Wireway and wiring trough frequently asked questions 25701 science park drive cleveland ohio 44122 216 464 3400.
Derating wires in a gutter.
Derating applies to current carrying conductors only.
In other words if you had a wireway with 31 ccc in the cross section for 1 derating is required.
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 factors depending on the installation environment of the cable ampacity ratings may need to be adjusted or derated to control heat flow.
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.
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 conductors for the environment where they are installed is an important consideration for selecting the type and size of conductor for a particular installation.
Adjustment factors for conductors in sheet metal auxiliary gutters are in 366 23 a and adjustment factors for conductors in metal wireways are in 376 22 b.
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.
And according to 240 4 they must be protected by a 15a fuse or circuit breaker.
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.
Ground wires are not considered current carrying conductors.
Each current carrying conductor of a paralleled set of conductors shall be counted as a current carrying conductor.
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.
Conductors cannot fill more than 20 of the interior cross sectional area of a wireway per 376 22 a.
At any cross section cable derating may be required see nec 362 5 note.
There are two important rules to remember when installing metal wireways.
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.
The adjusted ampacity for each conductor is 30 x 0 5 15.
