tap rule clarification

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hardworker

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I need you to refresh my memory concerning tap rule.

400 amp single phase service for a commercial building. 400 amp main fused disconnect is the first overload protection. I need to tap the 400 amp d/s to install a 200 amp fused disconnect downstream. What is the furthest the 200 amp d/s can be located from the 400 amp d/s?

Many thanks.
 
If the wires go on the outside of the building there is no limit.
If not, then 10 feet or 25 feet depending on which you qualify for based on wire size and other details.
 
Since you're using a 200 amp tap to a 400 amp feeder you can use the 25' tap rule which for the tap conductors is 1/3 the ampacity of the OCPD ahead of the tap. This is assuming that your tap will be on the load side of the service disconnect.
 
Keep in mind tap rules only apply if you use less then 400 amp conductor.

If you spliced in 400 amp conductors there would be no limitation on how far away you run them. Might need to reduce to smaller conductor if you are going to land at a 200 amp device, but tap rule would only apply to the section that is reduced.
 
GoldDigger said:
If the wires go on the outside of the building there is no limit.
If not, then 10 feet or 25 feet depending on which you qualify for based on wire size and other details.
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All this work is inside the building. The 200 amp d/s will be fed with 2/0 conductors. How many feet is my tap good for?
 
hardworker said:
All this work is inside the building. The 200 amp d/s will be fed with 2/0 conductors. How many feet is my tap good for?
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zero, as 2/0 is not large enough for 200 amps.

If you are supplying a dwelling unit (the entire dwelling load) 2/0 copper is acceptable for 200 amps via 310.15(B)(7). Not sure it is acceptable for a feeder tap though, would need to look harder before saying yes to that.

3/0 copper would be good for 25 feet of tap conductor.
 
kwired said:
zero, as 2/0 is not large enough for 200 amps.

If you are supplying a dwelling unit (the entire dwelling load) 2/0 copper is acceptable for 200 amps via 310.15(B)(7). Not sure it is acceptable for a feeder tap though, would need to look harder before saying yes to that.

3/0 copper would be good for 25 feet of tap conductor.
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I forgot about 2/0 not being heavy enough per code for commercial building. Sounds like I go with 3/0 and I can tap up to 25 feet.
 
hardworker said:
All this work is inside the building. The 200 amp d/s will be fed with 2/0 conductors. How many feet is my tap good for?
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Taps are not eligible for next-size up rule. As Kwired notes, 3/0 cu @75c is minimum size for 200A fuse.

35ft inside industrial building, only in industrial occupancies, where conditions of maintenance
and supervision ensure that only qualified persons service the equipment.
 
You could use #2/0 (minimum size is 400/3=133 amps) you would just need to use 175 amp fuses in the disconnect.
 
ramsy said:
Taps are not eligible for next-size up rule. As Kwired notes, 3/0 cu @75c is minimum size for 200A fuse.

35ft inside industrial building, only in industrial occupancies, where conditions of maintenance
and supervision ensure that only qualified persons service the equipment.
Click to expand...

Please explain next-size up rule. thanks
 
ramsy said:
Taps are not eligible for next-size up rule. As Kwired notes, 3/0 cu @75c is minimum size for 200A fuse.

35ft inside industrial building, only in industrial occupancies, where conditions of maintenance
and supervision ensure that only qualified persons service the equipment.
Click to expand...

Would you give example of the next-size up rule?
 
Say you're installing a 200 amp feeder to a 200 amp panel using #4/0 XHHW with an ampacity of 180 amps. Calculated load on the panel is 150 amps so you can go up to the next standard size OCPD (200 amps, 180 is not a standard size) from 240.6(A) to protect the feeder.

So you have 200 amp panel> 150 amp load> 180 amp conductor> 200 amp OCPD, all code compliant.
 
hardworker said:
Would you give example of the next-size up rule?
Click to expand...

Say you have a conductor that has an ampacity of 185A. Since 185A is not a standard size breaker or Fuse you are allowed to use the next largest breaker/fuse which would be 200A. However the load would be limited to 185A.

ETA: Sorry Rob, I was posting (slowly) while you were!
 
I am getting old and my memory is not as good as it use to be. I have a young electrician doing most of the work, but I like to check what he is doing. You guys have a lot of knowledge at your fingertips. Many thanks.
 
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