Question 1

How is wire size calculated in the US?

Accepted Answer

In the United States, wire size is measured using the American Wire Gauge (AWG) system for smaller sizes (from 14 AWG up to 4/0 AWG), and Thousand Circular Mils (kcmil) for larger sizes (starting at 250 kcmil). Sizing is governed by the National Electrical Code (NEC) Table 310.16, which specifies maximum allowable ampacities based on conductor material (copper or aluminum) and terminal temperature ratings (60°C, 75°C, or 90°C).

Question 2

What is the maximum allowed voltage drop?

Accepted Answer

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuit conductors, and a maximum combined drop of 5% across both feeders and branch circuits. Sizing wire larger to minimize voltage drop is crucial for long runs, as excessive voltage drop causes motors to run hot, lights to flicker, and electronic equipment to malfunction.

Question 3

Why does conductor material (copper vs. aluminum) affect sizing?

Accepted Answer

Copper has a lower electrical resistance than aluminum (resistivity K of copper is 12.9, whereas aluminum is 21.2). Because copper is a better conductor, a copper wire can carry more current than an aluminum wire of the same size. Aluminum wire is typically lighter and more cost-effective for large services, but it requires a larger physical gauge size to carry the same load as copper.

Question 4

How does temperature rating (60°C, 75°C, 90°C) affect ampacity?

Accepted Answer

The temperature rating specifies the maximum temperature the wire insulation can safely handle. Wires with higher temp ratings (like THHN at 90°C) have a higher raw ampacity limit. However, the final sizing must also take into account the temperature rating of the termination lugs on breakers and equipment, which are usually rated for 75°C. Thus, 75°C is the standard limit used for final sizing calculations.

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