Installing a 20KW Generator

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  #1  
Old 09-24-12, 08:36 AM
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Installing a 20KW Generator

Hi All,
I've done a bit of homework on connecting up a 20KW standby generator and was hoping that one of the experts here can perhaps validate my plan;

  • I will run 3 copper 3/0 gauge THHN conductors from my meter pan in 2" sched 40 PVC to my 200A Service Entrance ATS. The run is about 8 feet and in a protected area.
  • In installing my 200A Service Entrance rated ATS (Cummins Onan RSS200-6869), I need to bond neutral AND ground at that point and isolate them in my main load center where they are currently bonded together. I will simply move the #4 bare stranded ground that goes to the two ground rods and plumbing from the main load center to the ATS bonding lug.
  • I will run new #4 bare stranded from ATS bonding lug to Main Load Center ground. The feeder from the ATS to the Load Center (panels are adjacent) will be 3/0 THHN copper in 2" sched 40 PVC.
  • Wiring to GenSet will be run in 1" (for low voltage) and 2" (for line voltage) sched 40 PVC. Distance from panel is approx. 70 feet.
  • Genset is 20KW Cummins Onan RS20A and has a 100A breaker at the genset. I plan to run 3 individual #3 THHN Copper conductors and a #4 ground. **An outstanding question I have is if the ground needs to be insulated or if bare is permissible. The conduit is underground. I don't think it matters but want to be sure - I have a spool of #4 bare, so I am inclined to use that.

Does this all sound OK? Your input and/or advice is greatly appreciated!
 

Last edited by raywbo; 09-24-12 at 08:49 AM. Reason: text formatting
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  #2  
Old 09-24-12, 09:54 AM
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Welcome to the forums!

I plan to run 3 individual #3 THHN Copper conductors and a #4 ground. **An outstanding question I have is if the ground needs to be insulated or if bare is permissible. The conduit is underground.
No bare conductors may be run in conduit. The EGC from the genset needs to be insulated green.

How is load shedding accomplished to allow your 200A service to be backed up by a generator supplying 100A? I'm guessing, since your ATS is integrated with the Cummins Onan RS12000 genset, which only supplies 12KW, that this must be taken care of internally.
 
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Old 09-24-12, 05:53 PM
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Thanks for your reply Nashkat1,

The generator is rated for 20KW.. The load shedding is accomplished by opening up NC relays, which are managed by a "smart" controller in the generator. The genset opens the circuits on start up, monitors the load and closes them one at a time. If the load threshold reached 95% over a sustained period ot time, it sheds them off. There is some other logic built in to the load management and load monitoring, but that's the general idea.

I was going to use the two connections for my AC compressors (low voltage themostat wires). All the connections come back to a terminal block in the ATS.

So other than needing an insulated (green) ground instead of bare, does everything else in my plan soud proper?

I really appreciate your feedback.. Thanks!
 
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Old 09-24-12, 07:00 PM
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I will run new #4 bare stranded from ATS bonding lug to Main Load Center ground. The feeder from the ATS to the Load Center (panels are adjacent) will be 3/0 THHN copper in 2" sched 40 PVC.
I would run an insulated green #6 ground in the 2" conduit with the 3 - 3/0 THHN conductors and not run an external bare ground.

Wiring to GenSet will be run in 1" (for low voltage) and 2" (for line voltage) sched 40 PVC. Distance from panel is approx. 70 feet.
I believe I'd run no fewer than two 1" conduits (3/4" would be sufficient), one for low voltage control wiring and one for at least one 20A 120V circuit for the battery charger and a GFCI protected receptacle. Will there be a block and/or battery heater? If so, you probably need two 120 volt circuits. Will there be a remote annunciator? Some manufacturers are now using wireless annunciation, but hardwired annunciators sometimes need 10 to 12 wires.

I will simply move the #4 bare stranded ground that goes to the two ground rods and plumbing from the main load center to the ATS bonding lug.
#6 copper is all you need to the ground rods, but #4 copper is fine since you already have it. A #4 copper or #2 aluminum is, however, required to where the water service enters the house, but if any of it is outdoors you cannot use aluminum. Don't forget to jumper around water meters and pressure reducing valves.

How much calculated load do you have? If calculated load is less than the 100 amp output of the generator, load shedding would not be required.
 
  #5  
Old 09-24-12, 09:31 PM
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CasualJoe,
Your response and comments are much appreciated! There is indeed a block and alternator heater on this model and I was planning to run stranded #12's along with the 3- #3's and #6 ground.

I have so many holes coming into the utility area of my house (elec service, phone, cable, A/C lines, propane, Fuel Oil, sump pump, water softener discharge line and two more for genset!) so my rational here is to avoid another one, although running separate would be preferred otherwise.

In my 1" conduit I am running the 2 factory made wire harnesses; one for the remote controller/monitor and the other that runs to the terminal block in the ATS. The 2amp battery charger is in the ATS housing, so this harness carries that back in low voltage along with the start and load management wiring.

I am on a well, which is piped to the house in PVC, however the copper plumbing will be bonded to the ground in addition to the 2 outdoor rods.

My running load should be under 100 amps - we would be diligent about how and when heavy loads are used when on standby power. I doubt I'll initially set up the load shedding until I know I need it or have nothing else to do someday

I do have another question if you don't mind.. In running the 120V circuit for the block heater and perhaps a GFCI outlet, do I need to run a separate ground from my main panel housing the circuit breaker or can I share the ground from coming the ATS ?

Thanks again.. much appreciated!
 
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Old 09-25-12, 07:13 AM
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In running the 120V circuit for the block heater and perhaps a GFCI outlet, do I need to run a separate ground from my main panel housing the circuit breaker or can I share the ground from coming the ATS ?
Run 1 insulated ground in each conduit.
 
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Old 09-25-12, 06:38 PM
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I was planning to run stranded #12's along with the 3- #3's and #6 ground
That will work, but I don't usually recommend it. It's usually a royal PITA to run the small #12s with the larger conductors.

I would agree with Nash too, run an insulated ground in each conduit.
 
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Old 09-25-12, 08:15 PM
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Thanks to both of you. Your insights are much appreciated.
 
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