Fire Pump Conductor & Conduit Sizing Question?

[tnt8197]

I have a question with regards to a fire pump conductor and conduit sizing debate we are currently experiencing. The actual fire pump size is 75HP, 3PH, 208V, Wye. The jockey pump is a 1/3rdHP, same everything else. The distance from the main distribution board to the fire pump and jockey pump is 420'. The engineer has designed this on the plans with a 1600A Main circuit breaker, installed ahead of main switch in the main distribution board, and feeding it with four (4) four inch (4") conduits, each filled with three (3) 600kcMil, type RHH conductors, and one (1) 4/0kcMil, type RHH ground conductor. According to the 2002 NEC, section 695.4(B)(1) the overcurrent device need to be sized "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 when connected to this fire supply". This explains the main circuit breaker sizing, but according to the 2002 NEC, section 695.6(C)(1), the conductors will only need to be sized to "125% of the sum of the fire pump motor(s) and pressure maintenance motor(s) full-load current(s), and 100% of the associated fire pump accessory equipment?. According to my calculations, the FLA's of the fire pump and associated equipment is 267A, and even applying the 2002 NEC, section 695.7,voltage drop, we can not come up with the same conductor and conduit sizing as the engineer. Could someone please assist us as to the correct method to figure out the above situation? Thanking you in advance for your assistance.

edited by moderator to remove e-mail address. please use the PM function for private contacts

[ July 16, 2005, 10:20 AM: Message edited by: don_resqcapt19 ]

 

[bob]

Re: Fire Pump Conductor & Conduit Sizing Question?

TNT
Using 4 ckts of 600 kcm, 420 feet, 267 amps
I get 1.03%. At a LRA of 6 x 267 amps the VD = 6.5%. Did you use other figures?
The table in Annex C appears to show 5 600 RHH
in 4" EMT.

[ July 16, 2005, 12:06 PM: Message edited by: bob ]

 

[tnt8197]

Re: Fire Pump Conductor & Conduit Sizing Question?

Bob,

Isn't locked rotor calculation only taken into consideration when calculating overcurrent protection? NEC 2002 section 695.4(B)(1). I believe that this does not apply to conductor & conduit sizing. Thanks.

 

[bob]

Re: Fire Pump Conductor & Conduit Sizing Question?

You had noted the VD criteria in 695.7. That is why I list the LRA drop. What were the VD figures you caculated?
The conductors are sized at 125% of the FLA of the equipment and thus the necessary conduit is required to match.
I think I see your quandary now. Are you questioning the extra large conductors to be installed?
Apparently the engineer thinks the conductors are required to match the 1200 amp breaker.

[ July 16, 2005, 12:17 PM: Message edited by: bob ]

 

[tnt8197]

Re: Fire Pump Conductor & Conduit Sizing Question?

According to my calculations, 250kCMIL's will be at 4.7%VD, 300kCMIL will be at 3.9%VD, and 350kCMIL's will be at 3.3%VD. Each of the aforementioned only consisting of one set of conductors and conduit run, from the 1600A circuit breaker to the fire pump. Do these calculations add up the same for you? Thanks for the assistance.

 

[bob]

Re: Fire Pump Conductor & Conduit Sizing Question?

Using the 267 amps as stated My figures are
Line to Line VD
250 6.7% 14 volts
300 5.9% 12 volts
350 5.4% 11 volts
600 3.6% 7.3 volts
For 1 set of conductors.

[ July 16, 2005, 01:47 PM: Message edited by: bob ]

 

[iwire]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by tnt8197:
I have a question with regards to a fire pump conductor and conduit sizing debate we are currently experiencing.

Just curious if this debate is just for the learning experience or are you trying to use smaller conductors?

Bigger conductors equate to larger profits as long as you bid on them correctly.

 

[tnt8197]

Re: Fire Pump Conductor & Conduit Sizing Question?

Please see the attached link for Voltage Drop. This is where I am getting my figures from, including, several other voltage drop calculators that I have access to. Are you in agreement, based on my original post, that the FLA of the pumps in question is 267? Thanks for the assistance. web page

[ July 16, 2005, 02:32 PM: Message edited by: tnt8197 ]

 

[bob]

Re: Fire Pump Conductor & Conduit Sizing Question?

TNT
The voltage drop on this caculator is line to neutral VD. To get Line to Line VD multiply by your figures by 1.73. Change temp to 30C and you should get close to what I list. Line to Neutral does not mean much when the motor is connected Line to Line.
I have to take your figure of 267 amps. I have no way of knowing.

Iwire
I think he asked a good question as to why the Engr over sized the conductor. I was trying to answer it as best I could.

[ July 16, 2005, 03:26 PM: Message edited by: bob ]

 

[iwire]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by bob:
Iwire
I think he asked a good question as to why the Engr over sized the conductor. I was trying to answer it as best I could.

I did not think anything was woung with the question or answers.

I just am always amazed when people want to give away work / money that is already in the hand. I long ago gave up caring to know why an engineer substantially over sized a set of conductors. Most times they have their reasons for doing so.

I look this the same way as the phone company always wanting two 4" raceways to bring in one 25 pair cable.

Overkill but it has already been bid on and accepted that way.

 

[don_resqcapt19]

Re: Fire Pump Conductor & Conduit Sizing Question?

