Transfer Equipment

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nhee

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We are replacing some existing process equipment at an existing facility. The facility has a backup generator on site. The backup generator feeds a separate, backup power bus within their MCC which is isolated from the normal power bus.

Certain equipment (including the equipment we are replacing) is fed with dual kirk-key interlocked breakers so that the equipment can be run either on normal or backup power, but not both.

There is some newer equipment that was added approx 15 years ago. For this new equipment, dual 800A switches (one from normal one from generator) have their load side terminals "jumped" to feed a separate MCC. There is no interlock or method (other than operating procedure) to prevent paralleling the genset and utility power. There is no method to prevent backfeeding of utility with genset power.

The "newer" equipment is not part of our workscope, but I intended to discuss this arrangement with our customer. Most (all) of my genset installation experience is with break-before-make auto transfer switches.

1. I'm assuming that 702.6 prohibits this installation. Are there any sections/exceptions that would permit it?

2. Would it have been permitted in 1990 NEC?

3. Are there situations where "operating procedure" is an acceptable method (acceptable to NEC) to prevent inadvertant connection of backup to utility power?
 

wirenut1980

Senior Member
Location
Plainfield, IN
Re: Transfer Equipment

1. I'm assuming that 702.6 prohibits this installation. Are there any sections/exceptions that would permit it?
I believe Article 705 permits it. See the end of the section you cited 702.6. The installation you are talking about may or may not meet 705. I would be especially interested if they meet 705.16. Can't really talk on the 1990 NEC...I was only 10 at the time :D
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Re: Transfer Equipment

Originally posted by nhee:
...There is no interlock or method (other than operating procedure) to prevent paralleling the genset and utility power. There is no method to prevent backfeeding of utility with genset power.
...
This is the critical statement and, in all likelihood, the utility would prohibit the connection if they knew about it.

In 1990, the 1987 NEC was the edition most likely to be in effect. It would have been a violation at that time. The text and intent has remained fairly stable.

In addition, depending on close review of the system, it may violate 110.9 and 110.10. These sections have been relatively stable since 1975, at least.

edited 1975 NEC reference

[ January 12, 2005, 01:21 PM: Message edited by: rbalex ]
 
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