Could someone point me in the right direction of code if this is acceptable per NEC? We are looking at installing a 400A service with a 400A disconnect and fuses that feeds three 200A panels. Is it acceptable to run 4/0 Cu to those panels given that the upline overcurrent device is 400A fuses? The cable will never be overloaded due to the 200A panels. Two of the AC panels will be located within 10 feet of the disconnect. The third panel will be located ~50 feet away.
Service Overcurrent Protection
- Thread starter gemetzger
- Start date
The connections on the load side of your 400 amp disconnect would either need to be full size or meet one of the 240.21 tap rules.
The 10ft runs should not be a problem, the 50ft will be unless you meet the requirements of 240.21(B)(5) for outside taps.
The 10ft runs should not be a problem, the 50ft will be unless you meet the requirements of 240.21(B)(5) for outside taps.
Based on my understanding on your response, given two of these taps are <10 feet of the disconnect, we do meet all the criteria in 240.21.(B).(1). However, the third panel located 50' away is still a major question for me. The conductor is run via underground conduit under a building where it risers up to the third AC panel. Based on my interpretation on 240.21.(B).(5):
- The conductors are protected from physical damage
- They terminate at the main breaker in the AC panel which limits the load to the ampacity of the conductor
This brings up another question/details left out on the original question - the service is fed via a transformer and disconnect/fuses that are located ~100 feet from a building that houses a manual transfer switch (MTS). This conductor is adequately sized for 400A and runs in conduit. On the load side of the MTS, we are planning on breaking out into three runs going to each individual 200A panel of 200A minimum rated conductor. The definition of the tap and length we are considering would be from the MTS and not the disconnect, correct?
- The conductors are protected from physical damage
- They terminate at the main breaker in the AC panel which limits the load to the ampacity of the conductor
This brings up another question/details left out on the original question - the service is fed via a transformer and disconnect/fuses that are located ~100 feet from a building that houses a manual transfer switch (MTS). This conductor is adequately sized for 400A and runs in conduit. On the load side of the MTS, we are planning on breaking out into three runs going to each individual 200A panel of 200A minimum rated conductor. The definition of the tap and length we are considering would be from the MTS and not the disconnect, correct?
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To follow-up on the meeting 240.21.(B).(5).
- The conductors run under the building via a conduit so they are protected from physical damage. Is this an "approved" manner?
- They terminate at a single main breaker in the AC panel which limits the load to the ampacity of the conductor.
- The overcurrent device (fuses) are located inside the 400A disconnect. This is located approximately 150' from AC Panel 3.
- The 400A disconnect is located on a pole outside of the building ~100 feet away. The MTS is also located outside the building.
The above seems to meet the criteria in my opinion but would be open to any flaws in my thinking.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
1) You must have one disconnect location (with grouped disconnects or of single disconnect) AT the building. NEC 225.33
2) Your MTS should have OCPD. Otherwise there is no OCPD when you are using a generator. And the taps are illegal since there is no upstream OCPD. Maybe the Genset has OCPD integral to it.
If conductors are passing under the building they need to meet 230.6 or you are required to have the disconnect before it passes underneath (because then it fails to meet the requirements for being considered outside).
Your interpretation of 240.21(B)(5) looks correct. The 200A conductor must come off of a 400A conductor. You can not land the 200A conductors on the 400A MTS and run those 3 circuits direct to the panelboards. That would be a violation of the tap rule. It would be debatably cheaper to install a 400A panel board at the MTS and run the circuits from that rather than making splices in a handhole or gutter.
2) Your MTS should have OCPD. Otherwise there is no OCPD when you are using a generator. And the taps are illegal since there is no upstream OCPD. Maybe the Genset has OCPD integral to it.
If conductors are passing under the building they need to meet 230.6 or you are required to have the disconnect before it passes underneath (because then it fails to meet the requirements for being considered outside).
