One is about how many wires can fill the wireway while the other is about when derating or adjustment factors must be applied to those wires.
Derating wires in a gutter.
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.
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.
The adjusted ampacity for each conductor is 30 x 0 5 15.
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.
According to the 2014 nec.
Derating factors depending on the installation environment of the cable ampacity ratings may need to be adjusted or derated to control heat flow.
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.
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.
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.
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.
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.
At any cross section cable derating may be required see nec 362 5 note.
Seems like a big jump from 0 to 40 derating going from 30 to 31 ccc.
In other words if you had a wireway with 31 ccc in the cross section for 1 derating is required.
Derating applies to current carrying conductors only.
And according to 240 4 they must be protected by a 15a fuse or circuit breaker.
Wireway and wiring trough frequently asked questions 25701 science park drive cleveland ohio 44122 216 464 3400.
Ground wires are not considered current carrying conductors.
