Electrical
Shortz
Date: 1/2008
Author: Alan Manche
Document Number: 0100DB0711
Cedar Rapids, Iowa, USA
www.us.schneider-electric.com/codes
Keywords:
Fire Pump,
Instantaneous,
Circuit Breaker,
NEC 695
Fire Pump Circuit Breaker Feeders
Fire Pump Circuits and Instantaneous
Trip Circuit Breakers
The 2008 NEC establishes very specific requirements in order to ensure
continuity of power for a fire pump. NEC 695.4(B) permits an
overcurrent device between the remote power source and the fire pump
controller or transfer switch. It even gets more specific about selecting
and sizing the overcurrent device between the generator and the fire
pump controller.
…Overcurrent protective devices between an on-site standby
generator and a fire pump controller shall be selected and sized
according to 430.62 to provide short-circuit protection only…
A common misunderstanding is that “short-circuit protection only” is equated
to “instantaneous trip circuit breaker.” This drives an incorrect specification
for equipment which establishes an NEC compliance issue. During the
development of the 2002 NEC a proposal was made to permit
instantaneous trip circuit breakers for this requirement in the NEC. After
many comments explaining that instantaneous devices are Recognized
Components (Not Listed) and do not have a marked short-circuit current
rating by themselves, NEC Code Panel 15 rejected the idea of an
instantaneous circuit breaker for this application. The entire discussion can
be found in the Report on Comments for the 2002 NEC. NEC 430.52(C)(3)
also reinforces this same restriction on the instantaneous trip circuit breaker
where it is only permitted as part of a listed combination motor controller.
Sizing the Short Circuit Protective Device
for a Fire Pump on a Generator
How do I size the circuit breaker for a generator to comply with
NEC 695.4(B)? NEC 695.4(B) points back to NEC 430.62 for sizing the
overcurrent device for the generator fed fire pump. Remember that if the fire
pump is fed by a service, the circuit breaker is sized to carry locked rotor
(approx 6 times FLC) indefinitely. Where a generator serves the fire pump,
NEC 430.62 permits you to use the values in Table 430.52 instead of setting
the circuit breaker at locked rotor (approx 6 times FLC). Table 430.52 sizes
the circuit breaker at a maximum of 250 percent of the largest motor. Then
the FLC of all the other motors (jockey pumps) that are being fed by the
generator must be added.
NEC® Article 695 requires careful reading to ensure compliance of the fire
pump installation is addressed.
1
Fire Pump Circuit Breaker Feeders
Electrical Shortz
For More Information:
0100DB0711
Date: 1/2008
For more information, see the following references:
2008 NEC® Sections 430.52, 430.62 and 695.4
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3700 6th Street S.W.
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www.us.schneider-electric.com/codes/industrystandards/publications/electricalshortz
“Electrical Shortz” are produced by the Schneider Electric North America Codes and
Standards Group. These documents provide general guidance on a specific issue.
Circumstances regarding particular installation issues may need furter consideration.
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