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An Electrical Win-Win Solution

CONTRIBUTED BY: Syska Hennessy Group

August 1, 2014
KEYWORDS electrical
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Syska Hennessy is continually challenged to increase our client’s availability and reduce installed cost (CAPEX) at the same time. Here’s one best practice that we are using to meet this challenge. 
 
Background:
 
In Critical Facilities, the electrical load on the critical (UPS output) bus is a continuous load that rarely changes (except to increase!). That means that the critical bus power path (from the electrical service to the critical computer load) is a continuous load. Yesterday’s design practice has been to
use:
  • 100%-rated power circuit breakers with electronic trip units for circuits 800 amperes and larger,
  • Standard 80%-rated molded case circuit breakers with thermal-magnetic trip units for circuits smaller than 800 amperes.
 
A 100%-rated circuit breaker will carry 100% of its rating continuously, but an 80%-rated circuit breaker will carry only 80% of its rating continuously. Why are we specifying 80%-rated circuit breakers for continuous loads?
 
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