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#92466 03/17/05 10:22 PM
Joined: Feb 2004
Posts: 494
M
Member
Hi,
shortcircuit...

i guess your using the 90 degree wire column?

I SAID: "If I have a conduit running through a boiler at a power plant and it is ALWAYS 150 degrees in there do I have to worry about derating? NO! not if I LIMIT the number of conductors because as you mention the insualtion is designed to withstand certain temps.. it is ONLY when someone tries to put more than 3 current carrying conductors in a raceway that I am even worried about it."

i dont have to do that becasue I do not have MORE than 3 current carrying conductors. And because its in a power plant the NEC doesnt apply... I will use that if you prove me wrong!

-regards

Greg



[This message has been edited by mustangelectric (edited 03-17-2005).]

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#92467 03/17/05 10:37 PM
Joined: Jun 2004
Posts: 613
S
Member
Yes mustang...the 90 degree C column...

What if that boiler room in that nuc plant got up th 195 degrees fahenhiet, then how much current would that #10 be capable of carrying?

shortcircuit

#92468 03/17/05 10:59 PM
Joined: Feb 2004
Posts: 494
M
Member
Hi,
well i would just use the same calc you just presented:

#10 = 40amps x .67= 27 amps


unless i looked at the wrong table..

hey i am getting hungry and tired...i am almost off the clock...30 more minutes!


i am glad you have a lighter side!


-regards

greg

#92469 03/18/05 01:53 AM
Joined: Nov 2002
Posts: 794
Likes: 3
W
Member
Quote


Thanks but this topic is about DERATING NOT VOLTAGE DROP.

Though somewhat off topic, less voltage drop means that there's less waste heat lost in the wiring. And thus the wires won't get as hot. So you use heavier conductors in situations where many wires are bunched up together. Each wire contributes less waste heat then they would have if smaller given the same loads. Derating also gives a side benefit of less voltage drop for the customer/user.

[This message has been edited by wa2ise (edited 03-18-2005).]

#92470 03/18/05 05:53 AM
Joined: Jun 2004
Posts: 613
S
Member
No mustang...the correct answer is 0amps, yup thats right...a big fat goose egg...zilch...nada...zero amps dude.

You see, the temperature ratings at the top of the columns of table 310.16 are the MAXIMUM that the respective conductor is capable of enduring.

Read the last sentence of the heading above table 310.16...(30 degreesC and 86 degreesF)This is the temperature that the ampacities listed below in the table are capable of.

Any variation of that temperature and you MUST use the Correction Factors listed in the table below the ampacity table...

This is based on 3 conductors only. Add more in a pipe or cable and you must DERATE further according to table 310.15(B)(2)(a)

Also the 2005 Code recognizes that conductors in conduits exposed to direct sunlight experience a significant temperature rise also. See FPN #2 in 310.10 2005NEC. We may see a new table in 2008.

Here is a test question for you...What is allowable ampacity of #10 THHN installed in this example...

1) 8- #10 THHN/THWN current-carrying conductors in a conduit run 20feet through an area where the temperature is 120 degrees fahrenhiet?


shortcircuit



[This message has been edited by shortcircuit (edited 03-18-2005).]

#92471 03/18/05 06:45 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Greg I think you are confusing personal choice and code requirements. [Linked Image]

Quote
ten conductors in ANY raceway is bad design.

If you prefer to run separate raceways and your customer does not mind paying for higher labor costs you can do that and avoid derating for the number of conductors in the raceway.

The code is not a design manual, it is a minimum set of standards.

If someone wishes to place 50 conductors in a raceway the code has rules that make that installation safe.

I spend much of my time wiring office buildings, a typical electric rooms may have six 42 circuit panels all fully filled.

If I use your method of limiting each raceway to 3 current carrying conductors I would need to run 84 raceways out of that electric room.

There is no way I am going to do that, I will run two 2" EMTs out of each panel for a total of 12 raceways each with 21 current carrying conductors.

As these are typically all 20 amp circuits we will run 10 AWG. in the 2" raceways. (Note Massachusetts has a different derating table, in other states this installation would require 8 AWG.

Greg we can go back and forth on good design and bad design, the fact is much depends on the particulars of the job.

