Ampacity of the tap conductor

len149

Member
I have a 200 amp fuse protected 3/0 feeder ending in a 200 amp non-fusible disconnect. I plan to tap off of the load side of the disconnect and run 15Ft. of conductors in conduit to supply a 60 amp fusible disconnect which will supply other loads.

240.21(B) (2) (1) STATES: The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.

200amps / 3 = 67 amps required for the tap conductors ampacity. The 60 amp disconnect termination is listed 75 degrees F. The tap conductors being used will be THHN. The THHN 75 degree ampacity for # 6 is 65 Amps so it does not meet the 67 amps. But can’t the ampacity value of the #6 at 90 degree C. column of 75 amps be used to satisfy 1/3 rating of the tap conductors on the feeder? It seems the 60 amp overcurrent device will keep the conductor from exceeding the termination rating of the 60 amp disconnect. So would a number 6 conductor meet the requirement and be able to be used for a 60 Amp fusible disconnect in this case?
 

infinity

Moderator
Staff member
Location
New Jersey
Even if you landed the tap conductors on a 15 amp OCPD they still need to meet the minimum of 1/3 of the ampacity of the OCPD ahead of the feeder. You'll need #4 cu to meet the minimum of 67 amps.
 

iwire

Moderator
Staff member
Location
Massachusetts
It seems the 60 amp overcurrent device will keep the conductor from exceeding the termination rating of the 60 amp disconnect.
I agree with you it will, however there is more to it than a short or ground fault in the wiring beyond the 60 amp fuse.

Consider the raceway containing the tap conductors becomes damaged, in that case it will be the 200 amp overcurrent device that must open.

I believe that is the reason for the minimum percentage size rules in 240.21 sections.
 

len149

Member
Thanks for the reply I’ll run the #4 it’s no big deal I just thought: 75amps applied to the #6 THHN will heat the conductor up to 90 degrees F. without adverse effects to the conductor or insulation so 75 amps is the maximum amount of amps that can be placed on a #6 THHN conductor. The Tap rule does not state which temperature column must be used for the tap conductor ampacity it just states it must have a rating at 1/3 of the feeder fuse protection and can't be smaller than 1/10 or 20 amps so I thought it would meet the requirements. A #6 will also meet the equipment listed 75 degrees equipment termination requirements because the disconnect being fused at 60 amps will not allow the conductor to draw more than 60 amps thus keeping it below the 75 degrees. I just thought the maximum ampacity for the TAP conductor could be used for the 1/3 third rule which would allow for a #6. My bad. So if I got this right if by chance the conductor somehow where to draw more than 60 amps, which it shouldn’t do, because it is protected from doing so by the 60 amp fuse the temperature limitations of the equipment terminations would be violated. So the 75 degree ampacity must be used for the tap conductor?
 
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iwire

Moderator
Staff member
Location
Massachusetts
Len, you keep focusing on the 60 amp disconnect and that has nothing to do with it.

Use a 30, 60 or a 100 amp fused disconnect in all cases the minimum size conductor must be at least 1/3 of the 200 amp switch so a short circuit between the 200 amp and 60 amp overcurrent devices can open the 200 amp overcurrent device.

Also keep in mind if you chose to run a wire type EGC between the 200 amp and 60 amp overcurrent devices that EGC must be sized based on the 200 amp overcurrent device.
 

len149

Member
I understand that the conductor must have a one third of the over current protection rating of the feeder or in this case, greater than 67 amps. I guess I don't see why the 90 degree F. 75 amp ampacity of a #6 can't be used to size the tap conductor itself for 1/3 of the feeder fuse rating. Like I said I'll go with the #4. It's just that the actual rating of a number 6 THHN is 75 amps but, it seems it can't be used? It's just you are stating the 75 degree column for the TAP conductor ampacity has to be used and I'm just trying to figure out why the 90 Degree column ampacity couldn't be used for the calculation also. Thanks :)
 

jumper

Senior Member
I understand that the conductor must have a one third of the over current protection rating of the feeder or in this case, greater than 67 amps. I guess I don't see why the 90 degree F. 75 amp ampacity of a #6 can't be used to size the tap conductor itself for 1/3 of the feeder fuse rating. Like I said I'll go with the #4. It's just that the actual rating of a number 6 THHN is 75 amps but, it seems it can't be used? It's just you are stating the 75 degree column for the ampacity must be used for the tap conductor ampacity and I'm just trying to figure out why. Thanks :)
Because the terminal ratings on the equipment are limited to the 75C column, so the conductor is also. 110.14(C).
 

iwire

Moderator
Staff member
Location
Massachusetts
I guess I don't see why the 90 degree F. 75 amp ampacity of a #6 can't be used to size the tap conductor itself for 1/3 of the feeder fuse rating.
For the same reasons we cannot do that with any circuit, the code has no allowances for a tap conductor to be treated differently in this regard.
 

infinity

Moderator
Staff member
Location
New Jersey
Because the terminal ratings on the equipment are limited to the 75C column, so the conductor is also. 110.14(C).
For the same reasons we cannot do that with any circuit, the code has no allowances for a tap conductor to be treated differently in this regard.
I agree with both, the fact is that you only have a conductor ampacity based on the 75° C rating of the conductor.
 
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