Remote School Pump Station

[dcsva@aol.com]

We are installing a Duplex Suction Lift Waste Water Pump Station 950FT away from an elementary school. The pump station is the kind that is totally enclosed under a fiberglas hut. The hut is on a hinge to be opened for maintenance. We are getting 480V/3PH power (approx. 35 running amps) from the school and a telephone line from the school. Is a ground rod required at the pump station? If not required, would it be good practice to have installed anyway?

 

[bphgravity]

Re: Remote School Pump Station

This is considered a separate structure per the definition in Article 100. Per the requirements of section 250.32(A), a grounding electrode would need to be established at this location.

However, if the circuit you are running is a single branch circuit with an equipment grounding conductor, a grounding electrode is not required.

A ground rod is permitted to be installed at this location if desired but not mandatory, however, the benefit is questionable. It certainly can't hurt to have one there.

 

[dcsva@aol.com]

Re: Remote School Pump Station

bph,
The circuit we are running is a single 480VAC/3ph circuit with an equipment grounding conductor. (L1,L2,L3,G. The only items powered by this circuit are 2-10HP pumps and a 480/120 (5kva) single phase control circuit transformer.

Thanks for your response.

 

[bob]

Re: Remote School Pump Station

With a distance of 950 ft you need to do some caculations to determine the right size EGC. The one shown in 250.122 will not be adequate. What size OCP and conductors are you using? Are you using PVC conduit?

[ July 09, 2003, 01:10 PM: Message edited by: bob ]

 

[tom baker]

Re: Remote School Pump Station

If both pumps can run at the same time the feeder size would be 14 A x 1.25 + 14 + 5kw/480= 41 amps.
If this is a feeder then a disconnecting means is required.
If this is a branch circuit then a disconnect and grounding electrode system is required- you'd perhaps bond to the sewer pipe, and ground rods.
Also review the rules for motor disconnects in section 430.102, and the definitions of feeder, branch circuit and multiwire branch circuit in Art 100.

Size your circcuit for no more than 5% VD at 41 amps. If you increase the ungrounded conductors for volt drop the equipment grounding conductor is increased in size by the same amount.

Also assured bonding is required for any raceway containing circuits over 250 volts to ground.

 

[tom baker]

Re: Remote School Pump Station

Originally posted by tom baker:
If both pumps can run at the same time the feeder size would be 14 A x 1.25 + 14 + 5kw/480= 41 amps.
If this is a feeder then a disconnecting means is required.
If this is a branch circuit then a disconnect and grounding electrode system is not required- you'd perhaps bond to the sewer pipe, and ground rods.
Also review the rules for motor disconnects in section 430.102, and the definitions of feeder, branch circuit and multiwire branch circuit in Art 100.

Size your circcuit for no more than 5% VD at 41 amps. If you increase the ungrounded conductors for volt drop the equipment grounding conductor is increased in size by the same amount.

Also assured bonding is required for any raceway containing circuits over 250 volts to ground.

 

[dcsva@aol.com]

Re: Remote School Pump Station

Bob, have a 50Amp/3P breaker in a panelboard located in the school.
I have anticipated using 1/0 for L1,L2,L3 and #6 for the ground.
2" sched-80 PVC conduit, with concrete poured on top for protection.
We are going to put a 100amp fused disconnect with 50amp fuses at the pump station mounted on a 4"x6" post set in concrete. The reason we are using a 100amp disconnect is that the lugs can handle the size wire we are using.

 

[tom baker]

Re: Remote School Pump Station

DCSVA-your equipment grounding conductor is too small see the note to Table 250.122. With a 6 AWG EGC for a 1/0, the ground fault return path impedance will be much too high to open the breaker at the source quickly- you want it to open in under 1 cycle. Your proposed installation is a violation of 250.4(A)(3).

 

[dcsva@aol.com]

Re: Remote School Pump Station

Tom,
In order to adhere to 250.122(b), the EGC shall be adjusted proportionately in circular mil area.

