ground size when using tap rule

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neal

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A contractor installs a new 400amp disconnect with 400amp fuses. Fed from the building 2000amp service. He taps on load side of 2000amp service switch and runs 2 pipes each has 4-350cu and a 2cu ground to new disconnect. Using the 25ft. tap rule 240.21(B)(2)
This is a feeder tap.
1. How is the correct way to size this ground for each pipe when using the tap rule?

2.Looks like 350cu THHN is to small to meet 240.21(B)(2) breaker lugs is 75 degree rated?

Neal
 
Re: ground size when using tap rule

Neal in my opinion 250.122 requires a 250 kcmil CU EGC in each raceway for this tap of a 2000 amp bus.
 
Re: ground size when using tap rule

Item 1.
The tap rule is not relevant. You size the EGC to suit the overcurrent device. You have a set of 400 amp fuses, so a #3 is the minimum EGC. Therefore, the selected #2 is also acceptable.

Item 2.

I think I would agree. But I need to know one more thing. Is the feeder from the 2000 amp service switch protected by a 2000 amp overcurrent device? If so, the minimum tap conductor would need an ampacity of 1/3 of 2000, or 667 amps. Two sets of 350 would not answer. But if the feeder from the 2000 amp switch is served by an overcurrent device set at 1800 amps, then a pair of 350's would suffice.
 
Re: ground size when using tap rule

Originally posted by iwire: Neal in my opinion 250.122 requires a 250 kcmil CU EGC in each raceway for this tap of a 2000 amp bus.
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OK, Bob, we ruled this one differently. In my view the overcurrent device that protects the 400 amp feeder is the set of fuses at the load end, and not the 2000 amp fuses (or breaker) at the tap end. I say that because of 240.21(B)(2)(2). What's your view?
 
Re: ground size when using tap rule

Originally posted by charlie b:
Item 1.
The tap rule is not relevant. You size the EGC to suit the overcurrent device. You have a set of 400 amp fuses, so a #3 is the minimum EGC. Therefore, the selected #2 is also acceptable.
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Charlie I believe that Neal is asking about the EGC between the 2000 amp bus and the newly installed 400 amp disconnect.

If that is the case, and we have a ground fault in the raceway or 400 amp disconnect which OCPD will the EGC be required to open? ;)
 
Re: ground size when using tap rule

Originally posted by charlie b:
What's your view?
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Once we are on the load side of the 400 amp fuses I agree with you. :)

I think the wording in Table 250.122 supports my view.

Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)
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Re: ground size when using tap rule

Originally posted by iwire: Charlie I believe that Neal is asking about the EGC between the 2000 amp bus and the newly installed 400 amp disconnect.
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Let's pin this down more precisely. There is a 2000 amp switch. There is a feeder from that switch to an unnamed panel, an unnamed distance away. At some point along that feeder, we install a tap. We go from that tap point a distance of no more than 25 feet to a 400 amp disconnect.

I think the question has to do with the EGC that starts at the tap point, and goes to the 400 amp disconnect. I suspect (but I've never built one, nor even seen one) that this means that the phase conductors, the neutral conductors, and the EGCs are all tapped within the same enclosure (junction box?). So the EGC that runs from the 2000 amp switch to the unnamed panel may be a 250 MCM, but the EGC that taps into the 250 and that goes from the tap point to the disconnect only has to be a #3.
Originally posted by iwire: If that is the case, and we have a ground fault in the raceway or 400 amp disconnect which OCPD will the EGC be required to open?
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I don't have to answer that question! :D We are not designing for a short circuit within the conduit that goes from the tap point to the disconnect.

The whole point of having a tap rule is that we are taking extra steps to reduce the probability of that type of failure to an acceptable level. That is why the distance is limited. That is why the conductors must be protected (i.e., in conduit). That is why the ampacity of the tap conductors must be at least as high as the OCPD at the load end.
 
Re: ground size when using tap rule

Originally posted by charlie b:
Originally posted by iwire: If that is the case, and we have a ground fault in the raceway or 400 amp disconnect which OCPD will the EGC be required to open?
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I don't have to answer that question! :D We are not designing for a short circuit within the conduit that goes from the tap point to the disconnect.
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There lies the difference, I believe Table 250.122 requires us to design for a fault in the equipment containing the tap conductors.

It would make little sense not to.
 
Re: ground size when using tap rule

In that case, I would say that the tap rules themselves make no sense. Consider this: A fault within the conduit that serves the disconnect. The fault is not a perfect line to ground, but a breakdown in insulation. There is enough resistance in the fault path for the fault current to be above 400 amps (the limit of the tap conductors), but below 2000 amps (the protective limit of the upstream switch). What protects what?

The first answer is that the tap conductors will burn up, until such time as the fault current rises above 2000 amps.

The second (and better) answer is that this is beyond the design basis of the code, and that there need be no answer.

The whole point of a tap rule is that you can use conductors that are smaller than the 2000 amp upstream breaker, but only if you meet certain criteria. Why even write such criteria? What is their purpose, anyway? The only reasonable answer I can see is that the criteria make it so unlikely that a fault would occur in that short run of tap conductors that we can safely disregard the possibility of such a fault.

Now I need some help from the code authors. Anybody out there who can address the design intent of the tap rules?
 
Re: ground size when using tap rule

Originally posted by iwire: I think the wording in Table 250.122 supports my view.
Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)
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I concede that this statement supports your view. I would call this a mismatch between the requirements of 250.122 and the tap rules. It makes no sense to me otherwise. Do I have time to submit a code change to revise the title of that column of the table? Perhaps. Could I come up with a substantial substantiation in time? Doubtful.
 
Re: ground size when using tap rule

Originally posted by charlie b:
The first answer is that the tap conductors will burn up, until such time as the fault current rises above 2000 amps.
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I believe that would be the reason that the tap conductors must be 1/3 the rating of the OCP.

Using the 25' tap rule I can not tap a 2000 amp bus with 10 AWG to supply a 30 AMP OCP, I would have to use 400 kcmil CU to supply the the 30 amp OCP.

I believe this one third ampacity requirement and an EGC based on the up stream OCP ensures that the upstream OCP will open.

Admittedly this is JMO and other than the wording in Table 250.122 I have nothing to back it up. :)

Bob
 
Re: ground size when using tap rule

250.122 (G) Feeder Taps Equipment grounding conductors run with feeder taps shall not be smaller than shown in Table 250.122 based on the rating of the overcurrent device ahead of the feeder but shall not be required to be larger than the tap conductors.
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Don
 
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