10' tap rule

[jap]

10-ft feeder tap rule [240.21(B)(1)] ? You don't have to install an OCPD at the tap point of a feeder tap if its length doesn't exceed 10 ft and if it meets the following requirements


The ampacity of the tap conductor is not less than the computed load in accordance with Art. 220, and not less than the rating of the device supplied by the tap conductors or the OCPD at the termination of the tap conductors.

The tap conductors aren't extended beyond the equipment they supply.

The tap conductors are installed in a raceway if they leave the enclosure.

The tap conductors have an ampacity of no less than 10% of the ampacity of the OCPD from which the conductors are tapped.

My question is,
What does it mean by the tap conductors can't extend beyond the equipment they supply?

If a 400a fused disconnect was tapped from a 2000 amp buss with 500 mcm
Thhn within 10', would this installation be acceptable or not.

I'm reading this to mean that the definition of the tap conductor ends at the line side of the 400a Fused Disconnect.

 

[raider1]

I'm reading this to mean that the definition of the tap conductor ends at the line side of the 400a Fused Disconnect.

That would be correct. The tap conductors end where they terminate at an overcurrent protective device.

Chris

 

[charlie b]

My question is, What does it mean by the tap conductors can't extend beyond the equipment they supply? . . . I'm reading this to mean that the definition of the tap conductor ends at the line side of the 400a Fused Disconnect.

I am reading it the same way.

If a 400a fused disconnect was tapped from a 2000 amp buss with 500 mcm Thhn within 10', would this installation be acceptable or not.

Not acceptable. But not for the reason that led you to ask the question. If you had said 600 MCM, then it would have been acceptable. Here is why:

The ampacity of the tap conductor is not less than . . . the rating of the device supplied by the tap conductors or the OCPD at the termination of the tap conductors.

The ampacity of 500 MCM THHN is 380 amps. The rule says the ampacity cannot be less than the rating of the OCPD, and your OCPD is 400 amps. That makes your suggested installation a violation. It is commonly believed that you can always associate 500 MCM with 400 amp overcurrent protection, but that is not always true.

 

[david luchini]

If a 400a fused disconnect was tapped from a 2000 amp buss with 500 mcm
Thhn within 10', would this installation be acceptable or not.

No this would not be acceptable. See 240.21(B)(1)(1)(b) - "not less than the rating of the OCPD at the termination of the tap conductors."

You could use 600mcm and it would be OK, but not 500mcm.

Edit: Charlie types faster than me.

 

[iwire]

Don't forget the EGC will be based on the 2000 amp OCPD not the 400 OCPD

 

[jap]

The full 400 amp rating made me think that the 500 mcm would be a violation
and the "Next Higher" would not apply.

Thank you for the clarification.

Also I didnt realize the Grounding Conductor was to be based on the 2000 amp OCPD, That's also very good information that I will keep in mind in the future.

is that stated in the Tap Rule Section?

 

[david luchini]

is that stated in the Tap Rule Section?

No, in the ground and bonding section. See 250.122(G).

 

[iwire]

Keep in mind that the EGC is not required to be larger than the circuit conductors.

 

[charlie b]

Don't forget the EGC will be based on the 2000 amp OCPD not the 400 OCPD

Let's talk about that. My view is that the EGC associated with the 2000 amp conductors (i.e., upstream of the tap point) would be based on a 2000 amp OCPD, and that the EGC associated with the tap conductors themselves would be based on the 400 amp OCPD. I think that you are seeing it differently.

 

[LarryFine]

My view is that the EGC associated with the 2000 amp conductors (i.e., upstream of the tap point) would be based on a 2000 amp OCPD, and that the EGC associated with the tap conductors themselves would be based on the 400 amp OCPD. I think that you are seeing it differently.

I am, too. Read 250.122(G) again.

 

[iwire]

Let's talk about that. My view is that the EGC associated with the 2000 amp conductors (i.e., upstream of the tap point) would be based on a 2000 amp OCPD, and that the EGC associated with the tap conductors themselves would be based on the 400 amp OCPD. I think that you are seeing it differently.

The EGC must be sized to handle the current from the OCPD it is expected to open.

The EGC run with the tap conductors in this case must open the 2000 amp OCPD.

Once we get to the load side of the new 400 OCPD we have feeders not tap conductors so the EGC at that point will be based on the 400 amp OCPD.

 

[raider1]

I agree with Bob.

Chris

 

[charlie b]

Read 250.122(G) again.

When did they put that there? OK. I concede.

 

[LarryFine]

I agree with Bob.

Oh, stop that! :roll: grin

 

[jap]

In all my years, I never thought of it that way.

Thats some valuable information I-Wire that I would say many are not aware of.

Jap.

 

[iwire]

When did they put that there?

Honestly I think pretty recently.

250.122(G) aside IMO this wording in Table 250.122 .....

Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding

....has always required this and I think G was added to drive the point home.

 

[raider1]

Oh, stop that! :roll: grin

I do it because I know that it bothers you.

Chris

 

[LarryFine]

I do it because I know that it bothers you.

Oh. Well, in that case, keep it up.

 

[raider1]

Oh. Well, in that case, keep it up.

OK.

Chris

 

[jrohe]

The EGC must be sized to handle the current from the OCPD it is expected to open.

The EGC run with the tap conductors in this case must open the 2000 amp OCPD.

Once we get to the load side of the new 400 OCPD we have feeders not tap conductors so the EGC at that point will be based on the 400 amp OCPD.

I agree with iwire.