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Re: [TowerTalk] Grounding & Generator Questions

To: "Doug Rehman" <>,"'Towertalk'" <>
Subject: Re: [TowerTalk] Grounding & Generator Questions
From: "EUGENE SMAR" <>
Date: Thu, 4 Aug 2005 13:57:54 -0400
List-post: <>

     Your decision to supply the entire complex from TWO separate MG
(motor/generator) sets just escalated your design an order of magnitude.
You'll have to include a paralleling system of some kind to ensure that the
two generators are syncronized (in phase) with each other (running at the
same speed and phase within a couple of electrical degrees) before you close
them onto the loads.  This assumes you will feed the two loads with one
supply bus from the generators.

     If you decide to feed each meter/load from separate generators, you'll
now need to interlock the transfer switches so you don't backfeed the
utility feed from either generator.

     Asking us out here in Cyberland whether a two inch conduit is
sufficient to serve as a safety ground conductor is irrelevant.  Your local
electrical code holds sway in this design effort.  My experience has been
that you will need something called a derived ground system for your
generator complex.  I won't explain it here.

     BOTTOM LINE:  Have a licensed electrician or engineer design your
system for you.  At a minimum run your plans by the local AHJ (Authority
Having Jurisdiction) such as your permitting department.  Twenty-five KW is
an awful lot of generator capacity to fool around with.

Be safe and 73 de
Gene Smar  AD3F, P.E.

----- Original Message ----- 
From: "Doug Rehman" <>
To: "'Towertalk'" <>
Sent: Thursday, August 04, 2005 1:36 PM
Subject: [TowerTalk] Grounding & Generator Questions

> Below is a diagram of my house. (not to scale...)
> |
> |  Garage
> |
> |___________
> Meters|
> |Door
> |     Office/
> Driveway |      Shack
> |___________________
>                      Generators
> Tower
> The house has two electrical meters (main residence & in-law apt.) feeding
> two separate electrical panels. I run my business from the in-law apt. and
> also have the shack there. I'm going to set the house/office up for
> emergency power and from pricing generators, I can buy two 15 KW units for
> several thousand dollars less than one 25 KW unit.
> I've already done the study to know that it will take 25 KW for the
> house/office combination. I realize the 25 KW generator is probably a
> unit (liquid cooled versus air cooled, etc.), but the two 15 KW units will
> be easier to deal with (lighter to position, come with composite mounting
> pads versus building concrete pad, etc.). It also gives me backup to my
> backup. Other than hurricanes, we virtually never lose power so the
> generators won't get much run time.
> I'm planning on mounting the two auto transfer switches above the power
> meters (yes, they're NEMA 3- outdoor rated). My problem comes in routing
> power cables from the generators to the switches. I also have an issue
> connecting my shack ground to the power company ground at the meters. I
> think I can solve both problems with one conduit, but want to see if
> has any thoughts before I proceed.
> To get from the generators to the meters, I can either go straight up from
> each and through the attic or go up to the fascia and run the conduit
> the outside of the house. I'm leaning strongly towards going around the
> outside of the house. (I cannot bury the conduit because of the concrete
> driveway.)
> Would the metal conduit (probably 2" to accommodate the two lines from the
> generators) be satisfactory to use for connecting the shack ground to the
> power meter ground? Also, if you see any flaws in my plans, please let me
> know.
> Tnx,
> Doug
> _______________________________________________
> See:  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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