I would like to consider the use an emergency system common bus to supply an emergency system (100A) and optional stand-by systems (400A) with automatic selective load pickup and shedding on an existing 200 kW/250 kVA, 480/277 V generator (no room to replace with larger generator). However, 445.13(A) requires all feeders to be 115% of generator nameplate current rating. A #500 feeder for the 100A emergency system makes the common bus approach impractical. 445.13(B) allows a tap IAW 240.21(B) which appears to be shown in Exhibit 700.3 (single feeder that supplies multiple ATS). However, a common OCPD isn't really practical with automatic selective load pickup and shedding because the common OCPD doesn't need to be larger than the optional stand-by system OCPD (400A) and it would be better to avoid upstream OCPD when possible with an emergency system. Am I missing something in regards to 700 and 445.13? Do you have any recommendations in regards to providing Common OCPD or not?
Emergency System Common Bus with or without Common OCPD - NEC 700.10(B)(6)
- Thread starter NEC54
- Start date
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
The 115% feeder is from the alternator to the first OCPD (or bus). All downstream feeders of that are generally sized per the associated distribution OCPD
- Location
- Illinois
- Occupation
- retired electrician
This would be an example of what Roger said, with the 115% conductors only between the generator and the common OCPD. With many packaged generators, that OCPD would be installed by the generator manufacturer as part of the generator. A conductor sized to the rating of the OCPD as the "common bus", with taps to the various loads.
