Use Normal-Duty, Rated, or Locked-Rotor for Power Demand on Generator?

3/24/2023

Thanks for answering my previous questions; I have another one today.

Is the life safety electrical demand on the generator, for the electrical engineer, based on the normal-duty load (346 bhp), motor load (500 bhp), or the maximum locked-rotor road (572 bhp)?

Is there a governing standard that dictates this (IFC/NFPA 20/NFPA 70)?

We have a 2,000 gpm, 185 psi rated electric fire pump. Thanks in advance.

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

3 Comments

Dave Branson

3/24/2023 08:47:11 am

There are several portions of NFPA 20 that address your questions, depending on your specific configuration.

NFPA 20 Para. 9.2.3.4
Where the overcurrent protection permitted by 9.2.3 is installed, the overcurrent protection device shall be rated to carry indefinitely the sum of the locked rotor current of the largest pump motor and the full-load current of all of the other pump motors and accessory equipment.

NFPA 20 Para. 9.6.5.1 says:
Protective devices installed in the fire pump circuits of the on-site generator power source shall allow instantaneous pickup of the full pump room load and shall comply with NFPA 70, Section 700.28.

This should point your electrical engineer to a good starting place.

Todd E Wyatt

3/24/2023 09:24:57 am

“Duty” is referenced (72) times in 2023 NFPA 70 ... (0) times for “Normal Duty”, “Rated Duty”, and “Locked Duty”.
“Demand” is referenced (74) times in 2023 NFPA 70.
“Load calculations” is referenced (127) times in 2023 NFPA 70.
“Horsepower Ratings” is referenced (33) times in 2023 NFPA 70. As it relates to sizing the generator for the power source of the EES, see the following :

Chapter 4 Equipment for General Use
Article 445 Generators
445.1 Scope.
This article contains installation and other requirements for generators.

ENHANCED CONTENT
Article 445 covers the installation of generators. The following articles cover the use of generators in specific applications:
Article 695 for fire pumps
Article 700 for emergency systems
Article 701 for legally required standby systems
Article 702 for optional standby systems
Article 705 for interconnected electric power production sources
Article 708 for critical operations power systems

445.17 Generator Terminal Housings.
Generator terminal housings shall comply with 430.12. Where a HORSEPOWER rating is required to determine the required minimum size of the generator terminal housing, the full-load current of the generator shall be compared with comparable motors in Table 430.247 through Table 430.250. The higher HORSEPOWER rating of Table 430.247 and Table 430.250 shall be used whenever the generator selection is between two ratings.
Exception: This section shall not apply to generators rated over 600 volts.

The referenced “Article 700 Emergency Systems” states the following :

Article 700 Emergency Systems
Part I. General
700.1 Scope.
This article applies to the electrical safety of the installation, operation, and maintenance of emergency systems consisting of circuits and equipment intended to supply, distribute, and control electricity for illumination, power, or both, to required facilities when the normal electrical supply or system is interrupted.
ENHANCED CONTENT
Emergency systems are designed and installed to maintain a specific level of illumination for means of egress and to provide power for fire alarm systems, fire pumps, automatic doors, and similar equipment if the normal power supply fails.
Article 700 applies to the installation of emergency systems that are essential for safety to human life and are legally required by municipal, state, federal, or other codes or by a governmental agency having jurisdiction. Article 700 does not mandate whether emergency systems are required or where emergency or exit lights should be located. These determinations rely on NFPA 101, Life Safety Code, or the adopted building code. The systems covered by Article 700 are generally viewed as what is necessary for an orderly and safe evacuation from a building or other occupancy type that is required by the applicable codes to be provided with emergency power.
700.4 Capacity and Rating.
(A) Capacity.
An emergency system shall have adequate capacity in accordance with Parts I through IV of Article 220 or by another approved method. The system capacity shall be sufficient for the rapid load changes and transient power and energy requirements associated with any expected loads.
ENHANCED CONTENT
THE EMERGENCY SYSTEM MUST BE DESIGNED WITH ADEQUATE CAPACITY AND RATING TO SAFELY CARRY, AT ONE TIME, THE ENTIRE LOAD CONNECTED TO IT. The system must be capable of restarting emergency loads that have been interrupted, such as motors that may have stopped, and it must be suitable for the available fault current. See also 517.31(D) for the requirement covering acceptable methods to calculate the electrical demand of a hospital’s essential electrical system.

This references 517.31(D) ...

Chapter 5 Special Occupancies
Article 517 Health Care Facilities
517.31 Requirements for the Essential Electrical System.
(D) Capacity of Systems.
The essential electrical system shall have the capacity and rating to meet the maximum actual demand likely to be produced by the connected load.
Feeders shall be sized in accordance with 215.2 and Part III of Article 220. The alternate power source(s) required in 517.30 shall have the CAPACITY AND RATING TO MEET THE DEMAND PRODUCED BY THE LOAD AT ANY GIVEN TIME.
Demand calculations for sizing of the alternate power source(s) shall be based on any of the following:
(1) Prudent demand factors and historical data
(2) Connected load
(3) Feeder calculations
(4) Any combination of the above
The sizing requirements in 700.4 and 701.4 shall not apply to alternate sources.

Alex

3/24/2023 09:28:57 am

NFPA 20 (Section 9.6.2), the emergency power supply system should be capable of carrying the locked-rotor current of the largest motor and the full-load current of all other connected motors simultaneously. However, this requirement is typically applied to the starting condition of the motors, and once they are up and running, the generator must be sized to handle the actual motor load.

Use Normal-Duty, Rated, or Locked-Rotor for Power Demand on Generator?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES