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Short Circuit Protection of a tap conductor
https://brainfiller.com/arcflashforum/viewtopic.php?f=2&t=5323		 Page 1 of 1
Author:  saianguluri [ Fri Jun 19, 2020 6:32 am ]
Post subject:  Short Circuit Protection of a tap conductor
I have a low voltage(480V) OCPD feeding a 4/0 cable which feeds a junction box. From the junction box, three sets of 8AWG cables feed three motor starters. The OCPD protects 4/0 cable against short circuits but doesn't protect the 8AWG. When I referred NEC they recommend matching the ampacity of the feeder(4/0) but using a 1/0 is enough for the OCPD to protect the cable. Can anyone suggest what to choose?
Author:  JBD [ Fri Jun 19, 2020 7:40 am ]
Post subject:  Re: Short Circuit Protection of a tap conductor
What you are describing, the NEC calls a Tap conductor. You should look at the requirements in Article 240. In particular look at 240.2, 240.4(E), and 240.21(B).
Author:  saianguluri [ Mon Jun 22, 2020 8:51 am ]
Post subject:  Re: Short Circuit Protection of a tap conductor
Yes, I referred to those sections, and it is satisfying the requirements per NEC. But they are still not protected in TCC against the short circuit damage. So my question here is if the short circuit damage curve of a tap conductor is not protected by an OCPD do we consider NEC requirements and state them as protected?
Author:  mpparent [ Mon Jun 22, 2020 9:40 am ]
Post subject:  Re: Short Circuit Protection of a tap conductor
In section 240, it specifically says that you cannot have a tap conductor rated lower than 1/3 the rating of the OCPD protecting the feeder conductors (this assumes a feeder tap not over 25' long).

Since you have #4/0AWG conductors, I'll assume you have a 225A circuit breaker feeding them. That means your tap conductor ampacity has to be 75A or more. #8AWG doesn't cut it in that case.

Mike
Author:  saianguluri [ Tue Jun 23, 2020 7:16 am ]
Post subject:  Re: Short Circuit Protection of a tap conductor
So in this case you are recommending to replace my cable something above 75A. In that case, I changed my cable to 4 AWG, and its still not protected against short circuits in a TCC. So since 4 AWG is an NEC complaint can I just leave it or still make a change to protect it against short circuit?
Author:  JBD [ Tue Jun 23, 2020 8:18 am ]
Post subject:  Re: Short Circuit Protection of a tap conductor
saianguluri wrote:
...its still not protected against short circuits in a TCC.


600V conductor damage curves, shown on a TCC, are probably not applicable to your installation.
UL requires that overcurrent protective devices be tested up to their AIC value using insulated cables, sized per the NEC 75C tables. The cables can not be damaged during this short circuit test. This is likely because the protective device operates fast enough that the full test current is not be seen by the cable.
Author:  mpparent [ Tue Jun 23, 2020 12:27 pm ]
Post subject:  Re: Short Circuit Protection of a tap conductor
Can you post your TCC?

Mike
Author:  Jeff S [ Mon Jun 29, 2020 9:29 am ]
Post subject:  Re: Short Circuit Protection of a tap conductor
It doesn't sound like the tap rules are being applied correctly. 240.21 says overcurrent protection has to be located at the point in the circuit where the conductors receives their supply. So the overcurrent protection for the tap conductors should be in the junction box where the taps occur. But it does give some exceptions like the previously mentioned 25 ft tap rule 240.21(H). With that exception, you have to terminate the conductors in single set of fuses or circuit breaker that limits the load to the ampacity of the conductors [240.21(H)(2)(2)]. From what you described, it doesn't sound like you have that required overcurrent protection for the taps.
Author:  Flash [ Mon Jul 20, 2020 1:17 pm ]
Post subject:  Re: Short Circuit Protection of a tap conductor
saianguluri wrote:
I have a low voltage(480V) OCPD feeding a 4/0 cable which feeds a junction box. From the junction box, three sets of 8AWG cables feed three motor starters. The OCPD protects 4/0 cable against short circuits but doesn't protect the 8AWG. When I referred NEC they recommend matching the ampacity of the feeder(4/0) but using a 1/0 is enough for the OCPD to protect the cable. Can anyone suggest what to choose?


Read 240.21(1), the 10ft tap rule, that that section allows your installation where there is OCP at the termination of the tap conductor.

If you do install OCP for each tap, the OCP does not have to be in the enclosure where the tap is made, a fusible disconnect switch attached (conduit stub) to the enclosure is sufficient and is an accepted practice.

Perhaps more important than conductor protection is the lack of protection for the motor starter. In most cases a motor starter will have a max circuit protection rating associated with the UL listing, in this case OCP at the termination of the tap would make your circuit NEC compliant.
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