The short answer is that the CPD (Circuit Protection Device) should be rated to open at an amperage that is greater than the maximum load the circuit will carry and smaller than the rated amperage capacity of the wire in the circuit. We recommend choosing a size as close to, but not greater than, the amperage capacity of the wire.
Use our Circuit Wizard to find what circuit protection would work best for your application.
Fuses are thermal devices that open the circuit by utilizing a “fusible link” that melts at a known amperage in a known length of time. Circuit breakers can be either thermal or magnetic devices or a combination of the two.
There are two primary methods that CPD’s use to determine that excess amperage is flowing in a circuit. Thermal devices open to break the circuit and stop the current flow in response to heat generated by the excess amperage. Magnetic devices react to a magnetic field created by excess amperage.
When a circuit is activated, there is an initial surge of current (amperage) that the CPD must allow to pass without tripping the CPD. The two screen reprints below from the Blue Sea Systems’ testing system illustrate the difference in inrush currents between inductive (like motors) and resistive (like light bulbs) loads.
Notice that the ratio of normal running current (represented by the flat portion of the amperage line) between the inductive and the resistive graphs varies dramatically. The 17A inductive load initially drew 80 A or 470%, whereas the 44A resistive load initially drew only 126A or 286% of its normal operating current. Such inrush currents must be considered when sizing CPD’s. Each Blue Sea Systems’ CPD has a time/current chart shown on its catalog page.
We recommend sizing fuses and circuit breakers for 5X multiples for inductive loads and 3X multiples for resistive loads and assuming this inrush for approximately .5 seconds.