Don, Service sizing is an interesting affair. When people ask for a 200 amp service they are speaking of normal load, as anyone who's ever studied arc-faults knows a 200 amp service in close proximity to a transformer will easily deliver short circuit currents in excess of 200 amps, how much you ask, how about 20, 30, 40 THOUSAND Amps, yes as in 40,000. This is why the code requires the interrupting capacity of over current circuit devices to be equal to or greater than the short circuit current. Hence why circuit breakers also carry an Interrupting rating which is the available short circuit current (ASCC), usually breakers are marked with a yellow dot/star decal with the rating, 10K, 22K, etc.
The calculation is ASCC = TFLC (Total Full Load Current) / Z (Impendence), for example 241A/.015 = 16,067A This is also the reason fire pump controllers are SUSE Rated (SUITABLE for USE as SERVICE EQUIPMENT) So as you can see a service will deliver a hell of lot more than the nominal size of the service. Ok back to the service, the service conductors are only required to be sized for 125% of Full Load Current, yet the overcurrent devices are required to be sized at Locked Rotor Current typically 600%. That's the reason you see these 400 A, 600A, 800A OCPD Disconnects to accommodate the circuit breaker which is sized at locked rotor current. Locked Rotor Indefinitely, let's see how long those 125% feeders last in raceway at 600% (475% greater than they were sized for). The whole idea is to be sure a circuit breaker or fuse isn't going to trip should the motor start slow building counter emf, under heavy load or dragging its armature....run to destruction, including setting the feeder conductors afire. Check with your Utility and see how they want the service sized explaining it's a NEC 695 Fire Pump, give them 125% (continuous load) of FLC and the LRC, etc. Last but not least the fire pump service should only carry the controller and the pump. The rest of the stuff including the jockey goes on general purpose or emergency if they choose but not the fire pump service or fire pump transfer switch. I know 695 talks about calcs for pressure maintenance pumps and accessories but see what 695 covers and does not cover. (NEC 696.1(A) and (B) Hope that helps, John Drucker, CET Assistant Construction Official Fire Protection Subcode Official Building/Fire/Electrical Inspector Red Bank, New Jersey -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Don Lowry Sent: Saturday, April 5, 2014 1:11 PM To: [email protected] Subject: Fire Pump Electrical Service References from NFPA-20 (2010 edition) Situation: Fire Pump to be installed on ESFR system (75 hp) electric motor. 9.2.2 (1) or (5) On down through Chapter 9 to; 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 fire pump motor(s) and the pressure maintenance pump motor( s) and the full-load current of the associated fire pump accessory equipment. OK, so the locked rotor current is 543 amps, plus jockey pump, lights, heat, etc added would require a 600 amp over current protective device. This is what the Engineer and I determined would be the requirement for this dedicated service to the fire pump. When the engineer submitted his electrical service plan (600 amp service) to the city, the City Plan Reviewer states that we don't need a 600 amp service, but rather a 125 amp service. In reading through Article 695.4 of the NEC. It states: "Where the locked rotor current value does not correspond to a standard overcurrent device size, the next standard overcurrent device size shall be used in accordance with 240.6. The requirement to carry the locked rotor currents indefinitely shall not apply to conductors or devices other than overcurrent devices in the fire pump motor circuits." So, does this mean that the service itself (transformer) and related conductors can be per normal size (125 amp) with just a 600 amp fuse in the disconnect before the controller? In 30 + years this is the first time I have ever been asked about this. Before I was only asked about pump size etc , and when installed and checked at pump start-up there was indeed a fuse of appropriate size in the service disconnect. So I assumed all was correct. I'm just wanting to make sure the Engineer and I understand the requirements for the actual electrical service provided. I know it's not my area of expertise, nor my responsibility, just curious. Thanks in advance for any help. Don Lowry, CET, RME (TX) _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
