How to Choose the Right Wire Size: NEC Table 310.16 Guide
Choosing the correct wire size is one of the most fundamental tasks for any electrician. A wire that is too small can overheat, melt its insulation, and potentially start a fire. A wire that is too large wastes money and can be difficult to terminate in standard lugs. The process of sizing wire correctly relies heavily on NEC Table 310.16, but simply looking up a number in the chart is just the first step.
Step 1: Determine the Load
Before you can size a wire, you need to know how much current it will carry. This involves calculating the load and checking the rating of the overcurrent protection device (the circuit breaker or fuse). The wire's ampacity must be equal to or greater than the non-continuous load plus 125% of any continuous load.
Step 2: Check NEC Table 310.16
Once you know the required ampacity, you turn to Table 310.16. This table lists the maximum ampacity for various sizes of copper and aluminum conductors. However, you must pay attention to the temperature rating columns (60°C, 75°C, or 90°C).
Even if you are using THHN wire rated for 90°C, NEC 110.14(C) usually limits you to the 60°C or 75°C column based on the temperature rating of the terminals at the breaker and the equipment. For most commercial and residential applications with 100A or higher equipment, the 75°C column is the standard reference point.
Step 3: Apply Adjustment Factors (Derating)
The ampacities in Table 310.16 assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable. If your conditions differ, you must derate:
- Ambient Temperature: If the area is hotter than 86°F (like an attic or a roof), you must reduce the wire's ampacity using the correction factors at the bottom of Table 310.16.
- Conductor Bundling: If you bundle 4 or more current-carrying conductors in the same conduit, they can't dissipate heat as easily. You must apply an adjustment factor from NEC 310.15(C)(1).
This bundling adjustment is easy to confuse with a different rule that also involves conduit and conductor count: NEC Chapter 9 Table 1 conduit fill. Derating is about electrical capacity, whether the wire can carry the current once it's sharing a raceway with other hot conductors. Fill is about physical space, whether the conductors fit in the conduit at all. A run can pass one check and fail the other. For the fill side of the calculation, see our guide on NEC conduit fill calculation.
If the run is also long, run a voltage drop check separately. A wire sized correctly for ampacity and derating can still leave a load underpowered at the end of a 150-foot run. See our guide on calculating voltage drop for the formula and the NEC's 3% recommendation.
The Easy Way
Factoring in continuous loads, terminal ratings, temperature, and conductor bundling can get complicated quickly. Try using our Wire Size & AWG Calculator to quickly verify your math and ensure your installation is up to code.