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.
#92460 - 03/17/0506:12 PMRe: The purpose of Derating
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?
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!
[This message has been edited by mustangelectric (edited 03-17-2005).]
Electricity has no respect for ignorance!
#92461 - 03/17/0507:01 PMRe: The purpose of Derating
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).]
#92464 - 03/17/0510:06 PMRe: The purpose of Derating