derating factor
- Thread starter bill g
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
if they are loaded more than 15 amps
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Think about a 4 inch pipe run horizontally. All the wires will be bundled on top of each other creating heat. Sure there is more air but eventually that air will heat also.
we all follow Code rules we don't understand :grin:
I look at this like an oven. The larger the oven the longer it might take to heat, but eventually the temperature will be obtained.
:-?so with table c.1 page 705 it says a 4" emt is capable of a maximum 443 #12 thhn conductors. table 310.15(b)(2)(a) says 41 and above at 35% giving us 10.5 amps per . so aside from the fact that i can't type well, if we look below the table at fpn no.1 or a little further to 310.15(b)(2)(a) exception 1. confused . gray.?
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winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Another way to think of it is that the code rules are only simplified conservative approximations, not bleeding edge maximum performance calculations.
IMHO conduit size will make a difference, and conductors in a large conduit will have better cooling and operate at a lower temperature than the same conductors in a small conduit. But the code rules are based upon a reasonable worst case and don't make any allowance for improved cooling in a larger conduit.
Similarly the code rules say nothing about combinations of large and small current carrying conductors (you just count them up), conductors buried in thermal insulation in walls, or any of a number of circumstances which would change the current handling capacity of the conductors.
The code does permit 'engineering supervision' to be used to actually calculate the ampacity of the conductors in a given situation...but I suspect that in all but extreme cases the cost of getting a suitably trained engineer to do the calculations properly would exceed the cost of simply using larger wire.
-Jon
IMHO conduit size will make a difference, and conductors in a large conduit will have better cooling and operate at a lower temperature than the same conductors in a small conduit. But the code rules are based upon a reasonable worst case and don't make any allowance for improved cooling in a larger conduit.
Similarly the code rules say nothing about combinations of large and small current carrying conductors (you just count them up), conductors buried in thermal insulation in walls, or any of a number of circumstances which would change the current handling capacity of the conductors.
The code does permit 'engineering supervision' to be used to actually calculate the ampacity of the conductors in a given situation...but I suspect that in all but extreme cases the cost of getting a suitably trained engineer to do the calculations properly would exceed the cost of simply using larger wire.
-Jon
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Just remember that you have to derate them, if there are more than three calculator-carrying engineers in a cubicle.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I agree with Jon's answer, but will add that it would be too difficult to write, let alone enforce, a rule that said we have to derate unless the conduit is this size and the conductors are that size. Imagine all the possible combinations that would have to be described in detail, and recognize that for any given combination (e.g., not more than 7 #12 conductors combined with 2 #14 conductors in a 3 inch EMT), there would have to be a engineered calculation or a lab experiment to prove that that combination could be safely installed without having to derate in that particular set of circumstances. It is simply not practical to do anything other than say, count the current-carrying conductors, and then follow this table.
Cold Fusion
Senior Member
- Location
- way north
charlie, jon, roger -
I have never seen a math model, research, or paper on the heat dissipation depending on conduit size. Nehr-McGrath doesn't take conduit size into consideration.
I have no clues as to how one might figure this out. Does anyone have a reference or model?
cf
- Location
- Massachusetts
- Location
- Massachusetts
I also agree with that.
I think it is worth pointing out that the code is also aimed at the most common installations. I have been doing this job for a while and rarely would you end up with a lot of small conductors in a large conduit that also have to be derated.
The times I have seen large numbers of small conductors in large raceways they have been control circuits that we do not have to derate.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I have used a program called Ampcalc to obtain ampacities for underground ductbanks. It allows you to input the conduit size. But I have not done a test to see if it gives a different ampacity when you change the conduit size and make no other changes.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Is the derating the same if they have slide rules instead of electronic calculators?:roll:Just remember that you have to derate them, if there are more than three calculator-carrying engineers in a cubicle.
Cold Fusion
Senior Member
- Location
- way north
Lots heavier derate for the slie rule carriers. And there are at least two different slide rule rates that I know of. My 25cm bamboo Hemi heats up a lot quicker than my 15cm circular aluminum
cf
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