Service Rated ATS vs. ATS

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westgbazo

Member
Location
MI
Occupation
EE
Good Morning!

I have a question about service rated vs. non-service rated automatic transfer switches.

Here's the situation. Industrial site owns an existing 34.5kV fused switchgear lineup that will serve a new 3000kVA transformer (34.5kV-480V). The utility meter/demarcation point is before the existing MV switchgear.

This transformer will connect to a non-service rated ASCO 7000 ATS (which does not have a main breaker on the normal side of the ATS). The ATS then connects to the main breaker of the 480V switchboard.

My question is: Do I need a service entrance rated ATS between the transformer and the 480V switchboard main breaker? Isn't my 'service disconnecting means' the MV fused switch feeding the transformer?

Thanks in advance for your help.
 
What overcurrent protective device is protecting the normal side of the ATS? If there is overcurrent protection on the normal side, a standard ATS is acceptable. If there is no overcurrent protection on the normal side, a service rated ATS is required.
 
d0nut said:
What overcurrent protective device is protecting the normal side of the ATS? If there is overcurrent protection on the normal side, a standard ATS is acceptable. If there is no overcurrent protection on the normal side, a service rated ATS is required.
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No overcurrent protection between the transformer and the ATS. There are fuses on the primary of the transformer and a main breaker in the 480V switchboard that the ATS feeds.
 
It is not service conductors that it is switching so service rated shouldn't really be an issue. Possibly could still require overcurrent protection or even GFPE? Might depend on what primary side protection is as well?
 
kwired said:
It is not service conductors that it is switching so service rated shouldn't really be an issue. Possibly could still require overcurrent protection or even GFPE? Might depend on what primary side protection is as well?
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Primary side protection is 65E fuses in an S&C high profile switchgear lineup.

And that was my thought on service conductors. The service conductors would be from the utility's switch to the customer owned MV switchgear, so the ATS wouldn't be switching "service conductors".
 
In this case, the "service entrance" ATS is really just an ATS with a utility side breaker providing secondary protection for the MV-LV XFMR. Usually the MV-LV XFMR is outside and the secondary conductors can run long and need to land in a breaker per 240.21(C)(4), which would be your service entrance ATS, which is really just a convenient breaker position at the interior of the building inside nearest the entrance of the secondary conductors.
 
The rules in 240.21(C) will require that you use a service rated ATS. The 10' transformer secondary conductor rule requires that the secondary conductors land in equipment that contains an OCPD, the 25' transformer secondary conductor rule requires that the secondary conductors land on an OCPD.
 
don_resqcapt19 said:
The rules in 240.21(C) will require that you use a service rated ATS. The 10' transformer secondary conductor rule requires that the secondary conductors land in equipment that contains an OCPD, the 25' transformer secondary conductor rule requires that the secondary conductors land on an OCPD.
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Would 240.21(C)(3) be applicable here? This is an "Industrial Installation" and the secondary conductors are less than 25ft between transformer and ATS. Other conditions also satisfied.
 
You still need an OCPD to protect the ATS. You cannot use an OCPD on the primary side of a transformer with a delta primary and a wye secondary to protect equipment on the transformer's secondary.
 
d0nut said:
You still need an OCPD to protect the ATS. You cannot use an OCPD on the primary side of a transformer with a delta primary and a wye secondary to protect equipment on the transformer's secondary.
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Isn't that the intent of 240.21(C) though? To permit connection "to a transformer secondary, without overcurrent protection at the secondary" under the six different scenarios described in [240.21(C)(1) through (6)]?

My situation satisfies all conditions required in "Scenario 3" aka 240.21(C)(3), so secondary overcurrent protection should not be required between the transformer and the ATS. There is secondary protection then at the 480V switchboard (main breaker). That's my interpretation, at least.

Thanks everyone for the replies so far. I appreciate the discussion!
 
240.21(C)(3) is "For the supply of switchgear or switchboards in industrial installations only...". You are landing on an ATS, not switchgear or a switchboard, so I don't believe that section would apply to your scenario.
 
d0nut said:
240.21(C)(3) is "For the supply of switchgear or switchboards in industrial installations only...". You are landing on an ATS, not switchgear or a switchboard, so I don't believe that section would apply to your scenario.
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NEC definition of switchgear: "An assembly completely enclosed on all sides and top with sheet metal and containing primary power circuit switching, interrupting devices, or both, with buses and connections. The assembly may include control and auxiliary devices. Access to the interior of the enclosure is provided by doors, removable covers, or both."

The ATS does not contain interrupting devices but that is not a requirement to meet this definition.

However, I can see the other side of the argument that "transfer switch" has its own definition, therefore its excluded from 240.21(C)(3). In my opinion, I think it could also fit the definition of switchgear.
 
westgbazo said:
NEC definition of switchgear: "An assembly completely enclosed on all sides and top with sheet metal and containing primary power circuit switching, interrupting devices, or both, with buses and connections. The assembly may include control and auxiliary devices. Access to the interior of the enclosure is provided by doors, removable covers, or both."

The ATS does not contain interrupting devices but that is not a requirement to meet this definition.

However, I can see the other side of the argument that "transfer switch" has its own definition, therefore its excluded from 240.21(C)(3). In my opinion, I think it could also fit the definition of switchgear.
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What does the load side of the ATS connect to?
 
don_resqcapt19 said:
The rules in 240.21(C) will require that you use a service rated ATS. The 10' transformer secondary conductor rule requires that the secondary conductors land in equipment that contains an OCPD, the 25' transformer secondary conductor rule requires that the secondary conductors land on an OCPD.
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Doesn't require a service rated ATS though. Does require certain overcurrent protection and a service rated ATS is one way to get that.
 
westgbazo said:
The main breaker of a 480V switchboard.
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Should the switchboard maybe been ordered with ATS as part of it?

There likely needs to be GFPE on this as well. 3000kVA should definitely get you in that range, unless this secondary tap is not set up for anything over about 1/4 the capacity of the transformer. But even there you would likely need to pay closer attention to SCC and interrupt ratings.
 
kwired said:
Doesn't require a service rated ATS though. Does require certain overcurrent protection and a service rated ATS is one way to get that.
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It does if you want to land the transformer secondary conductors directly on the normal terminals of the ATS. That is my understanding of the proposed installation.
 
d0nut said:
You still need an OCPD to protect the ATS. .
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I just wanted to emphasize and expand on this. Although there is no specific code section requiring an ATS be protected at its rating by an ocpd, in practice you will find that a non-service rated ATS WILL require some sort of protection ahead of it per the manufacturer. Typically there will be multiple options, depending on the type of ocpd, OCPD size and available fault current. Typically no ocpd is not an option.
 
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