Certain phrases are used so commonly in the electrical industry that an explanation seems to be almost unnecessary. However, the definition might not specifically be found within the National Electrical Code. One phrase that comes to mind is “condition of use” in relation to conductors.
Determining the ambient temperature correction factors involving the installation and operation of conductors and the number of current-carrying conductors that may adversely affect the operation of the conductors is a major concern to electricians, electrical contractors and electrical inspectors. Nowhere within the NEC do we find the phrase “condition of use,” but the calculation involving this application can have some very definite and far-ranging effects on any electrical installation. Let’s explore and evaluate what is meant by this phrase and how it applies to an installation.
“Condition of use” applies to branch circuit, feeder and service conductors. Probably the best place to start evaluating this is Section 310.14, which deals with the selection of ampacities for conductors rated 0–2,000V, and where temperature limitation of conductors is discussed in 310.14(A)(3).
This subsection states that “no conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved.”
The following sentence further ensures that “in no case shall conductors be associated together in such a way, with respect to the type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.”
High temperature is a major issue with any insulated conductor and can cause major damage to insulation. Section 310.14(A)(3), Informational Note No. 1 explains that the “temperature rating of an insulated conductor is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time-period without serious degradation.”
There are four determining factors of operating temperature that must be taken into consideration. The first factor is ambient temperature, which can vary along the conductor length and with time. The second factor is the heat generated internally in the insulated conductor as the result of load current, including fundamental and harmonic currents. Factor number three is the rate at which any generated heat in the conductor, due to current flow and ambient temperature, dissipates into the ambient medium. Any thermal insulation that covers or surrounds current-carrying conductors affects heat dissipation from these encased, insulated conductors. The final factor deals with adjacent load-carrying conductors that have the dual effect of raising the surrounding ambient temperature and impeding heat dissipation.
These four factors require that an ambient temperature correction factor be applied to compensate or adjust the ampacity of tables 310.16 through 310.20, depending on the ambient temperature rating of the table. There are two ambient temperature tables, Table 310.15(B)(1) for tables using 30°C/86°F or Table 310.15(B)(2) for tables using 40°C/104°F.
Ampacity Table 310.16 is based on the following three specified conditions:
- an ambient temperature of 30°C/86°F
- up to three current-carrying conductors
- insulation rated at 60°C/140°F, 75°C/167°F and 90°C/194°F
I use Ampacity Table 310.16, Section 210.19(A)(1) for determining the minimum ampacity and size of a branch circuit conductor, and Section 210.20(A) for determining the size of the overcurrent device for that branch circuit, while at the same time addressing “condition of use.”
Let’s assume that we are installing a 1-inch EMT with six No. 6 AWG, copper XHHW insulated current-carrying conductors in an ambient temperature of 110°F (43°C) in a wet location. Use Table 310.4(A) to determine that XHHW is a 90°C insulation in a dry and damp location and a 75°C insulation in a wet location. In Table 310.16, a No. 6 XHHW copper conductor is rated at 55A in the 60°C column, 65A in the 75°C column, and 75A in the 90°C column. In a 43°C/110°F ambient temperature, Table 310.15(A) requires a derate of 18% or a multiplier of 0.82, using the 75°C column temperature rating for a wet location, or 65A times 0.82 equaling 53.3A, after taking into consideration the high ambient temperature.
Since there are six No. 6 AWG current-carrying conductors in the raceway and Table 310.16 is based on up to three current-carrying conductors, Table 310.15(C)(1) requires an adjustment factor of 80% times the 53.3A conductor, as it was ambient-temperature-compensated, equaling 42.64A.
In this example, the six No. 6 conductors have been calculated with the “condition of use” for high temperature and the number of current-carrying conductors.