Tap Rule

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sparkyand1

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Paden, OK
We have a situation where we are having to parallel two 1/0 conductors from a 3 phase 120/208 breaker in our switch gear and tap them to feed two disconnects. We have done our due dilligence in the code and we're just not sure if we're interpreting the tap rule correctly. We have a dishwasher @ 50 FLA and a 54 KW booster heater @ 150 FLA. We're considering them both continuous loads. We have two 1 1/4 conduits that we are paralleling 1/0 conductors through. Phase A, B and C in one and A,B and C in the other for a total of 300 amps. We've decided to go with a 250 amp breaker in the main switch gear to feed our paralled 1/0 conductors. The run is about 50 ft. We are going to terminate our 1 1/4 pipes in a gutter above the disconnects. from the gutter we are going to leave with a 2" conduit and about a 3 ft. run into the 200 amp disconnect from there we will use a chase nipple into the 100 amp disconnect. The 200 amp disconnect will power the booster heaters and will have 200 amp fuses. The 100 amp disconnect will have 70 amp fuses in it for the dishwasher. The main question we have is in reference to the tap rule. We are wanting to tap the paralled 1/0 conductors in the gutter with 3/0 and #3 conductors. the 3/0 will power the 200 amp disconnect and the #3 conductors will power the 100 amp disconnect. Everything we've seen in the code will allow us to do this, but everyone I've talked to just aren't sure about the tap rule. Please help.
Thank You
 
It seems to me that you are satisfying all of the conditions of 240.21(B)(1). I would say you are good to go.

Welcome to the forum.
 
OK. So much for the tap rules. But am I to understand that you will run 6 conductors (A, B, and C for the 3/0, and A, B, and C for the #3) all in the same 2" conduit that is 3 feet long? If so, I think you have a problem with the 3/0 conductors. You have 6 current-carrying conductors in the same raceway, and therefore have to reduce the ampacity to 80%. That brings the ampacity of the #3/0 down to 180 amps (presuming you are using 90C rated wire). Tap rule article 240.21(B)(1)(1)(b) requires the ampacity of the tap conductors to be at least as high as the rating of the device (i.e., the disconnect) or the overcurrent protection device (i.e., the fuses) to which they are connected. With a 180 amp wire and a 200 amp overcurrent device, you have a mismatch. The dishwasher circuit would be OK, since you have a derated ampacity of 88 amps and an overcurrent device of 70 amps.

One solution would be to run separate conduits from the gutter to the first disconnect. That would be cheaper than upsizing the heater circuit from 3/0 to 4/0.
 
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charlie b said:
OK. So much for the tap rules. But am I to understand that you will run 6 conductors (A, B, and C for the 3/0, and A, B, and C for the #3) all in the same 2" conduit that is 3 feet long? If so, I think you have a problem with the 3/0 conductors. You have 6 current-carrying conductors in the same raceway, and therefore have to reduce the ampacity to 80%. That brings the ampacity of the #3/0 down to 180 amps (presuming you are using 90C rated wire). Tap rule article 240.21(B)(1)(1)(b) requires the ampacity of the tap conductors to be at least as high as the rating of the device (i.e., the disconnect) or the overcurrent protection device (i.e., the fuses) to which they are connected. With a 180 amp wire and a 200 amp overcurrent device, you have a mismatch. The dishwasher circuit would be OK, since you have a derated ampacity of 88 amps and an overcurrent device of 70 amps.

One solution would be to run separate conduits from the gutter to the first disconnect. That would be cheaper than upsizing the heater circuit from 3/0 to 4/0.
Click to expand...

Charlie, Thank you very much. Yes, we had overlooked derating the wire. We had discussed that and planned on revisiting the 2" run but got side tracked by the tap issue. We will run 1 1/2" for the 3/0 and 1" for the #3's THANKS!
 
sparkyand1 said:
We have a situation where we are having to parallel two 1/0 conductors from a 3 phase 120/208 breaker in our switch gear and tap them to feed two disconnects. We have done our due dilligence in the code and we're just not sure if we're interpreting the tap rule correctly. We have a dishwasher @ 50 FLA and a 54 KW booster heater @ 150 FLA. We're considering them both continuous loads. We have two 1 1/4 conduits that we are paralleling 1/0 conductors through. Phase A, B and C in one and A,B and C in the other for a total of 300 amps. We've decided to go with a 250 amp breaker in the main switch gear to feed our paralled 1/0 conductors. The run is about 50 ft. We are going to terminate our 1 1/4 pipes in a gutter above the disconnects. from the gutter we are going to leave with a 2" conduit and about a 3 ft. run into the 200 amp disconnect from there we will use a chase nipple into the 100 amp disconnect. The 200 amp disconnect will power the booster heaters and will have 200 amp fuses. The 100 amp disconnect will have 70 amp fuses in it for the dishwasher. The main question we have is in reference to the tap rule. We are wanting to tap the paralled 1/0 conductors in the gutter with 3/0 and #3 conductors. the 3/0 will power the 200 amp disconnect and the #3 conductors will power the 100 amp disconnect. Everything we've seen in the code will allow us to do this, but everyone I've talked to just aren't sure about the tap rule. Please help.
Thank You
Click to expand...
I did not look at conductor ampacity or raceway fill but when you tap from parallel conductors your tap device must have all of the parallel conductors in it plus the tapped conductor. You did not mention exactly how you are making the tap and thought I should bring this up. The conductor you are "tapping" is the effective sum of all the parallel components. If in your example you would happen to tap one of your loads from one of the 1/0 feeder conductors and the other load from the other feeder conductor- that would be a violation, you must put all parallel components of one conductor in the same tap device to do it the correct way. Your 3/0 tap is a good example of one reason why, should you tap it from only one of the 1/0 conductors you will potentially have overloaded that 1/0 conductor.
 
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