EGC and GEC in an Enclosed Double-Generator Unit with ATS

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nsidebottom

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Alaska
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EE
Hi all,

I was wondering if anyone has some input on a somewhat strange system I am dealing with. I'm working on a project where we have two generators (4-wire three phase) in a conex shipping container meant to operate as a prime+backup power set. Both generators are wired into an ATS and are sitting on top of a large fuel tank+ tank containment which is itself bolted to the wood floor of the conex.

The ATS has a switched neutral, so each genset is an SDS and I am bonding the neutrals to grounds in the generator boxes on both units. Each genset has its own OCPD before running to the ATS which also has a breaker in it. EGC's run from the generators to each of the OCPD boxes for the individual generators, then EGC's run from those boxes to the ATS.

To my understanding this is all correct (please let me know if it isn't). My main concern is how to get the system setup with a proper GEC. From what I understand the GEC should be connected at the first disconnect. My intuition tells me I should connect it at the ATS, but I am not sure if this qualifies as the first disconnect since there are OCPD's upstream of it. I am also wondering if I should run EGC's from the generator tank/containment to the metal walls of the conex, then ground the ATS to the conex. If that is the case, would it be possible for me to terminate the GEC on the outside of the conex where a rod can be driven into the ground?

The ATS is on the interior of the conex, so being able to run the GEC from the outside of the conex would make things much simpler for us.

Also curious to know if any of this changes whether there is power on site or not. I don't believe they have utility there, but if they do, would the proper way to do the GEC be to run a strap from the ATS in our conex unit to the GEC that they would already have in the building from utility?

--Nicolas
 
Your grounding electrode conductor is going to connect wherever your neutral is bonded on one end and to the grounding electrode on the other end. I don't think the conex qualifies as a grounding electrode.

The ATS needs an EGC to a lug or a buss tagged to the enclosure. The metal walls of the conex should be bonded by virtue of the metal electrical equipment mounted to it.
 
It would connect at both generators main breaker line side neutral. The breakers are probably ground fault protection, so you definitely do not want to terminate the GEC at the transferswitch.
 
ActionDave said:
Your grounding electrode conductor is going to connect wherever your neutral is bonded on one end and to the grounding electrode on the other end. I don't think the conex qualifies as a grounding electrode.

The ATS needs an EGC to a lug or a buss tagged to the enclosure. The metal walls of the conex should be bonded by virtue of the metal electrical equipment mounted to it.
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The interior of the conex was spray-foamed and has had wood installed over top to insulate it for arctic conditions, so everything that is bolted to the walls for the most part is in wood, not steel. The ATS enclosure has an EGC running from the gensets and their breaker boxes to the ground lug in the ATS box.

As for the GEC connection, would that be in the transfer switch since the gensets are both SDS's, or should a GEC be run for each generator running from the MBJ to the ground rod, so I would have two separate GEC's?
 
hillbilly1 said:
It would connect at both generators main breaker line side neutral. The breakers are probably ground fault protection, so you definitely do not want to terminate the GEC at the transfer switch.
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So two separate GEC's both originating from the MBJ at each respective generator (before the breaker), then going to their own ground rods?
 
hillbilly1 said:
They both go to the same grounding electrode(s)
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What's the difference between that and running the GEC from the ATS enclosure ground lug if the ATS enclosure is connected to the generator case via EGC's? The generator case would be the aforementioned "location before the breaker", so effectively the same spot electrically speaking, no?

Apologies for the questions, I'm just new to NEC regulations as I usually do more equipment design and PCB's as opposed to installation of power electronics.
 
nsidebottom said:
What's the difference between that and running the GEC from the ATS enclosure ground lug if the ATS enclosure is connected to the generator case via EGC's? The generator case would be the aforementioned "location before the breaker", so effectively the same spot electrically speaking, no?
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GECs and EGCs are for different things. A EGC is all about clearing a fault and is what you need a bonding jumper for. GEC is simply a connection to the earth. They both end up at the main bonding jumper or system bonding jumper. You also don't want a parallel path for normal current so you put you GEC at the same place as your bonding jumper.
 
ActionDave said:
GECs and EGCs are for different things. A EGC is all about clearing a fault and is what you need a bonding jumper for. GEC is simply a connection to the earth. They both end up at the main bonding jumper or system bonding jumper. You also don't want a parallel path for normal current so you put you GEC at the same place as your bonding jumper.
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Thanks!

I'll go ahead and run the GEC from the MBJ's. Also, out of curiosity, what's with the "winged horses" thing I've seen on this forum with so many people referencing it as a location?
 
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