Mike Dolt Enterprises
Mike Dolt Enterprises
User: ElectricNewEngland, Location: New Hampshire
Asked on: Wednesday, April 20, 2011 at 4:33 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:27 am EDT
Question: I have a situation where we are tapping a feeder to two VFD units. The feeder is 3#1/0 cable running from a 200A disconnect (w/ 100A fuse) at MCC to a junction box. From the junction box, we tapped 3#6 cable to a 30A disconnect (w/ 25A fuse) for VFD.The problem is that the contractor is telling us that the run for the Tap Conductor is 50 feet or longer. I keep reading that Tap Conductors cannot be longer than 25 feet, although I cannot find that rule in the NEC codebook. I understand that Artical 240.21(B)(4) is for Tap Conductors over 25 feet, but it seems like that is only for rooms that are over 35 feet high, which our room isn't that high.Can I just provide a bigger sized conductor for the tap (such as 3#1/0)? Should I add another disconnect to the tap conductor? I'm confused, please help. Thanks.
NEC Rule
The length limit for tap conductors is not explicitly stated in the NEC for general applications. However, Article 240.21(B)(4) provides guidance for tap conductors in high ceilings (over 35 feet):> "In no case shall the length of the tap conductor exceed 25 ft (7.62 m), unless the tap conductor is supported at intervals not exceeding 4 ft (1.22 m) to prevent contact with the ceiling."Your Situation
In your case, the tap conductor run exceeds 25 feet. The NEC does not explicitly prohibit this, but it may raise safety and code compliance concerns.
Options Provide a larger conductor size
Increasing the conductor size, such as using 3#1/0, can reduce voltage drop and improve safety. However, this may not be a feasible solution if it exceeds the ampacity of the protective devices.
Add a disconnect to the tap conductor
Installing a disconnect at the junction box for the tap conductor can allow for isolation and easier maintenance. This would also comply with Article 240.21(B)(4) for high ceilings, although this is not required in your case.*
Support the tap conductor
If you choose to keep the existing tap conductor size, you must support it every 4 feet to ensure clearance from the ceiling. This may be impractical or difficult to accomplish.
Recommendation
The best solution depends on the specific conditions and equipment involved. It is recommended to consult with a qualified electrician and/or the manufacturer of the VFD units for guidance. They can assess the situation, perform calculations, and determine the most appropriate course of action to ensure compliance and safety. Flag for review