ECN Forum Forums Code Related Discussion NEC & other Code issues The purpose of Derating

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