fire pump conductor sizes

[ellen messer]

I have a 75 hp fire pump that is fed from the utility tranformer and the emergency switchgear. The emergency switchgear has a 600amp breaker in the gear for the fire pump in the event of a lose of normal power. It is my understanding that per NEC,the size of the breaker and conductor must be sized 123% higher than the actual fire pump ampherage. Meaning the minimum size of the breaker and conductor would be 125 AMP. I have been told, just cant find it, that NEC also says no higher than 250% which would mean a 250amp breaker.
the engineer has directed us to install the 600amp breaker but to use a conductor for 125amp.

is it true NEC has a max? and is the engineer asking us to install a system that doesnt meet code?

please advise
ellen

 

[davidbeach]

The rules for fire pumps are different than they are for any other motor application. Fire pumps are protected against system faults ahead of the motor, but are run to destruction for faults within the motor; every last drop of water it can pump is important.

I don't have the NEC handy, I'm sure someone else will be along to provide the proper code references.

 

[JuanBC]

Hi.

I've just search "fire pump" in the NEC 2014 and this is what I found:

ARTICLE 695 - Fire Pumps
...

I think you should read the whole article but I transcribe a part as an example:

(2) Overcurrent Device Selection. Overcurrent devices shall comply with 695.4(B)(2)(a) or (b).

[b](a) Individual Sources.[/b]
Overcurrent protection for individual sources shall comply with 695.4(B)(2)(a)(1) or (2).
(1) Overcurrent protective device(s) shall be rated to carry indefinitely the sum of the locked-rotor current of the
largest fire pump motor and the pressure maintenance pump motor(s) and the full-load current of all of the
other pump motors and associated fire pump accessory equipment when connected to this power supply.
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 circuit(s). [20:9.2.3.4]

(2) Overcurrent protection shall be provided by an assembly listed for fire pump service and complying with the
following:
a. The overcurrent protective device shall not open within 2 minutes at 600 percent of the full-load
current of the fire pump motor(s).
b. The overcurrent protective device shall not open with a re-start transient of 24 times the full-load current of the fire pump motor(s).
c. The overcurrent protective device shall not open within 10 minutes at 300 percent of the full-load current of the fire pump motor(s).
d. The trip point for circuit breakers shall not be field adjustable. [20:9.2.3.4.1]

(b) On-Site Standby Generators. Overcurrent protective devices between an on-site standby generator and a fire
pump controller shall be selected and sized to allow for instantaneous pickup of the full pump room load, but shall
not be larger than the value selected to comply with 430.62 to provide short-circuit protection only. [20:9.6.1.1]

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[jraef]

Yes, as pointed out above, you are wrong. 75HP 480V is 96A per the NEC tables. So a 600A breaker is correct and the conductors must be sized at 125% (not 123%) of the motor FLA from the tables. In addition, the allowed Voltage Drop is lower for a Fire Pump, so that often means FURTHER over sizing of the conductors, depending on the distance involved.

If you are this unfamiliar with the rather EXTENSIVE special requirements for Fire Pump circuits and devices, you are going to be well served to read Article 695 from front to back, paying close attention to all of the referenced articles along the way. There are other very important rules on routing of conductors, sizing of transformers, etc. etc. etc.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[7anoter4]

Let's say the fire pump motor is provided with a supply controller which is set up as per NEC Art.695 [as JuanBC already said]. Following IEEE 242/2001 chapter 9.5.2.4 Development of intermediate characteristics and using this formula:
IE/IN=SQRT((TE-To)/(TN-To)*(Io/In)^2*exp(-t/K)*(230+TN)/(230+TE)/(1-EXP(-t/K)))
and extracting TE=(A/B*230+To)/(1-A/B)
Where:
IE is emergency operating current rating,
IN is normal current rating,
Io is operating current prior to emergency,
TE is conductor emergency operating temperature,
TN is conductor normal operating temperature,
To is ambient temperature,
K is a constant, dependent on cable size and installation type (see Table 9-5),
230 is zero-resistance temperature value (234 for copper, 228 for aluminum),
e[exp in Excel] is base for natural logarithms.
A=(IE/IN)^2*(1-EXP(-t/K))
B=1/(TN-To)*(Io/IN)^2*EXP(-t/K)*(230+TN)
K=4 for underground ducts
Let's say IN=125 A TN=90oC To=25 TI=75oC
That will be #1 [NEC Table 310.15(B)(16)] copper conductor 75oC insulation 130 A.
Calculating at first IE1=24*100 A in 10 sec, then IE2=300 A in 10 min and IE3=600 A for 2 min
the final temperature will be 90oC.
Now if IE=600A for 4 min the temperature will be 90oC
If there is a provided protective system of the fire pump as per NEC then that it means 600 A and 3-4 min. delay circuit breaker will be suitable.
The above chosen cable could withstand the 24*100=2400 kA within 6 seconds. However the inrush current of 24 Imotorated should not continue more than 1 sec.in my opinion.

 

[controlsdude]

If I was you I would highlight the circuit as you pin it to a NEC article. If you find you can not find why the circuit is found in NEC then ask the provider of the equipment. Its your job as the end user to make sure what there providing is correct and up to standards. Also, get your companies insurance involved in this process, they usually have engineer on staff to look at equipment since they insure your company.