fire pump conductor sizes
fire pump conductor sizes
(OP)
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
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





RE: fire pump conductor sizes
I don't have the NEC handy, I'm sure someone else will be along to provide the proper code references.
RE: fire pump conductor sizes
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:
CODE --> NEC
JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"
RE: fire pump conductor sizes
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
RE: fire pump conductor sizes
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.
RE: fire pump conductor sizes