a big difference in the cost, then you might want to worry about refining it a bit.

Steve