I would like to see Article 450 addressed more completely - in particular, more details for Isolated Transformers of 600VAC and less (Section 450.3).
***QUALIFYING RANT***
One Specific Design Issue:
Dry-Type Transformers re-energizing, due to Utility Outage (i.e.: Drunk Driver hitting Primary Distribution Pole; Affected Primary Circuit isolated, Power restored).
Primary Inrush Current Levels (involving mainly KVAR) will average the following:
25x rated Primary FLA for 0.01 Second,
12x rated Primary FLA for 0.1 Second,
3x rated Primary FLA for 10.0 Seconds.
With most of the Transformers / SDS's in my typical Projects, the SDS will be a 4 Wire System; OCPDs will be placed on both sides (Primary OCPD and Secondary OCPD).
Secondary OCP will be set at 1.25 x the Secondary FLA for the Transformer; and the Secondary Conductors are sized to conform to the Secondary OCPD rating.
On the Primary side, the OCPD will be set anywhere from 1.75 to 2.50 x the Primary rated FLA.
This helps reduce Non-Selective Coordinated Primary Trip issues for Inrush and Secondary Faults.
If the Primary Feeders were allowed to be sized per the Primary FLA - instead of the OCPD, this would assist in reducing the Inrush level without increasing the length of the Feeders.
The Primary Feeders would be limited by the Maximum value found on the Secondary side - as the total load allowed would equal up to the rated Secondary FLA per the Transformer's rated KVA.
For Designs where Two Panelboards are fed from a Single Transformer, the combined OCPDs equal no more than 1.25 X the rated Secondary FLA.
The concept is similar to Article 430 - allowing for the Temporary Over Currents of Motor Starting.
In the past, I have Designed some Projects around this idea - with Approval by the local DBS (Plans Examiners plus Electrical Inspectors).
Any comments?
--Scott (EE)
Last edited by Scott35; 04/06/13 02:02 AM. Reason: typical spelling blunders!
