Hi,
What is the purpose/intent behind the derating rule?

Thanks for any replies.

-regards

Greg

It's all about heat.

When you have 3 conductors in a raceway or cable all them will have a side exposed to the ambient temperature so they can dissipate the heat created by current flow.

Now picture 10 conductors in a raceway or cable, many of these conductors will end up surrounded by other conductors, limiting the amount of heat dissipated on these inner conductors.

Derating is used to protect the insulation on the conductor from heat damage.

Anyway thats how I understand it.

In a home it is very unlikely that all the circuits would run maximum current at the same time.

In a commercial / industrial job it is very likely that all the circuits would be running near capacity for long periods of time.

Bob

Hi,
Thanks IWIRE...

If a system is properly designed there is there still a need for derating?

-regards

greg

Which de-rating rule? And how can "proper design" overturn physics?

Insulation is damaged by heat. The types of insulation are rated in terms of what temperature they are prooven to withstand, for extended periods, without damage. These rating are the basis fo the tables, such as 310-16.
Electricity, encountering resistance within the wire, generates heat. This, along with an assumed 'normal' room temperature, also provide a basis for 310-16.

Now, what if the temperature where the conduit is is not "normal?" We de-rate, or sometimes up-rate, accordingly.

What if the load is only momentary? We use a larger ampacity requirement if the use is "continuous."

Now, what about the number of conductors? Another fact about alternating current is that the current flowing in one wire afects the current flow in another wire. We call this "reactive inductance" or "impedance." Mike Holt is said to demonstrate this by "shorting" both ends of 500 ft. of #14 wire on a 20 amp circuit. Rather than draw the 80+ amps simple resistance calculations would lead yo to expect, he measures onlt 12 amps. Why? His 500 ft. of wire is still wound on the spool.
This is also apparent with multi-wire circuits. For example, two out-of-phase hot wires, drawing 10 amps each, and sharing a neutral, will have nothing flowing in the neutral. Effectively, there are only 20 amps going through that pipe. Run a separate neutral with each hot, and you have ten amps through each of four wires- with a heat-generating effect of 40 amps running inside that pipe (4 wires at 10 amps each).

Often, we inadvertantly oversize our wires and our pipes. The NEC is nothing if not conservative. In our calculations, we assume current based upon our breakers (usually). That almost every circuit in a house will never see anything near 20 amps of load, electricians often get away with having huge bundles of romex, and not de-rating. They're not following code, but they get away with it.

Hi,
Thanks for the replies.

derating is about LIMITING heat/current..there is NO benefit to installing a larger conductor...that same conductor is still in the same temp and the same amount of current...you need to LIMIT the amount of CURRENT..

ten conductors in ANY raceway is bad design.

i think i am not being clear enough here..

say I have 50 three phase motors lined up in a row..

these motors are 20 amps each.

I want to feed them from a panelboard..

how many conduits would you use?

1,2,3, 4?

Me I would use 9 1/2" conduits. 6 circuits in each except one..derating has no effect on this number of wires as far as wire size..

Some folks would use a 2-1/2" and stuff them all in i guess..

Have you ever noticed that most if not all of the tables in the code EVER refer to more than THREE conductors in a raceway or cable?

why do you think that is?

if i take those fifty motors and run two 2" pipes and tee off at each motor would you consider that a good design?

should I now upsize all my wire to a #10 or #8 then increase my conduit size?

NO that is a bad design...

just becasue I can get 562 wires in a 4 inch doesnt mean to do it!

what I should do is FOLLOW the code and LIMIT the number of conductors in a raceway accordingly.

some equipment is not meant to handle but only certain sizes of wire..what do you then?

you cant get a 350 MCM to fit a 200 AMP panel!

see what i mean?

the only reason i can figure is that this was done to LIMIT the current in those instances where too MANY wires are in a single raceway.

What is the largest MULTI-CONDUCTOR POWER CABLE you can buy?

