Tap clarification

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nickelec

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I have a 4000a switchboard. It has a main switch fused at 4000a. Bus is fully rated throughout the switchgear.

I have a 4000a/3600a fused generator panel that connects directly to the bus of the 4000a switchboard via lugs and 600mcm thhn conductors. 4000a main and generator panel have a Kirk key interlock so power can never be sourced from both

Is this considered a tap? Connection point on bus to the generator panel is over 25ft.

Do the conductors feeding the generator panel need to be rated at 3600a for the generator fuse, or 4000a for the main fuse? Conductors connected to the 4000a rated bus will be energized during normal operations.


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If the Kirk key interlock prevents the grid from ever energizing those conductors, so that they can only be energized from the generator, then the grid-side OCPD is irrelevant to sizing the conductors (and hence to whether they are a feeder tap). Only the generator-side OCPD matters.

Cheers, Wayne
 
wwhitney said:
If the Kirk key interlock prevents the grid from ever energizing those conductors, so that they can only be energized from the generator, then the grid-side OCPD is irrelevant to sizing the conductors (and hence to whether they are a feeder tap). Only the generator-side OCPD matters.

Cheers, Wayne
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Under normal operations those conductors are energized from the bus. The Kirk key only pervents personel from closing both switches at the same time .

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nickelec said:
Under normal operations those conductors are energized from the bus. The Kirk key only pervents personel from closing both switches at the same time .
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So if those conductors could have a fault and are protected from the grid only by the 4000A OCPD, they need to comply with 240.4 with respect to the 4000A OCPD size. That rules out 240.4(B), so they would need to comply with one of the tap rules in 240.21(B), or have an ampacity of 4000A.

Cheers, Wayne

Edit: I'm still a little unclear on the one-line diagram. If it is Grid - Interlocked Switch 1 - Bus Serving Loads - Interlocked Switch 2 - Generator, then the above would not apply to the conductors between Interlocked Switch 2 and the Generator, as my first answer would apply to those.
 
Basically conductors must be protected at their source, by OCPD or tap rules.
What is the source of these conductors? It sounds like they are connected to a 4000A source and then left 'open' at the 3600A transfer device.
 
jim dungar said:
Basically conductors must be protected at their source, by OCPD or tap rules.
What is the source of these conductors? It sounds like they are connected to a 4000A source and then left 'open' at the 3600A transfer device.
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2. Jim. That is correct. Conductors are rated for the generator breaker of 3600a and I have no concern during emergency power operations. My concern is only during normal power (although never a load connected to the conductors) the conductors are not rated for the 4000a main.


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nickelec said:
My concern is only during normal power (although never a load connected to the conductors) the conductors are not rated for the 4000a main.
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An appropriate concern, from the info so far your only option for NEC compliance is if 240.21(B)(4) applies to the conductors between the bus and the 3600A generator OCPD.

Cheers, Wayne
 
wwhitney said:
An appropriate concern, from the info so far your only option for NEC compliance is if 240.21(B)(4) applies to the conductors between the bus and the 3600A generator OCPD.

Cheers, Wayne
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10' is not possible due to field conditions.

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nickelec said:
10' is not possible due to field conditions.
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Not sure which NEC version you checked, in the 2017 and 2020 NEC 240.21(B)(4) starts off:

"(4) Taps over 7.5 m (25 ft) Long. Where the feeder is in a high bay manufacturing building over 11 m (35 ft) high at walls and the installation complies with all the following conditions:"

Cheers, Wayne
 
Believe it or not we're still on 2008. We're looking at leaning more towards. Taps of unlimited length. Basically the engineers and clients are trying to tell us that this is not a tap and to run conductors and conduit for 3600 amps

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Under the 2008 I can't see any justification for the conductors not to be rated based on the swithgear breaker as, even though they are supplied by the generator, when the kirkkeys are in "normal" they are only protected by the switchgear main.
You wound likely get some relief by applying 240.92(B)
 
augie47 said:
Under the 2008 I can't see any justification for the conductors not to be rated based on the swithgear breaker as, even though they are supplied by the generator, when the kirkkeys are in "normal" they are only protected by the switchgear main.
You wound likely get some relief by applying 240.92(B)
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So we're in agreement that this is definitely a tap. and should be sized per the breaker in the switchgear?

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nickelec said:
So we're in agreement that this is definitely a tap. and should be sized per the breaker in the switchgear?

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IMO, Yes
IT meets the definition. The fact that the generator breaker might be "Off" is not relevant. The conductors are protected by the switchgear main during "normal" operation.
 
nickelec said:
2. Jim. That is correct. Conductors are rated for the generator breaker of 3600a and I have no concern during emergency power operations. My concern is only during normal power (although never a load connected to the conductors) the conductors are not rated for the 4000a main.
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My point is: if you ignore the generator and the isolation switch, these conductors are only protected, against short circuit and ground faults, by the 4000A switch.

IMO, tap rules apply to these conductors.
 
This is from the engineers.

There is no tap involved on the generator side feeders as the main 4000A is set to trip at 3600 amps and the load side feeder ampacity is rated for 3780 amps. It will not be required to concrete encase the generator feeders as these are not considered service conductors.

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