Fyi Interesting Thank you, Ed Mayer
> On Apr 6, 2014, at 10:14 PM, "John Drucker - Home" <[email protected]> > wrote: > > 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 _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