The voltage drop has to be calculated based on locked rotor for this application. The code section, 695.7, specifies that the voltage drop not exceed 15% under motor starting conditions and not exceed 5% with the motor operating at 115% of FLA.
Don

 

[tnt8197]

Re: Fire Pump Conductor & Conduit Sizing Question?

Bob,

The calculator is set up for 208V, 3PH, 3W, which is exactly what is existing currently. If it were a 4W circuit than it would be line to neutral. I believe that you are multiplying your 1.73 twice? To calculate the load of the fire pump motor and the jockey pump motor, I have used the following equation:

Fire Pump - 75HP = 211A x 125% = 263.75
Jockey Pump - 1/3HP = 2.4A x 125% = 3A
Total for both pumps. = 266.75(267)

Using the voltage drop calculators that are readily available to all, I have come up with the VD scenarios in my previous posts. What I am trying to establish here is to see if my calculations are correct. Try this calculator as well Volt Drop Calculator. I appreciate your assistance Bob, and look forward to more of your positive feedback.

iwire wrote "I just am always amazed when people want to give away work / money that is already in the hand. I long ago gave up caring to know why an engineer substantially over sized a set of conductors. Most times they have their reasons for doing so."

It is our job, as extremely skilled tradesman, to continually question the means and methods of those affiliated with our profession. Only by asking the questions, and debating the questions meaningfully, will we be able to grow and progress in this rapidly changing, technologically advance industry. Believe it or not iwire, its not always about the money, sometimes its about furthering your education, and yourself as an educated, licensed, professional tradesman.

Not to get off of the subject here, but is anyone able to show me the calculations on how they came up with their figures? Thanking everyone in advance for their assistance.

 

[tnt8197]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by don_resqcapt19:
The voltage drop has to be calculated based on locked rotor for this application.

Don,

The section 695.7, voltage drop, does refer to your stated percantages for voltage drop. Where in the NEC 2002, does it state that the VD calculations has to be based on the locked rotor? The referenced section only states "controller-rated voltage", which is the fire pump control cabinet (208V), and the locked rotor application only applies to overcurrent protection? Thanks for your assistance Don.

 

[iwire]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by tnt8197:
It is our job, as extremely skilled tradesman, to continually question the means and methods of those affiliated with our profession. Only by asking the questions, and debating the questions meaningfully, will we be able to grow and progress in this rapidly changing, technologically advance industry. Believe it or not iwire, its not always about the money, sometimes its about furthering your education, and yourself as an educated, licensed, professional tradesman.

Well 99% of the time when people ask these types of questions on this forum it is because they want to know "whats the smallest possible conductor I can use and meet code".

Usually they do not want to provide what they bid on per the job specs.

So many times it is about the money.

 

[iwire]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by tnt8197:
The section 695.7, voltage drop, does refer to your stated percantages for voltage drop. Where in the NEC 2002, does it state that the VD calculations has to be based on the locked rotor?

Motor starting conditions are locked rotor.

695.7 Voltage Drop.
The voltage at the controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor starting conditions.........

 

[tnt8197]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by iwire:

[/qb]

Well 99% of the time when people ask these types of questions on this forum it is because they want to know "whats the smallest possible conductor I can use and meet code".

Usually they do not want to provide what they bid on per the job specs.

So many times it is about the money.

Were you able to come up with a calculation? Thanks!

 

[iwire]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by tnt8197:
I feel that it is unfair for you to assume and judge anyone, before you have the complete facts available to you.

That is true and probably why the first sentence of my first post in this thread was this.

Originally posted by iwire:
Just curious if this debate is just for the learning experience or are you trying to use smaller conductors?

I was trying to learn more facts.

As far as the calculations I see a LRC of approximately 1600 amps and voltage drop of 4.6% under starting conditions. But I did not take the time to add in the other loads.

Have you talked to the engineer to get their perspective and voice your concerns about building damage?

Edit 5.6% to 4.6%

[ July 16, 2005, 06:07 PM: Message edited by: iwire ]

 

[bob]

Re: Fire Pump Conductor & Conduit Sizing Question?

Iwire
What size conductor did you use? 4 600 kcm?

TNT
Using the 267 amps as stated My figures are
Line to Line VD
250 6.7% of 208 =14 volts
300 5.9% of 208 = 12 volts
350 5.4% of 208 = 11 volts
600 3.6% of 208 = 7.3 volts
For 1 set of conductors.
Line to Neutral
250 6.7% of 120 volts = 8v
300 5.9% 12 volts = 7v
350 5.4% 11 volts = 6.5v
600 3.6% 7.3 volts = 4.3v
Having a neutral has nothing to do with it. Its how the computer makes the caculations.

[ July 16, 2005, 06:18 PM: Message edited by: bob ]

 

[iwire]

Re: Fire Pump Conductor & Conduit Sizing Question?

Originally posted by bob:
Iwire
What size conductor did you use? 4 600 kcm?

No Bob I apparently entered three instead of four in my VD calculator.

Using 208 volts, 1600 amp load, four sets of 600 kcmil copper, 420' of length I come up with a figure of 3.49% drop.

 

[bob]

Re: Fire Pump Conductor & Conduit Sizing Question?

Iwire
Compute the VD using 267 amps, 208 v, 420 ft
1 350 per phase and see if you get the same results as I show, please.