Fire Pump Feeder Sizing
Fire Pump Feeder Sizing
(OP)
NEC requires 125%xFLA to be the minimum feeder size for the fire pump. But in order to carry the motor Locked Rotor Amps continuously, the breaker is usually sized to 600%xFLA. How is it possible for the feeder to carry Locked Rotor Current continuously if the feeder is under sized( i.e 125% x FLA)? I couldn’t find anything related to this scenario in the code. Really appreciate your help on this.





RE: Fire Pump Feeder Sizing
"Will work for (the memory of) salami"
RE: Fire Pump Feeder Sizing
Let’s take –for instance-a 100 HP squirrel cage rotor induction motor. Usually the protection time setting for 6-8 times rated current will be 1-5 sec.
The voltage 480 V.FLC as per NEC Table 430.250 is 124 A. The conductor ampacity has to be Amp=124*1.25=155 A and locked rotor current 8*124=992 A.
According to NEC Table 310.15(B)(16) copper conductor XLPE/EPR insulated [90 DGR.C.] 1/0 [ 170 A ampacity]. The maximum temperature for short-time [EMERGENCY] it is 130 oC. The conductor temperature reaches this limit in more than 15 sec. Even for 8*170=1360 A the time to reach the emergency temperature will be 10 sec. [The conductor heating was considered adiabatic phenomenon-no heat dissipation outside]
RE: Fire Pump Feeder Sizing
RE: Fire Pump Feeder Sizing
Also, as jraef points out if the motor draws LRA for very long, it is going to fail, most likely prior to a cable insulation failure.
RE: Fire Pump Feeder Sizing
RE: Fire Pump Feeder Sizing
Regards,
Dave
RE: Fire Pump Feeder Sizing
A) 125% of FLA for the wires to make sure pump can run continuously without overheating wiring.
B) 600% of FLA continuous rating of protection to make sure pump runs to destruction while still maintaining a small ability to interrupt the circuit in case of a short circuit in the wiring that might occur without a fire being present.
Makes sense to me. Like somebody's tagline, "there's probably a good reason it's always been done that way".
RE: Fire Pump Feeder Sizing
RE: Fire Pump Feeder Sizing
If an actual short-circuit will occur at motor terminals -for more than 600% rated current-the fault clearing time has to be limited to 1-5 sec. In my opinion it is also the spirit of NEC Art.695.4.
RE: Fire Pump Feeder Sizing
RE: Fire Pump Feeder Sizing
Therefore the cable sized for 125% FLA apparently can handle motor LRA for 20ms. I think 7anoter4 has a very good illustration in his first posting.
"Run to destruction" seems to be a myth or perhaps can be applied only when sizing the circuit breaker
You can download one of the Tornatech user manuals from below URL
http://www.tornatech.com/FichiersUpload/Manuals/GP...
RE: Fire Pump Feeder Sizing
That explanation doesn't really make any sense. The LRA will last for seconds not milliseconds. Perhaps he was talking about the motor inrush, which is not the same thing as LRA. 20 ms for asymmetrical current might be about right.
An NFPA approved fire pump controller should never trip on locked rotor amps unless there have been some drastic changes in the NEC lately that I am not aware of.
RE: Fire Pump Feeder Sizing
They don't go into why they do it, they just are saying "Do it because we said so." But in some commentary I read on the changes, the people providing input reiterated that the concept is to NOT trip on a Locked Rotor Current scenario in the motor circuit. But LRC is not the same as a short circuit, although we tend to over simplify them in that way. A bolted fault or ground fault will, as we know, allow the total available fault current in the circuit to flow into the fault, which can FAR exceed the motor LRC. But take that idea another step further; HOW is that going to happen? It is going to happen BECAUSE your conductor has ALREADY failed you. In that case, you are already deprived of that fire pump motor; if there is a bolted fault between the source and the motor, the motor is NOT going to get that current.
What might be behind this is an acknowledgement of the reality of what is available out there. Any circuit breaker, regardless of the trip features, comes with an overriding current release point of 10X (1000%) of the breaker FRAME rating. The frame rating is, in essence the maximum current that any breaker can carry; it is the current capacity of the conductive parts inside. The 10X trip feature is inherent even in Molded Case Switches, i.e. there are ostensibly NO trip elements inside at all, at least to the causal observer. But when you read the "fine print" there actually is, and the override release point is 10X the frame rating. So if you have a 600AF molded case switch / breaker, it CANNOT carry more than 6000A for very long not matter what you think the trip setting is (if any). By saying then that the OCPD in the fire pump circuit must be sized for continuously supplying the LRC of the motor, they are indirectly saying that the breaker will NOT trip under any circumstance if the motor shaft is locked, not if there is a short circuit in the conductors. If they did not word it that way and allowed the other OCPD rules to apply from article 430, then it might be possible for someone to install a device that will open up under that 10X override, which is NOT what they want to happen.
"Will work for (the memory of) salami"
RE: Fire Pump Feeder Sizing
http://files.engineering.com/getfile.aspx?folder=d...
RE: Fire Pump Feeder Sizing
RE: Fire Pump Feeder Sizing
That could be true. But since conductor installation should pass all other stringent requirements and the scenario is very rare, the code apparently does not mandate conductors to be larger than 125% FLA and 15% maximum starting voltage drop.
RE: Fire Pump Feeder Sizing