By Bert Markgraf | Updated March 24, 2022
Circuit breakers safeguard both the circuit and the cables they serve. They are specified by voltage, continuous current, and short‑circuit current. A breaking‑tripping curve shows how long a breaker will carry a given current before opening. When sizing a three‑phase breaker, you must consider the cable’s current‑carrying capacity, the system’s short‑circuit current, and the electrical characteristics of the connected loads.
Choose breakers that match the system voltage and its short‑circuit capability. For residential installations, the short‑circuit value is typically supplied by the utility and is uniform across a neighborhood. In commercial or industrial settings, an electrical engineer must perform a short‑circuit analysis to determine the available fault current.
Calculate the total connected load and multiply by 1.25 to allow for additional heat generated when the breaker sits beside others in a panel. Round up to the nearest standard rating and then select a cable that matches the breaker’s continuous‑current rating. Common breaker sizes are 15 A, 20 A, 30 A and 40 A, requiring AWG #14, #12, #10 and #8 conductors respectively.
Non‑linear loads such as motors or transformers draw high starting currents that can trip breakers even when the steady‑state load is safe. Review the nameplate or documentation for inrush values. If the data are unavailable, approximate the full‑load current by multiplying by six and consult the breaker curve to ensure the surge will not trip the device. Inrush currents last several seconds; if the peak exceeds the breaker’s curve, select a higher‑rated breaker and use the corresponding larger cable.