Your interpretation of 240.21(B)(5) looks correct. The 200A conductor must come off of a 400A conductor. You can not land the 200A conductors on the 400A MTS and run those 3 circuits direct to the panelboards. That would be a violation of the tap rule. It would be debatably cheaper to install a 400A panel board at the MTS and run the circuits from that rather than making splices in a handhole or gutter.
A couple follow-up questions to your response:
1) Given that the MTS is located on the outside of the building prior to entering the building, does this meet NEC 225.33 as a viable disconnect location? The MTS appears to meet this criteria in my opinion.
2) Regarding 230.6, the conductor is installed under >2" of concrete for the complete run EXCEPT for where it would tap up into the panel AND where it risers down as it enters the building through a conduit sleeve and then goes directly underground. There would be about a 2-4' section where I would probably considered it "inside" the building based on my reading of the code.
3) Why must the 200A conductor come off a 400A conductor versus landing the 200A conductor on a 400A MTS directly to be considered a tap? To me, it seems that running 5' or less of 400A conductor and splicing 200A conductors would cause more issues from a safety standpoint than just landing directly to the MTS? Similar argument could be made for adding a power distribution block and adding extra termination points.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
225.30(B) or one of the art.225 sections. I understand what you mean. There is only one service. But the code has been progressively updated to ensure that conductors entering a building have a disconnect in/on that building for it's supply. Whether it is a service or feeder or branch circuit.
Where it taps into a panelboard in the building it complies with the "closest point of entry". I try to think of it like a underground service where the conductors are allowed to be considered outside where it fits that criteria. If you have nfpa link or the handbook there is an image underneath that section that has conductors being considered outside and they rise into panels inside the building.
I have never taken the definition of Tap conductor or feeder conductor to be the points of connection on equipment. But I can be wrong. I don't think installing 3 circuits direct to the terminals of a transfer switch as being taps. I guess it more depends on how you terminate on the equipment.
For example, I would consider a bus bar as a conductor when tapped in a switchgear but I wouldn't consider a 400A breaker with three runs of 3/0 coming directly off of it as being an acceptable tap to feed 3 separate 200A panels (with MCB). And I don't extended that to equipment where the landings are preinstalled. That rating is dictated by the manufacturer.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
I don't know if I would consider the load side terminals as "conductors" that can be tapped. But I did find the above literature on it after googling methods to install taps on breakers.
Basically, they say:
You can't override the rating on the terminals. 400A terminals are not the same as saying 400A conductors. They have their own ratings for minimum size, temperature, etc. If you read through they always mention the minimum terminal rating and the number of terminals on the breaker. You can not install more than the runs available on the terminals and can not use a wire sized outside of the terminal ratings.
You can use wire to extend to a tap kit or buy a breaker with enough individual terminals to support the number of taps.
You can not just install 3 runs of 3/0 on a single terminal. (But I don't think that was what you were suggesting)
Now for this case, there is no OCPD at the transfer switch and will most likely have only single landings on the switch. The switch should say the size and number of available terminals for the load side connection. If there are two terminals available, then you would parallel 3/0 to a splice tap to increase the runs to 3. You would not splice a single 3/0 run with an additional 3/0. Nor would you double land on a single terminal. If the transfer switch comes with 3 terminals per phase for the load side connections then there is no issue (as far as I could tell).
The multiple terminal openings on the load side of the breaker may also be employed for deriving lower-current branch or feeder circuits from a breaker in accordance with the NEC and UL 508A tap rules.
The tap rule does seem to differ somewhat if tapping a breaker in a control panel to a power distribution block.
It seems If you extend tap conductors to a PDB inside a control cabinet those conductors must be rated at the amperage of the breaker.
I do agree that stacking more conductors in the terminals than what it was rated for would be violation.
Jap>
The tap rule does seem to differ somewhat if tapping a breaker in a control panel to a power distribution block.
It seems If you extend tap conductors to a PDB inside a control cabinet those conductors must be rated at the amperage of the breaker.
I do agree that stacking more conductors in the terminals than what it was rated for would be violation.
Jap>
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