Power to new Subpanel


  #1  
Old 08-28-01, 08:44 AM
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I am running service from an existing panel in my garage approximately 150 feet to a new pump house, which will be used to house a well pump and irrigation equipment. I need some advice on whether the neutral bus should be bonded to the ground at the service panel in the pump house. Here's the present set up --
Power comes into the meter outside the wall of the shop. From the meter, power is run to a panel with one 200 Amp breaker (outside). Directly behind the 200 Amp breaker panel (inside the shop now) is a service panel which houses the circuits for the shop (the neutral bus is not bonded in this panel). I'm planning to install a 100 Amp circuit breaker in the service panel in the shop and run two #2 wires as well as a #4 for the neutral to the pump house in underground conduit. At the pump house, the wires will feed a service panel that will have one 30 amp breaker for the pump and one or two 20 amp (120v) circuits for plug-ins, lights, etc.

Does the neutral bus at the pump house service panel need to be bonded to the ground? I am not running the ground from the panel in the shop, rather, I am grounding to the well casing outside the pump house and two 8 foot ground rods.

I've also heard that I need to run a ground from the subpanel in the garage to the subpanel in the pump house, but this doesn't seem quite right to me if I can ground the subpanel at the pump house (the garage is 150 feet away).

Thanks for your help.

 
  #2  
Old 08-28-01, 09:43 AM
J
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The only place in your electrical system where neutral is bonded to ground is in your main service disconnect enclosure. Any panel installed downstream from your main service disconnect must have the neutrals & grounds completely separated. Whoever installed the main breaker (outside) and the 200 amp breaker panel (inside the shop now) knew this and did it correctly. So the answer is no, don't bond them in the pump house panel.

As for the reduced neutral size, I'm wondering why? There is a calculation in Article 220 you can use, but many people recommend full sized neutrals anyway. You don't save an awful lot of money by dropping down two sizes on 150' of wire. Is this based on a direct-buried cable set you either bought already or have your eye on?

If you're providing a separately derived grounding electrode system (two rods to the well casing), then you do not have to bring your own ground from the shop panel. Keep in mind though that if you use two rods they are required to be a minimum of 6 feet apart.

Good luck.

Juice
 
  #3  
Old 08-28-01, 11:02 AM
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I'll cite the NEC article I think is relevant---Art.250-32 2 structures supplied from a common service,(b)grounded systems, (2)grounded conductor:where(1)an equiptment grounding conductor is not run to the structure and (3)GFI protection is (not at) the common service,the grounded circuit conductor(Neutral)run with the suppply to the structure shall be connected to the structure dis-connecting means and to the grounding electrodes and shall be used for bonding and grounding of equipment.I construe that 250-32 allows the option of a seperate grounding system at the pump house.One purpose of grounding is to have all metallic enclosures at ground potential.I suggest using #4 bare wire for connecting to the ground rods.IN THE trench drive a ground rod,say,10 ft. from the pump house and another 20 ft. past the first GR.The deeper the GR's and the more bare #4 wire in contact with the earth,the better.The other purpose of ground wires is to conduct fault-currents.With the Neutral connected to Ground at the pump-house the resistance of the fault-current path is minimal.You'r astute de-rating the Neutral-if you "balanced" a 120 volt 10 kw load the Neutral current would be Zero.
 
  #4  
Old 08-28-01, 01:51 PM
resqcapt19
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If the piping from the pump house to the house is metallic or if there are any other metallic paths between these buildings, you are not permitted to rebond the neutral. If this is the case an EGC must be installed with the circuit conductors between the 2 buildings. A grounding electrode is still required at the pump house and must be connected to the EGC conductor, but not the grounded (neutral) conductor. If there are no other metallic paths, a 3 wire circuit is permitted. In this second case the grounding electrode conductor at the pump house must be connected to the grounded conductor at this second building.
Don(resqcapt19)
 
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Old 08-28-01, 03:07 PM
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Thanks Don; I stand corrected.I omitted 250-32-(b)-(2)-Grounded Conductor, condition (2)of 3 conditions which is "there is no continuous metallic paths bonded to the grounding system in both structures." With appreciation, SM
 
 

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