Tap Rules - Is Wire Sized to Enclosure rating or OCP rating?

[CelectricB]

In the given one-line, I'm having trouble determining the cable ratings on both the supply and load sides of the 225A fuses that are in 400A rated disconnect enclosures. My current understanding of tap rules would mean that the supply side needs to be rated for the equipment size, or in this case 400A. Load side can be sized to 225A as it is not a tap.

The piece of code confusing me at the moment,

"The ampacity of the tap conductors is as follows:

• a.
  Not less than the combined calculated loads on the circuits supplied by the tap conductors
• b.
  Not less than the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors"
  
  
  
  

 

[infinity]

The #4/0 (230 amps) terminates on a 225 amp fuse so it is code complaint even with the 25' tap rule. The #4 EGC's are too small and must be sized based on the 600 amp OCPD ahead of the taps.

 

[Tulsa Electrician]

Why the single phase 200 amp 208.
Designers choice
Typo?
Would the same apply ( EGC size)to the 200
What about the #1 ground between CT main disconnect. Wonder what is going on there.

 

[don_resqcapt19]

This is a problem with the code language that happened when they tried to make it clear that the load end of the tap conductor can be landed on the line side terminals of a fusible disconnect. In this case the application of the 10' tap rule would require 400 amp conductors, but the application of the 25' tap rule would only require 225 amp conductors. That is not the intent of the code language, but it is what the code language requires when you use a fusible disconnect for a 10' tap.

The language in 240.21(B)(1)(1)(b) needs to be fixed to match the intent of the rule..

 

[wwhitney]

This is a problem with the code language that happened when they tried to make it clear that the load end of the tap conductor can be landed on the line side terminals of a fusible disconnect.

Based on the concern that the overcurrent device is just the fuse itself, not the whole fused disconnect? I don't think the definitions of "Overcurrent Device" and "Device" support that interpretation.

I (perhaps mistakenly) thought the reason behind the language was to allow, say, a 200A 10' tap to land on the main lugs of a 200A MLO panelboard with installed breakers totaling no more than 200A.

The language in 240.21(B)(1)(1)(b) needs to be fixed to match the intent of the rule..

Suggestions? I'm in PI mode.

Cheers, Wayne

 

[texie]

This is a problem with the code language that happened when they tried to make it clear that the load end of the tap conductor can be landed on the line side terminals of a fusible disconnect. In this case the application of the 10' tap rule would require 400 amp conductors, but the application of the 25' tap rule would only require 225 amp conductors. That is not the intent of the code language, but it is what the code language requires when you use a fusible disconnect for a 10' tap.

The language in 240.21(B)(1)(1)(b) needs to be fixed to match the intent of the rule..

I agree and often wondered why this has never been addressed.

 

[jaggedben]

It's that second clause...

"or not less than the rating of the overcurrent protective device at the termination of the tap conductors"

...that does it for you. That let's you ignore the first part that says "rating of the equipment".

Theoretically the 'equipment' clause would allow a 10ft tap to land on an MLO switchboard. Article 408 requires individual overcurrent protection for a panelboard, so you can't actually use it for a panelboard. But a switchboard doesn't have that requirement.

 

[wwhitney]

Article 408 requires individual overcurrent protection for a panelboard

Good point. Although I think 408.36 Exception 1 would allow an MLO panelboard with just two breakers in it to be protected by those breakers. Which would allow a 10' tap to land on the main lugs.

Cheers, Wayne

 

[don_resqcapt19]

It's that second clause...

"or not less than the rating of the overcurrent protective device at the termination of the tap conductors"

...that does it for you. That let's you ignore the first part that says "rating of the equipment".

Theoretically the 'equipment' clause would allow a 10ft tap to land on an MLO switchboard. Article 408 requires individual overcurrent protection for a panelboard, so you can't actually use it for a panelboard. But a switchboard doesn't have that requirement.

I read that as requiring the tap conductor to land directly on the overcurrent protective device, so that does not do it for me. That part would apply to a breaker, but not to a fuse.

 

[don_resqcapt19]

Good point. Although I think 408.36 Exception 1 would allow an MLO panelboard with just two breakers in it to be protected by those breakers. Which would allow a 10' tap to land on the main lugs.

Cheers, Wayne

Not sure how those two breakers would protect the panelboard unless the taps connected to the breaker and a breakers supplied the panelboard. I believe that exception was written for split bus panels, where the supply connects to the breakers and the breakers supply individual bus sections.

 

[wwhitney]

Not sure how those two breakers would protect the panelboard

For the busbars, I_in = I_out. So if you have a 200A amacity tap running to a 200A busbar supplying only (2) 100A breakers, then current on the tap is limited to 200A nominal. As I read it that complies with the 10' tap rule in 240.21 and 408.36 Exception 1.

I'm still not sure what the point is for the difference in language in 240.21(B)(1)(1)(b) and 240.21(B)(2)(2). What arrangement is intended to be allowed by the former that wouldn't satisfy the language in 240.21(B)(2)(2)?

