Ampacity refers to the amount of current (in Amperes) that can be carried by a particular conductor at a particular operating temperature. Ampacity is determined based on the size of the conductor (how much metal is available to carry current) and the installation conditions (how easily the heat generated by the current can dissipate out to its surroundings).
Determining ampacity is one of the most important calculations in electrical design. If a conductor with too low of an ampacity is selected, the installation will be dangerous - the conductor or equipment the conductor is connected to could overheat and start a fire. On the other hand, upsizing conductors unnecessarily leads to additional costs and may make projects too expensive.
The 2020 National Electrical Code addresses ampacity calculations in Articles 310 and 311. The NEC method for determining ampacity is as follows:
Determine the right table(s) to use, reflecting your installation conditions. NEC 310.16 is the most common table used for low voltage installations.
Determine the temperature rating of your conductor. Typically 60°C, 75°C, 90°C, or 105°C. The conductor temperature rating is related to the naming convention as described in NEC 310 and 311. The maximum operating temperature often varies depending on whether or not the location of use is wet or dry.
Determine if there are any additional considerations that warrant utilizing a lower temperature for any segment of your ampacity calculation.
Terminations are often rated lower than conductors. 60°C terminations are the minimum for low voltage equipment less than 100A and 75°C terminations are the minimum for low voltage equipment over 100A. Medium voltage terminations are almost always rated for 90°C. NEC Article 110 has more information on these requirements.
Is the raceway suitable for the temperature you are operating at? Raceways may not be rated for 105 degree Celsius.
Is additional margin required per your client or project needs? Are there any other special circumstances to be aware of?
Derate the ampacity tables based on the conditions of use throughout the run. Derating factors are numbers that multiply against table ampacities to modify them for specific conditions.
The 4 steps above seem pretty straightforward, but derating can make things complicated. The primary derating factors described by the NEC are bundling, ambient temperature, and burial depth. Here’s a deeper-dive on each one:
Bundling - when more than 3 current-carrying conductors are routed together without spacing for more than 24”, the NEC requires their ampacity to be derated based on exactly how many current-carrying conductors are in one area. The values for the bundling derating factor, x, are as follows:
4-6 conductors: x = .8
7-9 conductors: x = .7
10-20 conductors: x = .5
21-30 conductors: x = .45
31-40 conductors: x = .4
41 or more conductors: x = .35
Since ampacity tables in the NEC are all based on no more than 3 current-carrying conductors in each installation, it makes sense that additional derating is required for more wires bundled together. Additional conductors means additional heat. When bundled together this heat can’t easily be dissipated, leading to increased operating temperatures.
Ambient Temperature - When the air or soil surrounding a cable is warmer than the conditions listed in a table, the conductor won’t be able to carry as much current before reaching its temperature rating. Conversely, when the surroundings are cooler, the conductor can actually carry more current before the temperature limit is reached. The mathematical formula for the derating is:
y = √(TC-TA) / √(TC-TR)
Where:
y is the ambient temperature derating factor
TC is the conductor operating temperature limit in °C, as adjusted for any special conditions (including terminations) described above
TA is the actual ambient air or soil temperature in °C
TR is the reference ambient temperature in °C used in the relevant ampacity table from the NEC
A temperature derating factor greater than 1 is permissible. Derating for ambient temperature may be based on the conductor temperature instead of the termination temperature if the conductor temperature is higher.
Burial Depth - When a conductor is buried deeper than the conditions assumed for the NEC ampacity tables, the ampacity has to be decreased. The NEC does not allow a burial depth derating factor greater than 1. In particular, the NEC notes that this requires a derating of 6% per foot of difference in burial depth. Mathematically, this means:
z = .94 ^ ((DB-DR) / 12)
Where:
z is the burial depth derating factor
DB is the burial depth in inches
DR is the reference condition burial depth in inches from the relevant ampacity table from the NEC
Putting It All Together- The ampacity of a conductor A’ can be calculated:
A' = x y z A
Where A is the appropriate NEC table’s reference ampacity before correction factors at the conductor operating temperature limit (as adjusted down for terminations). A should be carefully selected for the correct design conditions as described in the 4-step process above. For multiple conditions of use throughout the circuit, the above equation can be extended to several values of A and its related derating factors.
A must be correctly selected to ensure overheating does not occur at the point of termination. Low voltage equipment must utilize NEC 310.16 for A. Medium voltage equipment may use the conditions of use tables throughout the circuit.
Example: What is the ampacity of (20) 8 AWG CU XHHW-2 current-carrying conductors run together through conduit in an outdoor area that sees ambient temperatures as high as 40°C? The equipment the wires terminate on have standard temperature ratings.
Solution: To solve this problem, we’ll follow the 4-step process outlined above and draw a picture to help ourselves understand the situation.
Step 1 is to determine the right NEC table to use. Low voltage conductors like XHHW-2 are covered by Article 310. Since the conductors are run in conduit, NEC Table 310.16 applies.
Step 2 is to determine the temperature rating of our conductor. XHHW-2 is rated for 90°C in wet or dry locations per NEC 310. The ampacity of XHHW-2 8 AWG at 90°C is 55A.
Step 3 is to determine if there are any special conditions that warrant limiting our ampacity to a lower temperature. Standard low voltage equipment under 100A has 60°C termination temperature limitations. The ampacity of conductors at 60°C is 40A, which we shall limit ourselves to instead.
Step 4 is to derate the reference ampacity for the conditions of use. We don’t have to consider a burial depth derating factor since the conductors are run through conduit in air-not underground.
The ambient temperature derating factor should be computed since the ambient air temperature could be as high as 40°C and NEC 310.16 assumes an ambient temperature of 30°C. The value of y can be computed against the 60°C operating limit of XHHW-2 because of the terminations:
y = √(60°C - 40°C) / √(60°C - 30°C) = .816
The bundling derating factor is also relevant since 20 conductors will be routed together in a single conduit. For 10-20 current-carrying conductors, x = .5.
The ampacity A’ can then be computed:
A’ = A x y = 40A * .816* .5 = 16.32A
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