I can get a 40 conductor CONTROL cable but I cant buy a 40 condcutor POWER cable...why? IT IS BAD DESIGN.

maybe I am missing something..

I NEVER even worry about derating myself because I do not overstuff pipes.

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.

there are tables for condcutors in free air and in raceways NONE of them say anything about MORE THAN 6.

310.15(B)(2)(a) covers the adjustment for more..this is used to CORRECT not to design!

only 400.5 covers this as far as i can remember.

somebody straighten me out!

-regards

greg



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

There is less voltage drop with an oversized conductor. Less light flicker when loads switch on and off.

Hi,

Thanks for the replies...

wa2ise- you said: "There is less voltage drop with an oversized conductor. Less light flicker when loads switch on and off."

hey...start your own thread!

Thanks but this topic is about DERATING NOT VOLTAGE DROP.

you took that sentence COMPLETELY out of context to make that point didnt you?

-regards

greg



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

The derating from heat is derived from Neher/McGrath calculations concerning heat loss factors(delta T) and formulae. It would not matter if the conduit was 24" in diameter, the delta T from the bundle would be the same. Additionally, as a current-carrying conductor with a typical Laminar flow cross-section, the additional generated magnetic fields(eddy currents from skining effects) further add to increase each conductors impedance...which generates more heat.

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

With wiring in a boiler room with a tempature of 150 degrees F...you would have to use a correction factor of .58 for a 90 degree C rated conductor.

#10 = 40amps x .58= 23.2 amps

The high temp significantly reduces the current carrying capacity of a conductor.

shortcircuit

Same topic, different containers....

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)

Why is it derating not applicable under 24"?

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

What about junction boxes?

Do switch legs and travelers count?

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.

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).]

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

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

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).]

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).]

Greg I think you are confusing personal choice and code requirements.


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).]

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?


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

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.


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


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.


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.


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).]

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

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.


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.

Bob

Hi,

I hate to say it but putting 50 wires in a conduit is not a good way to go..what it cost to do it right is not what is important..

sure the NEC will give you ways to pull that off in a safe way...all i am saying is that there is a better way...most of the jobs i work are already planned by the engineers...i have NEVER in my 27+ years run across a drawing that showed 50 conductors in a single raceway! i aint saying that it cant be done i am just talking best practice i guess..most of the jobs i am working MONEY IS NO OBJECT.

I am certainly not clear as to the intentions of derating and that is why I started the thread..I am just thinking outside the box to see if there is more behind the rules..

there is also such a thing as pull-ratio...I mean that 4 wires are easier to pull than 3 etc etc..this is part of conduit fill and not derating correct?

i certainly have seen conduits full of current carrying conductors and understand that the derating is for limiting the amount of heat that the conductor is designed for...

it just strikes me as a way to correct a mistake...besides the temp adjustment..

i am just trying to further my knowledge on the subject and to take a step back and look at the issue in a different perspective..

i guess there is no mystery to derating and it is very cut and dried to some.


anywho...thanks for the discussion and putting up with my nonsense!

there is always stuff to learn and to me anyone who is seeking knowledge is ok in my book, its the people who wont change or that see things with blinders on that irk me..

i cant wait unitl we discover how to transmit energy through the air and get rid of all these wires!

heres to the future!

thanks agian to all the good and bad replies!

-regards

greg



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

The derating tables are there so that engineering will not have to be done for each specific case.

In many cases more wires can be sefely run without derating.

Consider a 100amp service with only 50amp 240v breakers all using #6 wire (I think that is a correct size) and the #6 wires run in conduit.

2 circuits or a dozen circuits can only draw 100 amps nd any number of circuits produes the same amount of heat.

The temperature in the conduit is determined not by the number of wires or circuits but by the diameter of the conduit. Larger conduits will have a lower internl temperature. Make the conduit large enough and you do not need to derate.

