Tap Rules: Questions about Installation Details

[Spudley DoWright]

Hello Everyone,
I have often found the answers to questions on this forum for many years, and I have now joined.
I have been on a new assignment: Inspecting installation details in a facility that has been in place about 35 years.
My question is regarding Tap Rules:

Article 240..21 (B)(1-4) Feeder Taps, is being used to scrutinize an existing installation, where a fused bus plug, with Class RK5, FRS-R-100 fuses are installed, feeding #3 THHN to a Junction Box, where the #3 is then tapped in a four-way, with taps of 1#4 to a 60 amp fused disconnect and two #10 THHN to two separate 30 amp fused disconnects.
The fusing in the disconnects is appropriate as OCPD for the tap conductors (under 10 feet). 240.21(B)(4) specifies that since the tap conductors leave the enclosure in which the tap was made,that the tap conductors ampacity is not less than 1/10th of the rating of the OCPD protecting the feeder.

While my first casual observation suggested that these two #10 taps should be 8ga, the provision (B)(4), the ampacity of the 10ga THHN is 40 amps (per Table 310.16), which is clearly greater than 1/10th the 100 amp fuses for the feeder.

The concern is, while the 30 amp fuses will protect the tap conductors from a fault downstream of those fuses, it seems the tap conductors, from the 30 amp disconnects back to the Tap on the feeder, are under protected, should a fault, such as a short circuit at a rub point on the uninsulated or plastic bushed box connecter, develop.

The ICEA Table suggests that 100 amp Class RK5 fuses be connected to 8ga or larger (Conductor Short Circuit Protection, Minimum conductor size, Based on UL 248).

I haven't found out how to attach 4 photos to the post, so hope you can advise on that as well.

Thanks

The question is: Is there something I am missing here as far as determining whether the installation needs improved, or accept (B)(4), which indicates the installation meets requirements?

 

[infinity]

#3 THHN to a Junction Box, where the #3 is then tapped in a four-way, with taps of 1#4 to a 60 amp fused disconnect and two #10 THHN to two separate 30 amp fused disconnects.

This is code compliant under the 10' tap rule.

 

[electrofelon]

.

The concern is, while the 30 amp fuses will protect the tap conductors from a fault downstream of those fuses, it seems the tap conductors, from the 30 amp disconnects back to the Tap on the feeder, are under protected, should a fault, such as a short circuit at a rub point on the uninsulated or plastic bushed box connecter, develop.

That's exactly what a tap is, where you have conductors that are not protected at their ampacity. They are protected from overload, and due to their short length, a short circuit or ground fault will still clear the feeder ocpd upstream.