Do I need a 4 pole ATS?

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What is the secondary voltage of the transformer. i assume it is 208/120 3 phase 4-wire to match the service voltage.
 
steve66 said:
: then I don't see why you would bond the transformer ground and neutral. The neutral (at the transformer) would be solidly connected to the service neutral, and the ground (at the transformer) would be solidly connected to the service ground.

Seems to me the N-G bond at the service would be the only one required.

Steve
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Steve, the transformer is required to have the bonding jumper installed at the source of the SDS or at the first OCPD.
 
Here is a copy of an email regarding a 3 vs 4 pole transfer switch dated 1/18/08. Looks like the 4 pole sw is more by design unless the gen-set is required to have GFPE.

Jim

I?m still on the (standby, emergency, etc..) generators and transfer switches (excluding transformers). The question this time is there ever a required need for a separately derived system or would it be installed only by design? If by design what is be the purpose? Would not bonding the neutral winding to the generator frame accomplish the same purpose even when GFPE is installed?


Thanks in advance and have a good day.

(response)

Hi Mike:

Sorry for the delay in responding. I am currently in Paris at some meetings.

Design is generally the biggest driver. Sometimes products drive it - such as when a standard transformer is installed.

Sticking to your generator discussion.... One driver for separately derived can be GFPE. If you have the generator downstream from the GFPE and you rebond the neutral, you desensitize the GFPE. Also, If you have GFPE on the generator side as well - it can be a driver.l

Recognize that if you are putting the generator back at the service, much of the discussion we had previously repeats itself.

Best,

Jim
 
qcroanoke said:
That is correct.
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qcroanke, two things are happening with your arrangement. One is your loads from the generator are 3-wire and will not have a fault current return path if it is not set up as a SDS. The second is the transformer,. Since the secondary is a 4-wire the bonding jumper has to be installed. Since the transformer is before the transfer switch, the bonding jumper is phyiscally tied to the main bonding jumper in the service via the neutral and the ground. Although they are ran seperately they both have a point at which they are together. Without breaking the neutral in the transfer switch, they become parallel. the only way to eliminate this is to switch the neutral or install it on the load side of the transfer switch.

Rick
 
I might add that the transfer switch for the 240 volt 3-wire only loads, only needs to be a solid neutral switch since there is not any neutral conductors involved.

Rick
 
RUWired said:
Steve, the transformer is required to have the bonding jumper installed at the source of the SDS or at the first OCPD.
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If it has a solid neutral connection to the utility service, then I don't believe it is an SDS, so the bonding jumper isn't required at the transformer or the first OCPD.

Steve
 
steve66 said:
If it has a solid neutral connection to the utility service, then I don't believe it is an SDS, so the bonding jumper isn't required at the transformer or the first OCPD.

Steve
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Steve, that is thinking out of the box, but i don't think you can get around the definition of SDS. "A premises wiring system whose power is derived from a source of electric energy or equipment other than a service".(period)IMO since the transformer meets the definition, 250.30 applies.

Rick
 
RUWired said:
Steve, that is thinking out of the box, but i don't think you can get around the definition of SDS. "A premises wiring system whose power is derived from a source of electric energy or equipment other than a service".(period)IMO since the transformer meets the definition, 250.30 applies.

Rick
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I agree the Xfmr is a SDS.
 
RUWired said:
Steve, that is thinking out of the box, but i don't think you can get around the definition of SDS. "A premises wiring system whose power is derived from a source of electric energy or equipment other than a service".(period)IMO since the transformer meets the definition, 250.30 applies.

Rick
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But read the next sentence also:

"Such systems have no direct electrical connection, including a solidly connected grounded circuit conductors, to supply conductors originating in another system."

Steve
 
steve66 said:
But read the next sentence also:

"Such systems have no direct electrical connection, including a solidly connected grounded circuit conductors, to supply conductors originating in another system."

Steve
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I agree. If the grounded conductor off the transformer secondary is solidly-connected to that of the service system, it is not an SDS.
 
I'll have to agree with both Steve and Smart$ that it is directly tied to the service grounded conductor, but i bet it would be a hard sell to any inspector. Currently there are not articles writen for transformers if they are not a SDS. I think if there were it would closely follow article 250.35 filling in transformer instead of generator being that the transfer switch is dictating whether or not it is a SDS.
Rick
 
Smart $ said:
"Tomorrow" is here....

I drew up a one-line but it doesn't seem to indicate enough info...

So I drew up a connection diagram...

View attachment 3045
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Ok, that is it in a nut shell. So please explain how we can take a delta transformer, create a wye system and not have a seperately derived system. I see your point but like Rick said I believe I'll have a hard time selling it to the inspector.
 
I studied this diagram for a long time and almost agreed. Generator neutral is bonded but no neutral loads, this part okay. The transformer is in fact a separately derived system with neutral loads. The requirement to bond the grounded conductor to the grounding conductor at the source of the first disconnect of a SDS seems clear, 250.30. If this is true a 4 pole transfer switch would be required. As my grandmother would say “I have to study on this”. I'm not convinced either way at this time.

Nice drawing!
 
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Sorry for all the post but this is a very interesting thread with a lot of good comments.
In my previous post I stated the transformer is in fact a SDS. That is the old school of thinking in me. I often compared generators and transformers as the same. With a generator the system is defined by the transfer switch, wouldn?t a transformer be defined in same? 250.30 addresses SDS, nothing in 250.30 say a transformer is a SDS. With this I?m back to a 3 pole transfer switch.

opinions?
 
qcroanoke said:
Ok, that is it in a nut shell. So please explain how we can take a delta transformer, create a wye system and not have a seperately derived system. I see your point but like Rick said I believe I'll have a hard time selling it to the inspector.
Click to expand...

Not if you demonstrate that bonding the transformer will turn the entire grounding system into a parallel path for neutral current...

View attachment 3054

Yes, you could eliminate that problem by using a 4-pole ATS, but you are not required to do so.
 
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