Standard 50amp feeder
#8 for L1,L2,L3
#10 for EGC

Adjusted for Voltage drop at 950FT
1/0 for L1,L2,L3
#1 for EGC

Is this right for adjusting the size of the EGC.
I obtained this from Table 8 in Chapter 9 of NEC

Increase of Line Conductors:
#8 has Area in Cir Mil of 16510
1/0 has Area in Cir Mil of 105600
an increase of 639.62%

therefor:
#10 with a Area in Cir Mil of 10380 increased by 639.62% would = 66393 Cir Mil.
#1 wire minimum for the EGC.

 

[don_resqcapt19]

Re: Remote School Pump Station

The circut from the school to the pump station is a feeder and not a branch circuit, A grounding electrode system will be required at the pump station.
Don

 

[bob]

Re: Remote School Pump Station

DCSVA
You comment ragarding adjusting the EGC in proportion to the increase in the phase conductors
is basically correct. However, the table 250.122
is for circuits of about 100 ft. Anything over that distance should be checked. The impedance of the EGC conductor maybe too great to permit the OCP to open and clear the fault in a timely manner,usually 1 cycle if possible, as Tom stated.
Don has pointed out that this is a feeder and
needs a grounding system rather that a EGC.

 

[don_resqcapt19]

Re: Remote School Pump Station

Bob,

Don has pointed out that this is a feeder and
needs a grounding system rather that a EGC.

I didn't say that an EGC is not needed. In this case there is a second metallic path (the phone line) between the two structures so an EGC is required by 250.32(B)(2) and a grounding electrode system is required by 250.32(A).
Don

 

[dcsva@aol.com]

Re: Remote School Pump Station

Don and Bob,

When you say a "grounding system"
Does this mean?
1. Provide a ground rod at the pump station
2. Bond/Connect the EGC from the school to the GEC at the pump station
AND
3. Make the fused disconnect at the pump station a service rated device.

 

[tom baker]

Re: Remote School Pump Station

A grounding system is more than a ground rod. Review section 250.50, 52 and 54.
However it often ends up being a ground rod, but it most cases 2 ground rods are needed, unless the resistance of a single rod is 25 ohms or less.
Are you doing the installation yourself?
277/480 volt circuits are unforgiving and require the highest standard of workmanship.

 

[dcsva@aol.com]

Re: Remote School Pump Station

Tom,
No, I am not doing the work myself, we are hiring an Electrical Contractor, I am trying to educate myself by asking questions so that I can understand what is involved and why. I really appreciate all the responses.
You referenced 277/480. We are not carrying the neutral because we do not have any 277volt loads.
I figured we could treat it like a motor when it came to whether to pull the neutral or not.

 

[tom baker]

Re: Remote School Pump Station

Congratulations on your desire for knowledge!

 

[dcsva@aol.com]

Re: Remote School Pump Station

Tom or anyone,
where can I get the formula to determine the proper size EGC? Also, any comments on the Mike Holt grounding book, I am looking at buying this book.

 

[bphgravity]

Re: Remote School Pump Station

The earlier calculation you made by proportionaly increasing your EGC to the increased size of the ungrounded conductor was the correct formula.

I would suggest purchasing every and all books on grounding that you can accuire. The Mike Holt grounding book is realy good and has alot of color pictures to give a nice visual of code specifications.

I would also suggest Tom Henry's grounding workbook, Soares grounding workbook, and the IEEE "green book". The American Electricians Handbook also provides good grounding discussion. You will be suprised how many different interpretations there are of grounding theory and code intent. However, studying all viewpoints and expert opinions provides the most complete and accurate information on the proper grounding methods.

 

[tom baker]

Re: Remote School Pump Station

Mike holts grounding and bonding text is simply outstanding! For the average electrician it is far better than soares.
Start with Mikes text, and get his video as well.