Fire pump breaker

[ohmhead]

Well i have a question lets say a fire pump 125 hp pump 480 volts 3 phase lets not include every fact voltage drop ,type of motor just a general question.

Wire size at 125 % correct or incorrect ?

The transfer switch has the normal power from the utility trans .

The emergency is from the generator .

The breaker which is located in the generator does this need to be at locked rotor meaning lets just say a 900 amp or next higher at 1000 amp per art 695 nec ?

The engineer has this at 400 amps and one run of 500 mcm cu ?

 

[mcclary's electrical]

Well i have a question lets say a fire pump 125 hp pump 480 volts 3 phase lets not include every fact voltage drop ,type of motor just a general question.

Wire size at 125 % correct or incorrect ?

The transfer switch has the normal power from the utility trans .

The emergency is from the generator .

The breaker which is located in the generator does this need to be at locked rotor meaning lets just say a 900 amp or next higher at 1000 amp per art 695 nec ?

The engineer has this at 400 amps and one run of 500 mcm cu ?

125% of the motor(s) PLUS 100% of all associated accessory equipment
Yes, OCP should be figured for indefinate locked rotor.
Locked rotor for 150 hp comes to around 1080.

 

[ohmhead]

Well next question would you see a problem with this locked rotor rated breaker if installed in a switchboard of other loads on emergency .

Maybe a power outage issue like if generator failed do to a short circuit in fire pump lites out due high breaker size ?

Meaning all emergency lost ?

 

[mcclary's electrical]

Well next question would you see a problem with this locked rotor rated breaker if installed in a switchboard of other loads on emergency .

Maybe a power outage issue like if generator failed do to a short circuit in fire pump lites out due high breaker size ?

Meaning all emergency lost ?

I see your point, and even though can't put my finger directly on an article pertaining to it is 695.3 (a) (2) which refers you to NFPA 20,6.2.1, 6.2.2, 6.2.4.4.....which when you get into it says

"A requirement that one source not affect the reliability of another source. " But I'm not sure if you stretch that to fit here.

 

[Pierre C Belarge]

Well i have a question lets say a fire pump 125 hp pump 480 volts 3 phase lets not include every fact voltage drop ,type of motor just a general question.

The breaker which is located in the generator does this need to be at locked rotor meaning lets just say a 900 amp or next higher at 1000 amp per art 695 nec ?

The engineer has this at 400 amps and one run of 500 mcm cu ?

Overcurrent protection at locked rotor.
See Table 430.251(B). 908A - goto 1000A
and
the wire size is at 125% - 156A X 1.25 = 195A - 3/0 Copper

 

[ohmhead]

Well take a look at article 695.3 (B) -(1) generator capacity allow normal starting & running of the motor driving the fire pump while supplying all other loads .



Then read the next high lighted section where a alternative source of power is a on site generator fire pump not required to be sized for locked rotor current of the fire pumps motor .This is in the handbook not the paperback let me know what you think ?

 

[chris kennedy]

.This is in the handbook not the paperback let me know what you think ?

For those that don't have the NECHB here is the commentary following 695.3(B)(1).

Where the alternative source of power is an on-site standby generator, the alternative source disconnecting means and the alternative source overcurrent protective device(s) for the electric-drive fire pump are not required to be sized for locked-rotor current of the fire pump motor(s). The generator is required to be sized to allow the fire pump to start and run. The associated circuit components are required to be sized according to Article 430 and to ?allow instantaneous pickup of the full [fire] pump room load.? This requirement is also found in 9.6.5 in NFPA 20-2007, Standard for the Installation of Stationary Pumps for Fire Protection.

 

[hillbilly1]

For those that don't have the NECHB here is the commentary following 695.3(B)(1).

Yes, it would be a ridicously sized generator for locked rotor current. Though I believe it's not required, our engineers spec the fire pump connection to the generator ahead of the ocp on the generator. I designed a circuit to drop the building load when the fire pump is called for, and is on generator power. (building has battery back up emergency lighting) This way a smaller generator can be used because of load shedding.

 

[hurk27]

Well next question would you see a problem with this locked rotor rated breaker if installed in a switchboard of other loads on emergency .

Maybe a power outage issue like if generator failed do to a short circuit in fire pump lites out due high breaker size ?

Meaning all emergency lost ?

695.3(A) states we only need one reliable source if it can carry the lock rotor current to the fire pump indefinitely, and is protected from a fire in the building that it protects, listed in 695(A)(1) or (A)(2)

If not reliable then we must supply the fire system from more then one source listed in 695.3(B)

695(B)(1) Generator Capacity. An on-site generator(s) used to comply with this section shall be of sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load. Automatic shedding of one or more optional standby loads in order to comply with this capacity requirement shall be permitted. A tap ahead of the on-site generator disconnecting means shall not be required. The requirements of 430.113 shall not apply.

695(B)(1) require that if a generator is used, it must have the capacity to allow the normal starting and running of the pumps, and other loads or we are allowed to use automatic load shedding to achieve this.

Here is where the confusion get tougher.

695.4 Continuity of Power. in the paragraph under (3) it says for systems installed under the provisions of 695.3(B)(2) only!
But if we look at 695.3(B)(2) we find it is for feeders from separate utility services not generators?

So this leaves me to believe the second sentence in 695.4 paragraph is for 695.3(B)(1)

Over current 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. All disconnecting devices and over current protective devices that are unique to the fire pump loads shall comply with 695.4(B)(1) through (B)(4).

Which would require short circuit protection only of conductors sized in 430.62

Short circuit protection and over current protection are two different things and short circuit protection will be much higher for the same sized conductors.

 

[hurk27]

keep in mind that 430.62 will direct us to table 430.52 for short circuit protection.

 

[ohmhead]

Well thanks for the input good points made we will not change the breaker for locked rotor will use art 430 for this case .
As it is a 3000amp switchboard with lots of other load attached .

Thanks again .