Most houses around here have the majority of circuits running between studs above the main panel. 40 or 60 current carrying wires in an insulated space. Perhaps engineering or temperature measurments should be taken there.

---

The 50 motor problem. I would use sub panels.

George


The number of conductors and the current they are carrying will have a direct effect on the temperature of the conduits.

The metal conduit actually dissipates heat better than a cable, it works as a heat sink and has more surface area.

That aside there is no mention of the conduit sizing in the ampacity adjustment sections.

Ten 14 AWGs in a 6" raceway are required to be derated the same as ten 14 AWGs in a 3/4" raceway.

Bob

Greg I like to do things 'right' also but...


I will never be convinced it is my right to tell the customer how they want to spend their money.

If you go to get a brake job would you be impressed if they used all the most expensive parts without asking you what you wanted.

Most customers do want to spend extra money up front to do something as you describe and that is their choice.

Heck at this point most jobs go MC anyway, no one wants to pay the high labor for EMT when MC will get them a safe and workable electrical system.

JMO, Bob

I'm admittedly confused as to what your beliefs are, mustang.
You're not being targeted by me. I read carefully. You're certainly allowed your opinion, and I'm allowed to not share it.
Your thought that derating is correcting a mistake still leads me to believe that you think more than 3 wires in a conduit is a mistake.
So, how many conductors do you feel are sound engineering?
By using the ampacity adjustments, we are following the Code.

The purpose/intent of the Code is not to limit the # of wires, but to provide a safe installation regardless. (Remember, that was the question you asked)

Economics dictates what we do in the real World.
Like Bob, I would have no work if I did everything the way I wished it to be, and bid or billed a customer accordingly. There are some Code compliant things that I don't really care for either. Those are purely personal opinion, though.
mustang, unless you're yelling at everyone, please lighten up with the ALL CAPITALS.

Hey Bob, nice answers.

Anyway, I have come across some warm conduits and gutters before. But they had bigger problems than derating!

And I have heard before that the 24" limit is so heat will escape from the 12" of conduit in each direction. Something it would not be able to do if the heat had to travel further, say 100 feet to each end of the conduit. I guess the same for air movement with bundled or stacked cables.

As for the triplex thing, from what I understand it is much the same as the way Cat-5 works by reducing EMF, and lightly shielding the conductors from the effects of being parralel. And clock-wise is the way most tree-phase systems would run. Apprently if you reversed rotation on a tri-plexed run it would have a higher resistance, I assume. But I like the bending aspect you mentioned.

Now a set of travellers... one or the other is definately not a current carrying conductor. Its just dead fill....

Remember that if you load a conductor to its maximum permitted currect, the copper will be running close to the temperature that the ampacity rating is based on. (if you are using 75°C rated conductors at their maximum permitted ampacity, the copper will be close to 75°C)
Don

[This message has been edited by resqcapt19 (edited 03-19-2005).]

Exactly Don, you start with an amp clamp, only to find most of the circuits are at 90-110% of rated load. I find many resturaunts are like this. 18 - 22A on 20A circuits. You tell the customer about it, and they say, "well its been like that for years, how come it is just now popping?" And some places where ambient temps will just cook the insulation until it is dry and brittle over time. You go up to add a conduit for new circuits and find that its like an oven up near a cieling where the rest are run next to the tin roof. Or conduits on a roof covered in black roofing tar.

iwire ---

My comments on the number of wires was directod toward my example:

100amp service with a large number of 50amp 220v circuits.

Regardless of the number of circuits in the conduit, the heat poduction is limited by the 100amp service.

I guess derating in this case could be based on 4 current carrying wires regardless of the actual number of conductore. Always see your engineer before trying this at home.

George regardless of the main size you would have to count them all as current carrying conductors.

As much as what you say is true there is nothing in the derating sections that allows what you are suggesting.

Say I have ten - 20 amp lighting circuits in a conduit. It does not matter if each circuit has 1 amp of load or 20 amps of load, the NEC derating requirements are the same.

Bob