10 ft tap rule???

Status
Not open for further replies.
joeyww12000

joeyww12000

Senior Member
Location
Chatsworth GA
On my current job we have a trough set coming into our mechanical room with around sixty 3/4" emt conduits coming into the back of it with circuit HR's. We came out of the trough with numerous 1" and 3/4" emt conduits and hit troughs on both sides of mech room to get our HR's landed in the right panels. Each HR pipe coming into the first trough has the allowable amount of conductors in each pipe. Coming from the first trough to the others troughs inside the mech room is a different story. We have pipes filled to capacity. By capacity I mean more CCCs than should be in each pipe. Someone on the job said inside the mech room because of the limited distance it didnt matter. He said it fell under the 10ft tap rule. I thought code was anything over 24" you had to derate. Anyone heard of this tap rule? If so what is it?
 
From the description it sounds like you have derating issues in the wireways as well.

There is no 10' 'tap rule' for derating but there is more to it then 24"

Take a look at 310.5(A)(2) Exception.

That allows some leeway for up to 10' or 10% of the circuit length whichever is less.
 
Tap rule is a different animal than the 24" derating rule. You are correct. Longer than 24" and you must derate.

I think Bob means 310.15(A)(2)
 
joeyww12000 said:
While we are on the issue, what about the troughs? Is there a limit to the number of wires in a trough? How would you calculate that?
Click to expand...


Wireway, Joe, wireway. :smile: at least when you have to find the rules in the NEC.

Take a look at 376.22, basically your allowed 30 CCCs and then you get nailed.
 
iwire said:
Take a look at 376.22, basically your allowed 30 CCCs and then you get nailed.
Click to expand...

Your hard pressed to exceed the 20%. Even if it was a 4?4 OP would be allowed 240 #12 CCC's at any cross section.
 
chris kennedy said:
Your hard pressed to exceed the 20%. Even if it was a 4?4 OP would be allowed 240 #12 CCC's at any cross section.
Click to expand...

Chris, as soon as you pass 30 CCCs in a cross section you have to derate.
 
Mike has a good article in this months EC & M that mentions 310.15(A)(2), here's an excerpt:

http://ecmweb.com/nec/electric_conductor_size_matters/

808ecmCBfig3.jpg
 
This is a good time to throw in the question of, if your run is less then 24" do you still have a limit of 40% fill ? or can you exceed that for short runs ?
 
acrwc10 said:
This is a good time to throw in the question of, if your run is less then 24" do you still have a limit of 40% fill ? or can you exceed that for short runs ?
Click to expand...


Under 24" you can go up to 60% fill and still forget about ampacity adjustments.

See note 4 of Table 1 in chapter 9.
 
iwire said:
Chris, as soon as you pass 30 CCCs in a cross section you have to derate.
Click to expand...
(Chris aggreed.)

I've always had a hard time understanding this.

You cant exceed 20% in a cross section, thats clear enough. You cant exceed 30 CCCs without derating which is also clear.

But 376.22 and especially 366.22 does'nt say you ca'nt exceed 30 CCCs in a cross section. It only says you ca'nt exceed 30 CCCs without derating.

That would seem to limit the number of CCCs in the entire wireway to 30. I know that practically speaking this would be unreasonable when using only # 12 conductors. It would be good if these articles stated that you cant exceed 30 CCCs "in any cross section" without derating.
 
when it says the derating only applies where the number of conductrors exceeds 30. that you could have far more conductors in the gutter and not have to derate as long as in no cross sectional has more then 30 conductors. even then only the conductors passing thru the that area are the only ones required to be derated.
 
thetacon said:
when it says the derating only applies where the number of conductrors exceeds 30. that you could have far more conductors in the gutter and not have to derate as long as in no cross sectional has more then 30 conductors. even then only the conductors passing thru the that area are the only ones required to be derated.
Click to expand...

" Only WHERE (in the raceway) the # of CCCs exceeds 30. Thanks for that Thetacon. I can see how that would be another way of saying in any cross-sectional area.
 
Status
Not open for further replies.
Top