Enginator

[Nick]

Have you ever known something was a code requirement and then proceeded to do research biased on what you thought was right? Only to be wrong?
I am starting to detail a new job. It?s a central plant and information technology building for a new hospital. In the central plant there is an ?enginator.? A natural gas powered generator designed to run one chiller when electricity prices get to high. It is connected to two chillers via a series of transfer switches. Under normal conditions the two chillers run on utility power. If electricity rates climb the enginator starts and can run either chiller, not both.
Anyway the grounding system was all over the place on the drawings so I was putting together what was required. The enginator did not have a grounding electrode system. Even though it has a different name it is still just a generator (480/277Y) and is going to need to be grounded right? I look at the transfer switches and they are all 3 pole so my first thought is we are going to have to make sure the neutral point of the enginator is not bonded to the frame so we don?t create a down stream neutral to bond connection. All the feeds have only the phase conductors and an equipment grounding conductor so I am thinking all these feeds need a grounded conductor sized per 250.66 for a fault return path.
Then a light turns on in my head (thanks to someone else). We do not have any line to neutral loads so we are not pulling a neutral (obviously). By definition the enginator is now a separately derived system even with the 3 pole ATS? because there is no grounded conductor tying the service grounded conductor to the enginator neutral point. The enginator can have a neutral to ground bond and everything is just fine!

Am I missing something? Or have I got my head clear now.

PS: I realize since we are not serving line to neutral loads the system is not required to be grounded. I believe it was intended to be it was just missed.
Nick

 

[landelectric]

Re: Enginator

I can see from the text of your post that you are confusing the terms:

Grounded
v.
Grounding

This is an important distinction for interpreting the NEC

GROUNDED as in grounded conductor
meaning current carrying Neutral, white

Grounding as in Equipment
grounding conductor
meaning green, bare, earth, chassis, etc.

Eric Land

 

[Nick]

Re: Enginator

How am I Confusing the terms? I understand them and there difference very well. I re-read the post and it makes sense to me! Is my brain taking the weekened off already!?

 

[alex399]

Re: Enginator

"all these feeds need a grounded conductor sized per 250.66 for a fault return path. "

Is this the error you mean eric? He should say they need an equipment grounding conductor for a fault return path... No?

 

[Nick]

Re: Enginator

Well, the first part of that sentence states that the feeds have the phase conductors and equipment grounding conductors. Let me clarify.

When a 3 phase generator supplies a system through a 3 pole transfer switch and has an un switched grounded conductor supplying line to neutral loads the generator XO point cannot be bonded to the generator frame and equipment grounding conductor. Fault current when the generator is supplying the load would have to travel over the equipment grounding conductors through the main bonding jumper at the service switchgear and back to the generator XO over the grounded system conductor. All I was really saying was the absence of the grounded conductor in the feeders combined with a 3 pole transfer switch had me automatically thinking we did not have that fault return path and would have to add one sized per 250-66. It?s not often you see a generator that does not supply line to neutral loads. I have never seen grounding literature describe this scenario without it. Go figure.

 

[russ]

Re: Enginator

Nick:

I see what you mean.
You have to bond xo at the generator for a ground fault path. This will tie your your generator xo to the grounded conductor of the building service, thru the equipment ground between the two.
I think this is no diffrent, than bonding the grounded conductor of multiple transformers to the same grounding electrode. In this case I would think you should install a grounding conductor to the generator from the grounding electrode.

Russ

[ March 01, 2003, 08:21 AM: Message edited by: russ ]

 

[hornetd]

Re: Enginator

Is the generator Wye connected? Does the code require a grounded conductor to be run to the disconnecting means and bonded to it as it would be to a service disconnecting means?
--
Tom

 

[dereckbc]

Re: Enginator

Nick, with the generator neutral (Xo) bonded to the frame it will be considered SDS regardless if there is a neutral used or not. You will need a ground electrode connection and GEC. This is a common practice. Your fault path will be via EGC ran with generator phase conductor back to neutral bond.

[ March 01, 2003, 11:57 AM: Message edited by: dereckbc ]

 

[landelectric]

Re: Enginator

Hi Nick, Didn't mean to insult your knowledge,sorry.

I don't see any need to bond the XO point.

Eric Land

 

[gwz2]

Re: Enginator

Is all loads on the load side of the transfer switch(s) strictly 480V 3W?

If any of loads on the transfer switch(s) are for a 4W system, i.e. such as the OC devices or even a panelboard having a slash ( / ) marking such as 480/277V, then a grounded conductor must be part of the wiring method to that point.

 

[Nick]

Re: Enginator

Glenn,
Only one 2 chillers on the load side. No need for neutral. What you stated is why I thought there might be a problem. I was thinking that on a grounded wye system the grounded conductor had to be run anyway. Turns out that only applies to services. [250.24(B)]

 

[dereckbc]

Re: Enginator

Nick from what you describe there are only 3-phase loads therefore no need of a neutral. Configure the generator for SDS and use 3-pole transfer switch. Only problem you might have is access to the building ground electrode system to run a GEC from the generator.

Good Luck

DC