Overcurrent protection required for Transfer Equipment

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jcbabb

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Location
Norman, OK, USA
Is there a requirement (similar to 408.36) that requires an OCPD having a rating of not-greater-than that of the transfer equipment that it protects? I really feel like there should be, but I have not been able to find one.

Consider a situation where a generator has a 250 amp breaker feeding a 200 amp rated transfer switch. You could theoretically run a 250 amp feeder from genset-to-ATS, so the conductors would be protected. What I'm not sure about is if the overcurrent protection for the ATS is required by the NEC to be rated equal-to or less-than that of the ATS, much like the requirement in 408.36 regarding ocpd for panel boards.
 
Dsg319

Dsg319

Senior Member
Location
West Virginia
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Wv Master “lectrician”
Is the transfer equipment also the service disconnect?

If so what’s the difference between that and say a 600amp service into a gutter, with taps made from the conductors to 6 different 100amp meter mains.

I could be wrong but I would lean to think sizing the conductors to the transfer equipment size . 200amp conductors. Maybe have to follow tap rules?

Good question. I cannot answer, looking forward to see what others have to say.
 
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jcbabb

Member
Location
Norman, OK, USA
Dsg319 said:
Is the transfer equipment also the service disconnect?

If so what’s the difference between that and say a 600amp service into a gutter, with taps made from the conductors to 6 different 100amp meter mains.

I could be wrong but I would lean to think sizing the conductors to the transfer equipment size . 200amp conductors. Maybe have to follow tap rules?

Good question. I cannot answer, looking forward to see what others have to say.
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In this particular case, the ATS is the service disco, but I was hoping to make the question more broad-based to also cover the instance where it is not. You make a good point about the service taps...even though this is not a tap, I'll have to think more on that.
 
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jcbabb

Member
Location
Norman, OK, USA
ron said:
404.14 indicates that switches shall be used within their ratings.
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Good catch with the reference to Article 404 - thanks! The load is under the rating of the transfer switch, so it is technically being used within its rating. However, that section says nothing with respect to OCPD requirements.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
ron said:
404.14 indicates that switches shall be used within their ratings.
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that is for snap switches.

No a transfer switch need not be protected at its rating unless required by the manufacturer (I could find no such requirement for the Kohler RXT series). HOWEVER, depending on the available fault current, you may need a certain type and size breaker to meet the SCCR rating of the switch, and yes SOME OF THESE ARE ALLOWED TO EXCEED THE SWITCH RATING. For example, again with the kohler RXT series 400A ATS, for certain breakers they can be up to 800A. See page 5 here if you want to see for yourself:

https://www.electricgeneratorsdirect.com/manuals/kohler-5c25b963ca4f07861b2082a56eb0e5de.pdf)
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Here is another protection sheet for cummins transfer switches. As you can see, depending on the fuse class, the fuse can be up to twice the rating of the switch. The Document that lists the specific breakers and their allowed ratings is very hard to find. I actually have a call into cummins right now to try and get the document as I am needing it to design a 600A ATS system.
 

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kwired

kwired

Electron manager
Location
NE Nebraska
I see no requirements in art 404 that deal with overcurrent protection.

Listing of a specific device could still require it though.

How often do we install 15 amp switches on 20 amp circuits? Is allowed if the load is not over 15 amps.

Even have installed non fused safety switches on circuits with higher OCPD on them. Thing is they are usually motor disconnects and still falls within rated HP of the switch. One more common one I run into is a 30 amp non fused switch supplied by a 60 amp breaker for 20 HP 480 volt motor applications.
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
electrofelon said:
Here is another protection sheet for cummins transfer switches. As you can see, depending on the fuse class, the fuse can be up to twice the rating of the switch. The Document that lists the specific breakers and their allowed ratings is very hard to find. I actually have a call into cummins right now to try and get the document as I am needing it to design a 600A ATS system.
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That table is to achieve short circuit protection. I don't believe it helps with overcurrent protection, unless you bring it down to the switch rating.
 
S

shortcircuit2

Senior Member
Location
South of Bawstin
I do not see any requirement in the Code for overcurrent protection of transfer equipment used under article 702 of the 2020 NEC. New section 230.82(11) allows meter-mounted transfer switches connected to the supply side of the Service Disconnect. The transfer switch only needs to have a short-circuit current rating for the available fault current.
 
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