According to the 2014 nec.
Derating wires in a gutter.
When installing a system on a rooftop special consideration must be given to the ambient temperature inside the wire way or conduit that contain the conductors to prevent heating beyond the rating of the insulation of the.
Conductors cannot fill more than 20 of the interior cross sectional area of a wireway per 376 22 a.
The adjusted ampacity for each conductor is 30 x 0 5 15.
At any cross section cable derating may be required see nec 362 5 note.
Therefore if a conduit has four wires in it and one of those wires is a ground wire the conductors do not require derating since only three carry current.
If more than 30 current carrying conductors are present recommended wireway size recommendation wireway size by max conductor type wireway size by cable fill wireway cross sectional requirements in2 cable cross sectional area in2 3 o calculations 2 o 1 o largest.
There are two important rules to remember when installing metal wireways.
Each current carrying conductor of a paralleled set of conductors shall be counted as a current carrying conductor.
Seems like a big jump from 0 to 40 derating going from 30 to 31 ccc.
Derating factors depending on the installation environment of the cable ampacity ratings may need to be adjusted or derated to control heat flow.
Ground wires are not considered current carrying conductors.
If these are all two wire circuits hot and neutral the number of current carrying conductors in the emt is 12 and these wires must be derated to 50 percent of their ampacity shown in table 310 16.
In auxiliary gutters and metal wireways it is not required to derate the ampacity of conductors if the number of current carrying conductors is 30 or less.
Derating conductors for the environment where they are installed is an important consideration for selecting the type and size of conductor for a particular installation.
The rate at which heat is dissipated to the environment depends on many factors but the two main requirements in the nec for derating ampacity are ambient.
The derating tables are based on the amount of heat the current will produce plus the ambient temperature and the temperature of other wires close by.
Derating applies to current carrying conductors only.
And according to 240 4 they must be protected by a 15a fuse or circuit breaker.
In other words if you had a wireway with 31 ccc in the cross section for 1 derating is required.