We would quickly start losing work if we bid all work as separate raceways.

That said I agree with you that many times it is a better installation to run separate raceways. If you have a fault in a raceway with many circuits in it you interrupt service to a large area as opposed to just one circuit.

IMO the design is more often influenced by installation costs rather than long term service.

Bob




[This message has been edited by iwire (edited 03-18-2005).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#92472 03/18/05 08:06 AM
Joined: Dec 2000
Posts: 4,294
Member
The whopping # of conduits that Iwire posted would be a minimum for 1Ø system or line-line 3Ø loads only.

In the case of a 4W Y system, you'd have to tack on another third for the "hangers" that would have to be 2 wire circuits.
How does one limit the number of conductors in a raceway to 3 in the case of 3Ø - 4Wire services and feeders?

Is a 4 wire system inherently bad engineering and design as well? [Linked Image]


[This message has been edited by electure (edited 03-18-2005).]

#92473 03/18/05 08:38 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
e57 I will take a stab at your questions.

Keep in mind these are just my opinions and I have nothing to back it up. [Linked Image]

Quote
So why is it you can have up to 30 conductors in any cross-section of gutter/wireway before the need for derating? (under 20% fill of course)

My guess is that the conductors will not be in a tight bundle like in a raceway or cable.

Quote
Why is it derating not applicable under 24"?

What about all those conductors on both sides of every panel?

What about junction boxes?

I believe the answer to those 3 question is heat conductivity.

Copper along with being a very good electrical conductor is an excellent conductor of heat.

With unbundled conductors on each side of the nipple, Jbox or panel acting as heat sinks the up to 24" long section of conductors that are tightly packed can not get much warmer than the conductors on each side.

Quote
Do switch legs and travelers count?

IMO they do although I doubt it matters much as generally derating does not get in the way for 15, 20 and 30 amp circuits until you exceed 9 conductors.

Quote
Ever wonder why 3 or more conductor cable is triplexed? (Twisted clock-wise) PS - you can get 12-8 w/ Gnd MC! And all are twisted the same way.

There may be other reasons but that 12-8 MC (which along with 10-8 MC we use often) would be much stiffer to bend without the twisting.

If all the conductors were simple parallel in the cable when you tried to bend the cable it would be difficult as the conductors on the outside of the bend need to be longer than the conductors on the inside of the bend.

By putting the twists in the cable it makes the bend much easer.

Try bending eight solid 12 AWGs taped together in a parallel bundle.

Then try the same thing with a twist.

Even quicker experiment, try to bend a 12/2 NM cable on its side. In other words with one conductor on the outside radius and one conductor on the inside radius.

Unless the conductors slide by each other or stretch it is imposable.

Bob


[This message has been edited by iwire (edited 03-18-2005).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#92474 03/18/05 01:24 PM
Joined: Feb 2004
Posts: 494
M
Member
Hi,
I NEVER said the following:

"limiting each raceway to 3 current carrying conductors"

what i did say: "what I should do is FOLLOW the code and LIMIT the number of conductors in a raceway accordingly."

what you are probably referring to: "was my example for discussion regardig the conduit passing through a boiler at a powerhouse"..

i can see that i am turning into a target becasue I have opinions and ideas...so i will bail out of this thread too so as not to bother anyone.

it really pays to read carefully.

anyway...interesting discussion.

thanks for the comments

-regards

greg

#92475 03/18/05 01:40 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Greg you are not a target and your opinions are welcome just do not expect we will all agree with them just as I do not expect all to agree with me. [Linked Image]

Quote
what i did say: "what I should do is FOLLOW the code and LIMIT the number of conductors in a raceway accordingly."

Well what code are you speaking of, code wise I can put 50 current carrying conductors in a raceway.

If I choose to do so I will have to use the derating tables.

At some point well before 50 conductors it becomes more costly to bump up the conductor size than to run additional raceways.

For me here in MA with our derating table this point of diminishing return is 24 current carrying conductors. More than 24 current carrying conductors in a raceway will result in the use of 8 AWG for 20 amp branch circuits. At that point I would rather add a raceway.

Again these decisions are design considerations that will vary from job to job.

I agree it is an intresting topic. [Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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