Cheers, Wayne

 

[jaggedben]

I read that as requiring the tap conductor to land directly on the overcurrent protective device, so that does not do it for me. That part would apply to a breaker, but not to a fuse.

That logic leads to some really weird conclusions. Like, that one can never land a 25ft tap on a fused disconnect. Or that one can only do so if one lands the conductors the wrong way, i.e. on the load end with the fuses.

 

[don_resqcapt19]

That logic leads to some really weird conclusions. Like, that one can never land a 25ft tap on a fused disconnect. Or that one can only do so if one lands the conductors the wrong way, i.e. on the load end with the fuses.

The 25' tap rule does not use the same language as the 10' tap rule.

 

[don_resqcapt19]

For the busbars, I_in = I_out. So if you have a 200A amacity tap running to a 200A busbar supplying only (2) 100A breakers, then current on the tap is limited to 200A nominal. As I read it that complies with the 10' tap rule in 240.21 and 408.36 Exception 1.
...

Cheers, Wayne

The breakers only provide overload protection for the panel board...the code rule requires overcurrent protection.

 

[wwhitney]

The breakers only provide overload protection for the panel board...the code rule requires overcurrent protection.

The wording of 408.36 Exception No 2 first sentence is "Individual protection shall not be required for a panelboard protected on its supply side by two main circuit breakers or two sets of fuses having a combined rating not greater than that of the panelboard." I guess you can debate whether the protection is on the supply side, but the arrangement I described would be protected by two circuit breakers.

I'm pretty unclear on the reason that 408.36 Exception No 2 exists. If it's related to split bus panels, what's wrong with simply calling those two panelboards in a common enclosure? Then each set of busbars (panelboard) would still be individual protected, if I understand correctly the typical connectivity used in a split bus panelboard.

Cheers, Wayne

 

[wwhitney]

The 25' tap rule does not use the same language as the 10' tap rule.

Yes, why?

In other words, say we changed the wording in the 10' tap rule 240.21(B)(1)(1)(b) to match that in the 25' tap rule 240.21(B)(2)(2). So now 240.21(B)(1)(1)(b) reads "(The ampacity of the tap conductors is) not less than the rating of the single circuit breaker or single set of fuses on which the tap conductors terminate." What configuration would now be ruled out by 240.21(B)(1)(1)(b) that was previously allowed?

Cheers, Wayne

 

[wwhitney]

The 25' tap rule does not use the same language as the 10' tap rule.

P.S. If the distinction you are drawing is between "the rating of the overcurrent protective device at the termination of the tap conductors" and "the tap conductors terminate in a single circuit breaker or a single set of fuses," I don't see those as describing different sets of possible configurations. What difference do you see in those two phrases?

Cheers, Wayne

 

[don_resqcapt19]

The wording of 408.36 Exception No 2 first sentence is "Individual protection shall not be required for a panelboard protected on its supply side by two main circuit breakers or two sets of fuses having a combined rating not greater than that of the panelboard." I guess you can debate whether the protection is on the supply side, but the arrangement I described would be protected by two circuit breakers.

I'm pretty unclear on the reason that 408.36 Exception No 2 exists. If it's related to split bus panels, what's wrong with simply calling those two panelboards in a common enclosure? Then each set of busbars (panelboard) would still be individual protected, if I understand correctly the typical connectivity used in a split bus panelboard.

Cheers, Wayne

There is no short circuit or ground fault protection of the panelboard. The defined term of overcurrent protection includes short circuit, ground fault, and overload. The breakers on the bus only provide overload protection for the panelboard.

After some digging, I have no idea what exception #1 is talking about. That was the main rule until 2008 code where the main rule was changed to require a single protective device and the 2 device rule was made into an exception. How do you supply a panelboard with two main breakers?

The split bus I was thinking of is really exception #2 ...dwelling unit services with 6 two pole service disconnects, 5 of which fed 2 pole loads and the 6th feeding the other part of the bus that serviced the 120 volt loads.

 

[don_resqcapt19]

P.S. If the distinction you are drawing is between "the rating of the overcurrent protective device at the termination of the tap conductors" and "the tap conductors terminate in a single circuit breaker or a single set of fuses," I don't see those as describing different sets of possible configurations. What difference do you see in those two phrases?

Cheers, Wayne

I am looking at (B)(1) and seeing a major difference between the two clauses of (B)(1)(1)(b). The first requiring the tap conductor to have an ampacity equal to the rating of the equipment where the OCPD is installed. This would require 400 amp conductors for 10' tap landed in a fusible disconnect with 225 amp fuses. The second clause relates to the OCPD at the termination of the tap conductors. I see this as only applying to a circuit breaker.
I see no reason for that language and no reason why that language should not be the same as in (B)(2) for the 25' tap rule.

 

[Tulsa Electrician]

Not less than the rating of the equipment containing an overcurrent device(s) supplied by the tap conductors

or not less than the rating of the overcurrent protective device at the termination of the tap conductors

One has to read slowly and make the distinction.